v8.h

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// Copyright 2012 the V8 project authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
 
/** \mainpage V8 API Reference Guide
 *
 * V8 is Google's open source JavaScript engine.
 *
 * This set of documents provides reference material generated from the
 * V8 header file, include/v8.h.
 *
 * For other documentation see https://v8.dev/.
 */
 
#ifndef INCLUDE_V8_H_
#define INCLUDE_V8_H_
 
#include <stddef.h>
#include <stdint.h>
#include <stdio.h>
 
#include <memory>
#include <string>
#include <type_traits>
#include <utility>
#include <vector>
 
#include "cppgc/common.h"
#include "v8-internal.h"                 // NOLINT(build/include_directory)
#include "v8-version.h"                 // NOLINT(build/include_directory)
#include "v8config.h"                 // NOLINT(build/include_directory)
 
// We reserve the V8_* prefix for macros defined in V8 public API and
// assume there are no name conflicts with the embedder's code.
 
/**
 * The v8 JavaScript engine.
 */
namespace v8 {
 
class AccessorSignature;
class Array;
class ArrayBuffer;
class BigInt;
class BigIntObject;
class Boolean;
class BooleanObject;
class CFunction;
class Context;
class Data;
class Date;
class External;
class Function;
class FunctionTemplate;
class HeapProfiler;
class ImplementationUtilities;
class Int32;
class Integer;
class Isolate;
template <class T>
class Maybe;
class MicrotaskQueue;
class Name;
class Number;
class NumberObject;
class Object;
class ObjectOperationDescriptor;
class ObjectTemplate;
class Platform;
class Primitive;
class Promise;
class PropertyDescriptor;
class Proxy;
class RawOperationDescriptor;
class Script;
class SharedArrayBuffer;
class Signature;
class StartupData;
class StackFrame;
class StackTrace;
class String;
class StringObject;
class Symbol;
class SymbolObject;
class PrimitiveArray;
class Private;
class Uint32;
class Utils;
class Value;
class WasmModuleObject;
template <class T> class Local;
template <class T>
class MaybeLocal;
template <class T> class Eternal;
template<class T> class NonCopyablePersistentTraits;
template<class T> class PersistentBase;
template <class T, class M = NonCopyablePersistentTraits<T> >
class Persistent;
template <class T>
class Global;
template <class T>
class TracedGlobal;
template <class T>
class TracedReference;
template <class T>
class TracedReferenceBase;
template<class K, class V, class T> class PersistentValueMap;
template <class K, class V, class T>
class PersistentValueMapBase;
template <class K, class V, class T>
class GlobalValueMap;
template<class V, class T> class PersistentValueVector;
template<class T, class P> class WeakCallbackObject;
class FunctionTemplate;
class ObjectTemplate;
template<typename T> class FunctionCallbackInfo;
template<typename T> class PropertyCallbackInfo;
class StackTrace;
class StackFrame;
class Isolate;
class CallHandlerHelper;
class EscapableHandleScope;
template<typename T> class ReturnValue;
 
namespace internal {
enum class ArgumentsType;
template <ArgumentsType>
class Arguments;
template <typename T>
class CustomArguments;
class FunctionCallbackArguments;
class GlobalHandles;
class Heap;
class HeapObject;
class ExternalString;
class Isolate;
class LocalEmbedderHeapTracer;
class MicrotaskQueue;
class PropertyCallbackArguments;
class ReadOnlyHeap;
class ScopedExternalStringLock;
struct ScriptStreamingData;
class ThreadLocalTop;
 
namespace wasm {
class NativeModule;
class StreamingDecoder;
}  // namespace wasm
 
}  // namespace internal
 
namespace metrics {
class Recorder;
}  // namespace metrics
 
namespace debug {
class ConsoleCallArguments;
}  // namespace debug
 
// --- Handles ---
 
/**
 * An object reference managed by the v8 garbage collector.
 *
 * All objects returned from v8 have to be tracked by the garbage
 * collector so that it knows that the objects are still alive.  Also,
 * because the garbage collector may move objects, it is unsafe to
 * point directly to an object.  Instead, all objects are stored in
 * handles which are known by the garbage collector and updated
 * whenever an object moves.  Handles should always be passed by value
 * (except in cases like out-parameters) and they should never be
 * allocated on the heap.
 *
 * There are two types of handles: local and persistent handles.
 *
 * Local handles are light-weight and transient and typically used in
 * local operations.  They are managed by HandleScopes. That means that a
 * HandleScope must exist on the stack when they are created and that they are
 * only valid inside of the HandleScope active during their creation.
 * For passing a local handle to an outer HandleScope, an EscapableHandleScope
 * and its Escape() method must be used.
 *
 * Persistent handles can be used when storing objects across several
 * independent operations and have to be explicitly deallocated when they're no
 * longer used.
 *
 * It is safe to extract the object stored in the handle by
 * dereferencing the handle (for instance, to extract the Object* from
 * a Local<Object>); the value will still be governed by a handle
 * behind the scenes and the same rules apply to these values as to
 * their handles.
 */
template <class T>
class Local {
 public:
  V8_INLINE Local() : val_(nullptr) {}
  template <class S>
  V8_INLINE Local(Local<S> that)
      : val_(reinterpret_cast<T*>(*that)) {
    /**
     * This check fails when trying to convert between incompatible
     * handles. For example, converting from a Local<String> to a
     * Local<Number>.
     */
    static_assert(std::is_base_of<T, S>::value, "type check");
  }
 
  /**
   * Returns true if the handle is empty.
   */
  V8_INLINE bool IsEmpty() const { return val_ == nullptr; }
 
  /**
   * Sets the handle to be empty. IsEmpty() will then return true.
   */
  V8_INLINE void Clear() { val_ = nullptr; }
 
  V8_INLINE T* operator->() const { return val_; }
 
  V8_INLINE T* operator*() const { return val_; }
 
  /**
   * Checks whether two handles are the same.
   * Returns true if both are empty, or if the objects to which they refer
   * are identical.
   *
   * If both handles refer to JS objects, this is the same as strict equality.
   * For primitives, such as numbers or strings, a `false` return value does not
   * indicate that the values aren't equal in the JavaScript sense.
   * Use `Value::StrictEquals()` to check primitives for equality.
   */
  template <class S>
  V8_INLINE bool operator==(const Local<S>& that) const {
    internal::Address* a = reinterpret_cast<internal::Address*>(this->val_);
    internal::Address* b = reinterpret_cast<internal::Address*>(that.val_);
    if (a == nullptr) return b == nullptr;
    if (b == nullptr) return false;
    return *a == *b;
  }
 
  template <class S> V8_INLINE bool operator==(
      const PersistentBase<S>& that) const {
    internal::Address* a = reinterpret_cast<internal::Address*>(this->val_);
    internal::Address* b = reinterpret_cast<internal::Address*>(that.val_);
    if (a == nullptr) return b == nullptr;
    if (b == nullptr) return false;
    return *a == *b;
  }
 
  /**
   * Checks whether two handles are different.
   * Returns true if only one of the handles is empty, or if
   * the objects to which they refer are different.
   *
   * If both handles refer to JS objects, this is the same as strict
   * non-equality. For primitives, such as numbers or strings, a `true` return
   * value does not indicate that the values aren't equal in the JavaScript
   * sense. Use `Value::StrictEquals()` to check primitives for equality.
   */
  template <class S>
  V8_INLINE bool operator!=(const Local<S>& that) const {
    return !operator==(that);
  }
 
  template <class S> V8_INLINE bool operator!=(
      const Persistent<S>& that) const {
    return !operator==(that);
  }
 
  /**
   * Cast a handle to a subclass, e.g. Local<Value> to Local<Object>.
   * This is only valid if the handle actually refers to a value of the
   * target type.
   */
  template <class S> V8_INLINE static Local<T> Cast(Local<S> that) {
#ifdef V8_ENABLE_CHECKS
    // If we're going to perform the type check then we have to check
    // that the handle isn't empty before doing the checked cast.
    if (that.IsEmpty()) return Local<T>();
#endif
    return Local<T>(T::Cast(*that));
  }
 
  /**
   * Calling this is equivalent to Local<S>::Cast().
   * In particular, this is only valid if the handle actually refers to a value
   * of the target type.
   */
  template <class S>
  V8_INLINE Local<S> As() const {
    return Local<S>::Cast(*this);
  }
 
  /**
   * Create a local handle for the content of another handle.
   * The referee is kept alive by the local handle even when
   * the original handle is destroyed/disposed.
   */
  V8_INLINE static Local<T> New(Isolate* isolate, Local<T> that);
  V8_INLINE static Local<T> New(Isolate* isolate,
                                const PersistentBase<T>& that);
  V8_INLINE static Local<T> New(Isolate* isolate,
                                const TracedReferenceBase<T>& that);
 
 private:
  friend class Utils;
  template<class F> friend class Eternal;
  template<class F> friend class PersistentBase;
  template<class F, class M> friend class Persistent;
  template<class F> friend class Local;
  template <class F>
  friend class MaybeLocal;
  template<class F> friend class FunctionCallbackInfo;
  template<class F> friend class PropertyCallbackInfo;
  friend class String;
  friend class Object;
  friend class Context;
  friend class Isolate;
  friend class Private;
  template<class F> friend class internal::CustomArguments;
  friend Local<Primitive> Undefined(Isolate* isolate);
  friend Local<Primitive> Null(Isolate* isolate);
  friend Local<Boolean> True(Isolate* isolate);
  friend Local<Boolean> False(Isolate* isolate);
  friend class HandleScope;
  friend class EscapableHandleScope;
  template <class F1, class F2, class F3>
  friend class PersistentValueMapBase;
  template<class F1, class F2> friend class PersistentValueVector;
  template <class F>
  friend class ReturnValue;
  template <class F>
  friend class Traced;
  template <class F>
  friend class TracedGlobal;
  template <class F>
  friend class TracedReferenceBase;
  template <class F>
  friend class TracedReference;
 
  explicit V8_INLINE Local(T* that) : val_(that) {}
  V8_INLINE static Local<T> New(Isolate* isolate, T* that);
  T* val_;
};
 
 
#if !defined(V8_IMMINENT_DEPRECATION_WARNINGS)
// Handle is an alias for Local for historical reasons.
template <class T>
using Handle = Local<T>;
#endif
 
 
/**
 * A MaybeLocal<> is a wrapper around Local<> that enforces a check whether
 * the Local<> is empty before it can be used.
 *
 * If an API method returns a MaybeLocal<>, the API method can potentially fail
 * either because an exception is thrown, or because an exception is pending,
 * e.g. because a previous API call threw an exception that hasn't been caught
 * yet, or because a TerminateExecution exception was thrown. In that case, an
 * empty MaybeLocal is returned.
 */
template <class T>
class MaybeLocal {
 public:
  V8_INLINE MaybeLocal() : val_(nullptr) {}
  template <class S>
  V8_INLINE MaybeLocal(Local<S> that)
      : val_(reinterpret_cast<T*>(*that)) {
    static_assert(std::is_base_of<T, S>::value, "type check");
  }
 
  V8_INLINE bool IsEmpty() const { return val_ == nullptr; }
 
  /**
   * Converts this MaybeLocal<> to a Local<>. If this MaybeLocal<> is empty,
   * |false| is returned and |out| is left untouched.
   */
  template <class S>
  V8_WARN_UNUSED_RESULT V8_INLINE bool ToLocal(Local<S>* out) const {
    out->val_ = IsEmpty() ? nullptr : this->val_;
    return !IsEmpty();
  }
 
  /**
   * Converts this MaybeLocal<> to a Local<>. If this MaybeLocal<> is empty,
   * V8 will crash the process.
   */
  V8_INLINE Local<T> ToLocalChecked();
 
  /**
   * Converts this MaybeLocal<> to a Local<>, using a default value if this
   * MaybeLocal<> is empty.
   */
  template <class S>
  V8_INLINE Local<S> FromMaybe(Local<S> default_value) const {
    return IsEmpty() ? default_value : Local<S>(val_);
  }
 
 private:
  T* val_;
};
 
/**
 * Eternal handles are set-once handles that live for the lifetime of the
 * isolate.
 */
template <class T> class Eternal {
 public:
  V8_INLINE Eternal() : val_(nullptr) {}
  template <class S>
  V8_INLINE Eternal(Isolate* isolate, Local<S> handle) : val_(nullptr) {
    Set(isolate, handle);
  }
  // Can only be safely called if already set.
  V8_INLINE Local<T> Get(Isolate* isolate) const;
  V8_INLINE bool IsEmpty() const { return val_ == nullptr; }
  template<class S> V8_INLINE void Set(Isolate* isolate, Local<S> handle);
 
 private:
  T* val_;
};
 
 
static const int kInternalFieldsInWeakCallback = 2;
static const int kEmbedderFieldsInWeakCallback = 2;
 
template <typename T>
class WeakCallbackInfo {
 public:
  typedef void (*Callback)(const WeakCallbackInfo<T>& data);
 
  WeakCallbackInfo(Isolate* isolate, T* parameter,
                   void* embedder_fields[kEmbedderFieldsInWeakCallback],
                   Callback* callback)
      : isolate_(isolate), parameter_(parameter), callback_(callback) {
    for (int i = 0; i < kEmbedderFieldsInWeakCallback; ++i) {
      embedder_fields_[i] = embedder_fields[i];
    }
  }
 
  V8_INLINE Isolate* GetIsolate() const { return isolate_; }
  V8_INLINE T* GetParameter() const { return parameter_; }
  V8_INLINE void* GetInternalField(int index) const;
 
  // When first called, the embedder MUST Reset() the Global which triggered the
  // callback. The Global itself is unusable for anything else. No v8 other api
  // calls may be called in the first callback. Should additional work be
  // required, the embedder must set a second pass callback, which will be
  // called after all the initial callbacks are processed.
  // Calling SetSecondPassCallback on the second pass will immediately crash.
  void SetSecondPassCallback(Callback callback) const { *callback_ = callback; }
 
 private:
  Isolate* isolate_;
  T* parameter_;
  Callback* callback_;
  void* embedder_fields_[kEmbedderFieldsInWeakCallback];
};
 
 
// kParameter will pass a void* parameter back to the callback, kInternalFields
// will pass the first two internal fields back to the callback, kFinalizer
// will pass a void* parameter back, but is invoked before the object is
// actually collected, so it can be resurrected. In the last case, it is not
// possible to request a second pass callback.
enum class WeakCallbackType { kParameter, kInternalFields, kFinalizer };
 
/**
 * An object reference that is independent of any handle scope.  Where
 * a Local handle only lives as long as the HandleScope in which it was
 * allocated, a PersistentBase handle remains valid until it is explicitly
 * disposed using Reset().
 *
 * A persistent handle contains a reference to a storage cell within
 * the V8 engine which holds an object value and which is updated by
 * the garbage collector whenever the object is moved.  A new storage
 * cell can be created using the constructor or PersistentBase::Reset and
 * existing handles can be disposed using PersistentBase::Reset.
 *
 */
template <class T> class PersistentBase {
 public:
  /**
   * If non-empty, destroy the underlying storage cell
   * IsEmpty() will return true after this call.
   */
  V8_INLINE void Reset();
  /**
   * If non-empty, destroy the underlying storage cell
   * and create a new one with the contents of other if other is non empty
   */
  template <class S>
  V8_INLINE void Reset(Isolate* isolate, const Local<S>& other);
 
  /**
   * If non-empty, destroy the underlying storage cell
   * and create a new one with the contents of other if other is non empty
   */
  template <class S>
  V8_INLINE void Reset(Isolate* isolate, const PersistentBase<S>& other);
 
  V8_INLINE bool IsEmpty() const { return val_ == nullptr; }
  V8_INLINE void Empty() { val_ = 0; }
 
  V8_INLINE Local<T> Get(Isolate* isolate) const {
    return Local<T>::New(isolate, *this);
  }
 
  template <class S>
  V8_INLINE bool operator==(const PersistentBase<S>& that) const {
    internal::Address* a = reinterpret_cast<internal::Address*>(this->val_);
    internal::Address* b = reinterpret_cast<internal::Address*>(that.val_);
    if (a == nullptr) return b == nullptr;
    if (b == nullptr) return false;
    return *a == *b;
  }
 
  template <class S>
  V8_INLINE bool operator==(const Local<S>& that) const {
    internal::Address* a = reinterpret_cast<internal::Address*>(this->val_);
    internal::Address* b = reinterpret_cast<internal::Address*>(that.val_);
    if (a == nullptr) return b == nullptr;
    if (b == nullptr) return false;
    return *a == *b;
  }
 
  template <class S>
  V8_INLINE bool operator!=(const PersistentBase<S>& that) const {
    return !operator==(that);
  }
 
  template <class S>
  V8_INLINE bool operator!=(const Local<S>& that) const {
    return !operator==(that);
  }
 
  /**
   * Install a finalization callback on this object.
   * NOTE: There is no guarantee as to *when* or even *if* the callback is
   * invoked. The invocation is performed solely on a best effort basis.
   * As always, GC-based finalization should *not* be relied upon for any
   * critical form of resource management!
   *
   * The callback is supposed to reset the handle. No further V8 API may be
   * called in this callback. In case additional work involving V8 needs to be
   * done, a second callback can be scheduled using
   * WeakCallbackInfo<void>::SetSecondPassCallback.
   */
  template <typename P>
  V8_INLINE void SetWeak(P* parameter,
                         typename WeakCallbackInfo<P>::Callback callback,
                         WeakCallbackType type);
 
  /**
   * Turns this handle into a weak phantom handle without finalization callback.
   * The handle will be reset automatically when the garbage collector detects
   * that the object is no longer reachable.
   * A related function Isolate::NumberOfPhantomHandleResetsSinceLastCall
   * returns how many phantom handles were reset by the garbage collector.
   */
  V8_INLINE void SetWeak();
 
  template<typename P>
  V8_INLINE P* ClearWeak();
 
  // TODO(dcarney): remove this.
  V8_INLINE void ClearWeak() { ClearWeak<void>(); }
 
  /**
   * Annotates the strong handle with the given label, which is then used by the
   * heap snapshot generator as a name of the edge from the root to the handle.
   * The function does not take ownership of the label and assumes that the
   * label is valid as long as the handle is valid.
   */
  V8_INLINE void AnnotateStrongRetainer(const char* label);
 
  /** Returns true if the handle's reference is weak.  */
  V8_INLINE bool IsWeak() const;
 
  /**
   * Assigns a wrapper class ID to the handle.
   */
  V8_INLINE void SetWrapperClassId(uint16_t class_id);
 
  /**
   * Returns the class ID previously assigned to this handle or 0 if no class ID
   * was previously assigned.
   */
  V8_INLINE uint16_t WrapperClassId() const;
 
  PersistentBase(const PersistentBase& other) = delete;  // NOLINT
  void operator=(const PersistentBase&) = delete;
 
 private:
  friend class Isolate;
  friend class Utils;
  template<class F> friend class Local;
  template<class F1, class F2> friend class Persistent;
  template <class F>
  friend class Global;
  template<class F> friend class PersistentBase;
  template<class F> friend class ReturnValue;
  template <class F1, class F2, class F3>
  friend class PersistentValueMapBase;
  template<class F1, class F2> friend class PersistentValueVector;
  friend class Object;
 
  explicit V8_INLINE PersistentBase(T* val) : val_(val) {}
  V8_INLINE static T* New(Isolate* isolate, T* that);
 
  T* val_;
};
 
 
/**
 * Default traits for Persistent. This class does not allow
 * use of the copy constructor or assignment operator.
 * At present kResetInDestructor is not set, but that will change in a future
 * version.
 */
template<class T>
class NonCopyablePersistentTraits {
 public:
  typedef Persistent<T, NonCopyablePersistentTraits<T> > NonCopyablePersistent;
  static const bool kResetInDestructor = false;
  template<class S, class M>
  V8_INLINE static void Copy(const Persistent<S, M>& source,
                             NonCopyablePersistent* dest) {
    static_assert(sizeof(S) < 0,
                  "NonCopyablePersistentTraits::Copy is not instantiable");
  }
};
 
 
/**
 * Helper class traits to allow copying and assignment of Persistent.
 * This will clone the contents of storage cell, but not any of the flags, etc.
 */
template<class T>
struct CopyablePersistentTraits {
  typedef Persistent<T, CopyablePersistentTraits<T> > CopyablePersistent;
  static const bool kResetInDestructor = true;
  template<class S, class M>
  static V8_INLINE void Copy(const Persistent<S, M>& source,
                             CopyablePersistent* dest) {
    // do nothing, just allow copy
  }
};
 
 
/**
 * A PersistentBase which allows copy and assignment.
 *
 * Copy, assignment and destructor behavior is controlled by the traits
 * class M.
 *
 * Note: Persistent class hierarchy is subject to future changes.
 */
template <class T, class M> class Persistent : public PersistentBase<T> {
 public:
  /**
   * A Persistent with no storage cell.
   */
  V8_INLINE Persistent() : PersistentBase<T>(nullptr) {}
  /**
   * Construct a Persistent from a Local.
   * When the Local is non-empty, a new storage cell is created
   * pointing to the same object, and no flags are set.
   */
  template <class S>
  V8_INLINE Persistent(Isolate* isolate, Local<S> that)
      : PersistentBase<T>(PersistentBase<T>::New(isolate, *that)) {
    static_assert(std::is_base_of<T, S>::value, "type check");
  }
  /**
   * Construct a Persistent from a Persistent.
   * When the Persistent is non-empty, a new storage cell is created
   * pointing to the same object, and no flags are set.
   */
  template <class S, class M2>
  V8_INLINE Persistent(Isolate* isolate, const Persistent<S, M2>& that)
    : PersistentBase<T>(PersistentBase<T>::New(isolate, *that)) {
    static_assert(std::is_base_of<T, S>::value, "type check");
  }
  /**
   * The copy constructors and assignment operator create a Persistent
   * exactly as the Persistent constructor, but the Copy function from the
   * traits class is called, allowing the setting of flags based on the
   * copied Persistent.
   */
  V8_INLINE Persistent(const Persistent& that) : PersistentBase<T>(nullptr) {
    Copy(that);
  }
  template <class S, class M2>
  V8_INLINE Persistent(const Persistent<S, M2>& that) : PersistentBase<T>(0) {
    Copy(that);
  }
  V8_INLINE Persistent& operator=(const Persistent& that) {
    Copy(that);
    return *this;
  }
  template <class S, class M2>
  V8_INLINE Persistent& operator=(const Persistent<S, M2>& that) { // NOLINT
    Copy(that);
    return *this;
  }
  /**
   * The destructor will dispose the Persistent based on the
   * kResetInDestructor flags in the traits class.  Since not calling dispose
   * can result in a memory leak, it is recommended to always set this flag.
   */
  V8_INLINE ~Persistent() {
    if (M::kResetInDestructor) this->Reset();
  }
 
  // TODO(dcarney): this is pretty useless, fix or remove
  template <class S>
  V8_INLINE static Persistent<T>& Cast(const Persistent<S>& that) {  // NOLINT
#ifdef V8_ENABLE_CHECKS
    // If we're going to perform the type check then we have to check
    // that the handle isn't empty before doing the checked cast.
    if (!that.IsEmpty()) T::Cast(*that);
#endif
    return reinterpret_cast<Persistent<T>&>(const_cast<Persistent<S>&>(that));
  }
 
  // TODO(dcarney): this is pretty useless, fix or remove
  template <class S>
  V8_INLINE Persistent<S>& As() const {  // NOLINT
    return Persistent<S>::Cast(*this);
  }
 
 private:
  friend class Isolate;
  friend class Utils;
  template<class F> friend class Local;
  template<class F1, class F2> friend class Persistent;
  template<class F> friend class ReturnValue;
 
  explicit V8_INLINE Persistent(T* that) : PersistentBase<T>(that) {}
  V8_INLINE T* operator*() const { return this->val_; }
  template<class S, class M2>
  V8_INLINE void Copy(const Persistent<S, M2>& that);
};
 
 
/**
 * A PersistentBase which has move semantics.
 *
 * Note: Persistent class hierarchy is subject to future changes.
 */
template <class T>
class Global : public PersistentBase<T> {
 public:
  /**
   * A Global with no storage cell.
   */
  V8_INLINE Global() : PersistentBase<T>(nullptr) {}
 
  /**
   * Construct a Global from a Local.
   * When the Local is non-empty, a new storage cell is created
   * pointing to the same object, and no flags are set.
   */
  template <class S>
  V8_INLINE Global(Isolate* isolate, Local<S> that)
      : PersistentBase<T>(PersistentBase<T>::New(isolate, *that)) {
    static_assert(std::is_base_of<T, S>::value, "type check");
  }
 
  /**
   * Construct a Global from a PersistentBase.
   * When the Persistent is non-empty, a new storage cell is created
   * pointing to the same object, and no flags are set.
   */
  template <class S>
  V8_INLINE Global(Isolate* isolate, const PersistentBase<S>& that)
      : PersistentBase<T>(PersistentBase<T>::New(isolate, that.val_)) {
    static_assert(std::is_base_of<T, S>::value, "type check");
  }
 
  /**
   * Move constructor.
   */
  V8_INLINE Global(Global&& other);
 
  V8_INLINE ~Global() { this->Reset(); }
 
  /**
   * Move via assignment.
   */
  template <class S>
  V8_INLINE Global& operator=(Global<S>&& rhs);
 
  /**
   * Pass allows returning uniques from functions, etc.
   */
  Global Pass() { return static_cast<Global&&>(*this); }  // NOLINT
 
  /*
   * For compatibility with Chromium's base::Bind (base::Passed).
   */
  typedef void MoveOnlyTypeForCPP03;
 
  Global(const Global&) = delete;
  void operator=(const Global&) = delete;
 
 private:
  template <class F>
  friend class ReturnValue;
  V8_INLINE T* operator*() const { return this->val_; }
};
 
 
// UniquePersistent is an alias for Global for historical reason.
template <class T>
using UniquePersistent = Global<T>;
 
/**
 * Deprecated. Use |TracedReference<T>| instead.
 */
template <typename T>
struct TracedGlobalTrait {};
 
/**
 * A traced handle with copy and move semantics. The handle is to be used
 * together with |v8::EmbedderHeapTracer| and specifies edges from the embedder
 * into V8's heap.
 *
 * The exact semantics are:
 * - Tracing garbage collections use |v8::EmbedderHeapTracer|.
 * - Non-tracing garbage collections refer to
 *   |v8::EmbedderHeapTracer::IsRootForNonTracingGC()| whether the handle should
 *   be treated as root or not.
 *
 * Note that the base class cannot be instantiated itself. Choose from
 * - TracedGlobal
 * - TracedReference
 */
template <typename T>
class TracedReferenceBase {
 public:
  /**
   * Returns true if this TracedReferenceBase is empty, i.e., has not been
   * assigned an object.
   */
  bool IsEmpty() const { return val_ == nullptr; }
 
  /**
   * If non-empty, destroy the underlying storage cell. |IsEmpty| will return
   * true after this call.
   */
  V8_INLINE void Reset();
 
  /**
   * Construct a Local<T> from this handle.
   */
  Local<T> Get(Isolate* isolate) const { return Local<T>::New(isolate, *this); }
 
  template <class S>
  V8_INLINE bool operator==(const TracedReferenceBase<S>& that) const {
    internal::Address* a = reinterpret_cast<internal::Address*>(val_);
    internal::Address* b = reinterpret_cast<internal::Address*>(that.val_);
    if (a == nullptr) return b == nullptr;
    if (b == nullptr) return false;
    return *a == *b;
  }
 
  template <class S>
  V8_INLINE bool operator==(const Local<S>& that) const {
    internal::Address* a = reinterpret_cast<internal::Address*>(val_);
    internal::Address* b = reinterpret_cast<internal::Address*>(that.val_);
    if (a == nullptr) return b == nullptr;
    if (b == nullptr) return false;
    return *a == *b;
  }
 
  template <class S>
  V8_INLINE bool operator!=(const TracedReferenceBase<S>& that) const {
    return !operator==(that);
  }
 
  template <class S>
  V8_INLINE bool operator!=(const Local<S>& that) const {
    return !operator==(that);
  }
 
  /**
   * Assigns a wrapper class ID to the handle.
   */
  V8_INLINE void SetWrapperClassId(uint16_t class_id);
 
  /**
   * Returns the class ID previously assigned to this handle or 0 if no class ID
   * was previously assigned.
   */
  V8_INLINE uint16_t WrapperClassId() const;
 
  template <class S>
  V8_INLINE TracedReferenceBase<S>& As() const {
    return reinterpret_cast<TracedReferenceBase<S>&>(
        const_cast<TracedReferenceBase<T>&>(*this));
  }
 
 protected:
  /**
   * Update this reference in a thread-safe way
   */
  void SetSlotThreadSafe(T* new_val) {
    reinterpret_cast<std::atomic<T*>*>(&val_)->store(new_val,
                                                     std::memory_order_relaxed);
  }
 
  /**
   * Get this reference in a thread-safe way
   */
  const T* GetSlotThreadSafe() const {
    return reinterpret_cast<std::atomic<const T*> const*>(&val_)->load(
        std::memory_order_relaxed);
  }
 
 private:
  enum DestructionMode { kWithDestructor, kWithoutDestructor };
 
  /**
   * An empty TracedReferenceBase without storage cell.
   */
  TracedReferenceBase() = default;
 
  V8_INLINE static T* New(Isolate* isolate, T* that, void* slot,
                          DestructionMode destruction_mode);
 
  T* val_ = nullptr;
 
  friend class EmbedderHeapTracer;
  template <typename F>
  friend class Local;
  friend class Object;
  template <typename F>
  friend class TracedGlobal;
  template <typename F>
  friend class TracedReference;
  template <typename F>
  friend class ReturnValue;
};
 
/**
 * A traced handle with destructor that clears the handle. For more details see
 * TracedReferenceBase.
 */
template <typename T>
class TracedGlobal : public TracedReferenceBase<T> {
 public:
  using TracedReferenceBase<T>::Reset;
 
  /**
   * Destructor resetting the handle.
   */
  ~TracedGlobal() { this->Reset(); }
 
  /**
   * An empty TracedGlobal without storage cell.
   */
  TracedGlobal() : TracedReferenceBase<T>() {}
 
  /**
   * Construct a TracedGlobal from a Local.
   *
   * When the Local is non-empty, a new storage cell is created
   * pointing to the same object.
   */
  template <class S>
  TracedGlobal(Isolate* isolate, Local<S> that) : TracedReferenceBase<T>() {
    this->val_ = this->New(isolate, that.val_, &this->val_,
                           TracedReferenceBase<T>::kWithDestructor);
    static_assert(std::is_base_of<T, S>::value, "type check");
  }
 
  /**
   * Move constructor initializing TracedGlobal from an existing one.
   */
  V8_INLINE TracedGlobal(TracedGlobal&& other) {
    // Forward to operator=.
    *this = std::move(other);
  }
 
  /**
   * Move constructor initializing TracedGlobal from an existing one.
   */
  template <typename S>
  V8_INLINE TracedGlobal(TracedGlobal<S>&& other) {
    // Forward to operator=.
    *this = std::move(other);
  }
 
  /**
   * Copy constructor initializing TracedGlobal from an existing one.
   */
  V8_INLINE TracedGlobal(const TracedGlobal& other) {
    // Forward to operator=;
    *this = other;
  }
 
  /**
   * Copy constructor initializing TracedGlobal from an existing one.
   */
  template <typename S>
  V8_INLINE TracedGlobal(const TracedGlobal<S>& other) {
    // Forward to operator=;
    *this = other;
  }
 
  /**
   * Move assignment operator initializing TracedGlobal from an existing one.
   */
  V8_INLINE TracedGlobal& operator=(TracedGlobal&& rhs);
 
  /**
   * Move assignment operator initializing TracedGlobal from an existing one.
   */
  template <class S>
  V8_INLINE TracedGlobal& operator=(TracedGlobal<S>&& rhs);
 
  /**
   * Copy assignment operator initializing TracedGlobal from an existing one.
   *
   * Note: Prohibited when |other| has a finalization callback set through
   * |SetFinalizationCallback|.
   */
  V8_INLINE TracedGlobal& operator=(const TracedGlobal& rhs);
 
  /**
   * Copy assignment operator initializing TracedGlobal from an existing one.
   *
   * Note: Prohibited when |other| has a finalization callback set through
   * |SetFinalizationCallback|.
   */
  template <class S>
  V8_INLINE TracedGlobal& operator=(const TracedGlobal<S>& rhs);
 
  /**
   * If non-empty, destroy the underlying storage cell and create a new one with
   * the contents of other if other is non empty
   */
  template <class S>
  V8_INLINE void Reset(Isolate* isolate, const Local<S>& other);
 
  template <class S>
  V8_INLINE TracedGlobal<S>& As() const {
    return reinterpret_cast<TracedGlobal<S>&>(
        const_cast<TracedGlobal<T>&>(*this));
  }
 
  /**
   * Adds a finalization callback to the handle. The type of this callback is
   * similar to WeakCallbackType::kInternalFields, i.e., it will pass the
   * parameter and the first two internal fields of the object.
   *
   * The callback is then supposed to reset the handle in the callback. No
   * further V8 API may be called in this callback. In case additional work
   * involving V8 needs to be done, a second callback can be scheduled using
   * WeakCallbackInfo<void>::SetSecondPassCallback.
   */
  V8_INLINE void SetFinalizationCallback(
      void* parameter, WeakCallbackInfo<void>::Callback callback);
};
 
/**
 * A traced handle without destructor that clears the handle. The embedder needs
 * to ensure that the handle is not accessed once the V8 object has been
 * reclaimed. This can happen when the handle is not passed through the
 * EmbedderHeapTracer. For more details see TracedReferenceBase.
 *
 * The reference assumes the embedder has precise knowledge about references at
 * all times. In case V8 needs to separately handle on-stack references, the
 * embedder is required to set the stack start through
 * |EmbedderHeapTracer::SetStackStart|.
 */
template <typename T>
class TracedReference : public TracedReferenceBase<T> {
 public:
  using TracedReferenceBase<T>::Reset;
 
  /**
   * An empty TracedReference without storage cell.
   */
  TracedReference() : TracedReferenceBase<T>() {}
 
  /**
   * Construct a TracedReference from a Local.
   *
   * When the Local is non-empty, a new storage cell is created
   * pointing to the same object.
   */
  template <class S>
  TracedReference(Isolate* isolate, Local<S> that) : TracedReferenceBase<T>() {
    this->val_ = this->New(isolate, that.val_, &this->val_,
                           TracedReferenceBase<T>::kWithoutDestructor);
    static_assert(std::is_base_of<T, S>::value, "type check");
  }
 
  /**
   * Move constructor initializing TracedReference from an
   * existing one.
   */
  V8_INLINE TracedReference(TracedReference&& other) {
    // Forward to operator=.
    *this = std::move(other);
  }
 
  /**
   * Move constructor initializing TracedReference from an
   * existing one.
   */
  template <typename S>
  V8_INLINE TracedReference(TracedReference<S>&& other) {
    // Forward to operator=.
    *this = std::move(other);
  }
 
  /**
   * Copy constructor initializing TracedReference from an
   * existing one.
   */
  V8_INLINE TracedReference(const TracedReference& other) {
    // Forward to operator=;
    *this = other;
  }
 
  /**
   * Copy constructor initializing TracedReference from an
   * existing one.
   */
  template <typename S>
  V8_INLINE TracedReference(const TracedReference<S>& other) {
    // Forward to operator=;
    *this = other;
  }
 
  /**
   * Move assignment operator initializing TracedGlobal from an existing one.
   */
  V8_INLINE TracedReference& operator=(TracedReference&& rhs);
 
  /**
   * Move assignment operator initializing TracedGlobal from an existing one.
   */
  template <class S>
  V8_INLINE TracedReference& operator=(TracedReference<S>&& rhs);
 
  /**
   * Copy assignment operator initializing TracedGlobal from an existing one.
   */
  V8_INLINE TracedReference& operator=(const TracedReference& rhs);
 
  /**
   * Copy assignment operator initializing TracedGlobal from an existing one.
   */
  template <class S>
  V8_INLINE TracedReference& operator=(const TracedReference<S>& rhs);
 
  /**
   * If non-empty, destroy the underlying storage cell and create a new one with
   * the contents of other if other is non empty
   */
  template <class S>
  V8_INLINE void Reset(Isolate* isolate, const Local<S>& other);
 
  template <class S>
  V8_INLINE TracedReference<S>& As() const {
    return reinterpret_cast<TracedReference<S>&>(
        const_cast<TracedReference<T>&>(*this));
  }
 
  /**
   * Returns true if this TracedReference is empty, i.e., has not been
   * assigned an object. This version of IsEmpty is thread-safe.
   */
  bool IsEmptyThreadSafe() const {
    return this->GetSlotThreadSafe() == nullptr;
  }
};
 
 /**
 * A stack-allocated class that governs a number of local handles.
 * After a handle scope has been created, all local handles will be
 * allocated within that handle scope until either the handle scope is
 * deleted or another handle scope is created.  If there is already a
 * handle scope and a new one is created, all allocations will take
 * place in the new handle scope until it is deleted.  After that,
 * new handles will again be allocated in the original handle scope.
 *
 * After the handle scope of a local handle has been deleted the
 * garbage collector will no longer track the object stored in the
 * handle and may deallocate it.  The behavior of accessing a handle
 * for which the handle scope has been deleted is undefined.
 */
class V8_EXPORT HandleScope {
 public:
  explicit HandleScope(Isolate* isolate);
 
  ~HandleScope();
 
  /**
   * Counts the number of allocated handles.
   */
  static int NumberOfHandles(Isolate* isolate);
 
  V8_INLINE Isolate* GetIsolate() const {
    return reinterpret_cast<Isolate*>(isolate_);
  }
 
  HandleScope(const HandleScope&) = delete;
  void operator=(const HandleScope&) = delete;
 
 protected:
  V8_INLINE HandleScope() = default;
 
  void Initialize(Isolate* isolate);
 
  static internal::Address* CreateHandle(internal::Isolate* isolate,
                                         internal::Address value);
 
 private:
  // Declaring operator new and delete as deleted is not spec compliant.
  // Therefore declare them private instead to disable dynamic alloc
  void* operator new(size_t size);
  void* operator new[](size_t size);
  void operator delete(void*, size_t);
  void operator delete[](void*, size_t);
 
  internal::Isolate* isolate_;
  internal::Address* prev_next_;
  internal::Address* prev_limit_;
 
  // Local::New uses CreateHandle with an Isolate* parameter.
  template<class F> friend class Local;
 
  // Object::GetInternalField and Context::GetEmbedderData use CreateHandle with
  // a HeapObject in their shortcuts.
  friend class Object;
  friend class Context;
};
 
 
/**
 * A HandleScope which first allocates a handle in the current scope
 * which will be later filled with the escape value.
 */
class V8_EXPORT EscapableHandleScope : public HandleScope {
 public:
  explicit EscapableHandleScope(Isolate* isolate);
  V8_INLINE ~EscapableHandleScope() = default;
 
  /**
   * Pushes the value into the previous scope and returns a handle to it.
   * Cannot be called twice.
   */
  template <class T>
  V8_INLINE Local<T> Escape(Local<T> value) {
    internal::Address* slot =
        Escape(reinterpret_cast<internal::Address*>(*value));
    return Local<T>(reinterpret_cast<T*>(slot));
  }
 
  template <class T>
  V8_INLINE MaybeLocal<T> EscapeMaybe(MaybeLocal<T> value) {
    return Escape(value.FromMaybe(Local<T>()));
  }
 
  EscapableHandleScope(const EscapableHandleScope&) = delete;
  void operator=(const EscapableHandleScope&) = delete;
 
 private:
  // Declaring operator new and delete as deleted is not spec compliant.
  // Therefore declare them private instead to disable dynamic alloc
  void* operator new(size_t size);
  void* operator new[](size_t size);
  void operator delete(void*, size_t);
  void operator delete[](void*, size_t);
 
  internal::Address* Escape(internal::Address* escape_value);
  internal::Address* escape_slot_;
};
 
/**
 * A SealHandleScope acts like a handle scope in which no handle allocations
 * are allowed. It can be useful for debugging handle leaks.
 * Handles can be allocated within inner normal HandleScopes.
 */
class V8_EXPORT SealHandleScope {
 public:
  explicit SealHandleScope(Isolate* isolate);
  ~SealHandleScope();
 
  SealHandleScope(const SealHandleScope&) = delete;
  void operator=(const SealHandleScope&) = delete;
 
 private:
  // Declaring operator new and delete as deleted is not spec compliant.
  // Therefore declare them private instead to disable dynamic alloc
  void* operator new(size_t size);
  void* operator new[](size_t size);
  void operator delete(void*, size_t);
  void operator delete[](void*, size_t);
 
  internal::Isolate* const isolate_;
  internal::Address* prev_limit_;
  int prev_sealed_level_;
};
 
 
// --- Special objects ---
 
/**
 * The superclass of objects that can reside on V8's heap.
 */
class V8_EXPORT Data {
 private:
  Data();
};
 
/**
 * A container type that holds relevant metadata for module loading.
 *
 * This is passed back to the embedder as part of
 * HostImportModuleDynamicallyCallback for module loading.
 */
class V8_EXPORT ScriptOrModule {
 public:
  /**
   * The name that was passed by the embedder as ResourceName to the
   * ScriptOrigin. This can be either a v8::String or v8::Undefined.
   */
  Local<Value> GetResourceName();
 
  /**
   * The options that were passed by the embedder as HostDefinedOptions to
   * the ScriptOrigin.
   */
  Local<PrimitiveArray> GetHostDefinedOptions();
};
 
/**
 * An array to hold Primitive values. This is used by the embedder to
 * pass host defined options to the ScriptOptions during compilation.
 *
 * This is passed back to the embedder as part of
 * HostImportModuleDynamicallyCallback for module loading.
 *
 */
class V8_EXPORT PrimitiveArray {
 public:
  static Local<PrimitiveArray> New(Isolate* isolate, int length);
  int Length() const;
  void Set(Isolate* isolate, int index, Local<Primitive> item);
  Local<Primitive> Get(Isolate* isolate, int index);
};
 
/**
 * The optional attributes of ScriptOrigin.
 */
class ScriptOriginOptions {
 public:
  V8_INLINE ScriptOriginOptions(bool is_shared_cross_origin = false,
                                bool is_opaque = false, bool is_wasm = false,
                                bool is_module = false)
      : flags_((is_shared_cross_origin ? kIsSharedCrossOrigin : 0) |
               (is_wasm ? kIsWasm : 0) | (is_opaque ? kIsOpaque : 0) |
               (is_module ? kIsModule : 0)) {}
  V8_INLINE ScriptOriginOptions(int flags)
      : flags_(flags &
               (kIsSharedCrossOrigin | kIsOpaque | kIsWasm | kIsModule)) {}
 
  bool IsSharedCrossOrigin() const {
    return (flags_ & kIsSharedCrossOrigin) != 0;
  }
  bool IsOpaque() const { return (flags_ & kIsOpaque) != 0; }
  bool IsWasm() const { return (flags_ & kIsWasm) != 0; }
  bool IsModule() const { return (flags_ & kIsModule) != 0; }
 
  int Flags() const { return flags_; }
 
 private:
  enum {
    kIsSharedCrossOrigin = 1,
    kIsOpaque = 1 << 1,
    kIsWasm = 1 << 2,
    kIsModule = 1 << 3
  };
  const int flags_;
};
 
/**
 * The origin, within a file, of a script.
 */
class ScriptOrigin {
 public:
  V8_INLINE ScriptOrigin(
      Local<Value> resource_name,
      Local<Integer> resource_line_offset = Local<Integer>(),
      Local<Integer> resource_column_offset = Local<Integer>(),
      Local<Boolean> resource_is_shared_cross_origin = Local<Boolean>(),
      Local<Integer> script_id = Local<Integer>(),
      Local<Value> source_map_url = Local<Value>(),
      Local<Boolean> resource_is_opaque = Local<Boolean>(),
      Local<Boolean> is_wasm = Local<Boolean>(),
      Local<Boolean> is_module = Local<Boolean>(),
      Local<PrimitiveArray> host_defined_options = Local<PrimitiveArray>());
 
  V8_INLINE Local<Value> ResourceName() const;
  V8_INLINE Local<Integer> ResourceLineOffset() const;
  V8_INLINE Local<Integer> ResourceColumnOffset() const;
  V8_INLINE Local<Integer> ScriptID() const;
  V8_INLINE Local<Value> SourceMapUrl() const;
  V8_INLINE Local<PrimitiveArray> HostDefinedOptions() const;
  V8_INLINE ScriptOriginOptions Options() const { return options_; }
 
 private:
  Local<Value> resource_name_;
  Local<Integer> resource_line_offset_;
  Local<Integer> resource_column_offset_;
  ScriptOriginOptions options_;
  Local<Integer> script_id_;
  Local<Value> source_map_url_;
  Local<PrimitiveArray> host_defined_options_;
};
 
/**
 * A compiled JavaScript script, not yet tied to a Context.
 */
class V8_EXPORT UnboundScript {
 public:
  /**
   * Binds the script to the currently entered context.
   */
  Local<Script> BindToCurrentContext();
 
  int GetId();
  Local<Value> GetScriptName();
 
  /**
   * Data read from magic sourceURL comments.
   */
  Local<Value> GetSourceURL();
  /**
   * Data read from magic sourceMappingURL comments.
   */
  Local<Value> GetSourceMappingURL();
 
  /**
   * Returns zero based line number of the code_pos location in the script.
   * -1 will be returned if no information available.
   */
  int GetLineNumber(int code_pos);
 
  static const int kNoScriptId = 0;
};
 
/**
 * A compiled JavaScript module, not yet tied to a Context.
 */
class V8_EXPORT UnboundModuleScript : public Data {
  // Only used as a container for code caching.
};
 
/**
 * A location in JavaScript source.
 */
class V8_EXPORT Location {
 public:
  int GetLineNumber() { return line_number_; }
  int GetColumnNumber() { return column_number_; }
 
  Location(int line_number, int column_number)
      : line_number_(line_number), column_number_(column_number) {}
 
 private:
  int line_number_;
  int column_number_;
};
 
/**
 * A compiled JavaScript module.
 */
class V8_EXPORT Module : public Data {
 public:
  /**
   * The different states a module can be in.
   *
   * This corresponds to the states used in ECMAScript except that "evaluated"
   * is split into kEvaluated and kErrored, indicating success and failure,
   * respectively.
   */
  enum Status {
    kUninstantiated,
    kInstantiating,
    kInstantiated,
    kEvaluating,
    kEvaluated,
    kErrored
  };
 
  /**
   * Returns the module's current status.
   */
  Status GetStatus() const;
 
  /**
   * For a module in kErrored status, this returns the corresponding exception.
   */
  Local<Value> GetException() const;
 
  /**
   * Returns the number of modules requested by this module.
   */
  int GetModuleRequestsLength() const;
 
  /**
   * Returns the ith module specifier in this module.
   * i must be < GetModuleRequestsLength() and >= 0.
   */
  Local<String> GetModuleRequest(int i) const;
 
  /**
   * Returns the source location (line number and column number) of the ith
   * module specifier's first occurrence in this module.
   */
  Location GetModuleRequestLocation(int i) const;
 
  /**
   * Returns the identity hash for this object.
   */
  int GetIdentityHash() const;
 
  typedef MaybeLocal<Module> (*ResolveCallback)(Local<Context> context,
                                                Local<String> specifier,
                                                Local<Module> referrer);
 
  /**
   * Instantiates the module and its dependencies.
   *
   * Returns an empty Maybe<bool> if an exception occurred during
   * instantiation. (In the case where the callback throws an exception, that
   * exception is propagated.)
   */
  V8_WARN_UNUSED_RESULT Maybe<bool> InstantiateModule(Local<Context> context,
                                                      ResolveCallback callback);
 
  /**
   * Evaluates the module and its dependencies.
   *
   * If status is kInstantiated, run the module's code. On success, set status
   * to kEvaluated and return the completion value; on failure, set status to
   * kErrored and propagate the thrown exception (which is then also available
   * via |GetException|).
   */
  V8_WARN_UNUSED_RESULT MaybeLocal<Value> Evaluate(Local<Context> context);
 
  /**
   * Returns the namespace object of this module.
   *
   * The module's status must be at least kInstantiated.
   */
  Local<Value> GetModuleNamespace();
 
  /**
   * Returns the corresponding context-unbound module script.
   *
   * The module must be unevaluated, i.e. its status must not be kEvaluating,
   * kEvaluated or kErrored.
   */
  Local<UnboundModuleScript> GetUnboundModuleScript();
 
  /**
   * Returns the underlying script's id.
   *
   * The module must be a SourceTextModule and must not have a kErrored status.
   */
  int ScriptId();
 
  /**
   * Returns whether the module is a SourceTextModule.
   */
  bool IsSourceTextModule() const;
 
  /**
   * Returns whether the module is a SyntheticModule.
   */
  bool IsSyntheticModule() const;
 
  /*
   * Callback defined in the embedder.  This is responsible for setting
   * the module's exported values with calls to SetSyntheticModuleExport().
   * The callback must return a Value to indicate success (where no
   * exception was thrown) and return an empy MaybeLocal to indicate falure
   * (where an exception was thrown).
   */
  typedef MaybeLocal<Value> (*SyntheticModuleEvaluationSteps)(
      Local<Context> context, Local<Module> module);
 
  /**
   * Creates a new SyntheticModule with the specified export names, where
   * evaluation_steps will be executed upon module evaluation.
   * export_names must not contain duplicates.
   * module_name is used solely for logging/debugging and doesn't affect module
   * behavior.
   */
  static Local<Module> CreateSyntheticModule(
      Isolate* isolate, Local<String> module_name,
      const std::vector<Local<String>>& export_names,
      SyntheticModuleEvaluationSteps evaluation_steps);
 
  /**
   * Set this module's exported value for the name export_name to the specified
   * export_value. This method must be called only on Modules created via
   * CreateSyntheticModule.  An error will be thrown if export_name is not one
   * of the export_names that were passed in that CreateSyntheticModule call.
   * Returns Just(true) on success, Nothing<bool>() if an error was thrown.
   */
  V8_WARN_UNUSED_RESULT Maybe<bool> SetSyntheticModuleExport(
      Isolate* isolate, Local<String> export_name, Local<Value> export_value);
  V8_DEPRECATE_SOON(
      "Use the preceding SetSyntheticModuleExport with an Isolate parameter, "
      "instead of the one that follows.  The former will throw a runtime "
      "error if called for an export that doesn't exist (as per spec); "
      "the latter will crash with a failed CHECK().")
  void SetSyntheticModuleExport(Local<String> export_name,
                                Local<Value> export_value);
};
 
/**
 * A compiled JavaScript script, tied to a Context which was active when the
 * script was compiled.
 */
class V8_EXPORT Script {
 public:
  /**
   * A shorthand for ScriptCompiler::Compile().
   */
  static V8_WARN_UNUSED_RESULT MaybeLocal<Script> Compile(
      Local<Context> context, Local<String> source,
      ScriptOrigin* origin = nullptr);
 
  /**
   * Runs the script returning the resulting value. It will be run in the
   * context in which it was created (ScriptCompiler::CompileBound or
   * UnboundScript::BindToCurrentContext()).
   */
  V8_WARN_UNUSED_RESULT MaybeLocal<Value> Run(Local<Context> context);
 
  /**
   * Returns the corresponding context-unbound script.
   */
  Local<UnboundScript> GetUnboundScript();
};
 
 
/**
 * For compiling scripts.
 */
class V8_EXPORT ScriptCompiler {
 public:
  /**
   * Compilation data that the embedder can cache and pass back to speed up
   * future compilations. The data is produced if the CompilerOptions passed to
   * the compilation functions in ScriptCompiler contains produce_data_to_cache
   * = true. The data to cache can then can be retrieved from
   * UnboundScript.
   */
  struct V8_EXPORT CachedData {
    enum BufferPolicy {
      BufferNotOwned,
      BufferOwned
    };
 
    CachedData()
        : data(nullptr),
          length(0),
          rejected(false),
          buffer_policy(BufferNotOwned) {}
 
    // If buffer_policy is BufferNotOwned, the caller keeps the ownership of
    // data and guarantees that it stays alive until the CachedData object is
    // destroyed. If the policy is BufferOwned, the given data will be deleted
    // (with delete[]) when the CachedData object is destroyed.
    CachedData(const uint8_t* data, int length,
               BufferPolicy buffer_policy = BufferNotOwned);
    ~CachedData();
    // TODO(marja): Async compilation; add constructors which take a callback
    // which will be called when V8 no longer needs the data.
    const uint8_t* data;
    int length;
    bool rejected;
    BufferPolicy buffer_policy;
 
    // Prevent copying.
    CachedData(const CachedData&) = delete;
    CachedData& operator=(const CachedData&) = delete;
  };
 
  /**
   * Source code which can be then compiled to a UnboundScript or Script.
   */
  class Source {
   public:
    // Source takes ownership of CachedData.
    V8_INLINE Source(Local<String> source_string, const ScriptOrigin& origin,
                     CachedData* cached_data = nullptr);
    V8_INLINE Source(Local<String> source_string,
                     CachedData* cached_data = nullptr);
    V8_INLINE ~Source();
 
    // Ownership of the CachedData or its buffers is *not* transferred to the
    // caller. The CachedData object is alive as long as the Source object is
    // alive.
    V8_INLINE const CachedData* GetCachedData() const;
 
    V8_INLINE const ScriptOriginOptions& GetResourceOptions() const;
 
    // Prevent copying.
    Source(const Source&) = delete;
    Source& operator=(const Source&) = delete;
 
   private:
    friend class ScriptCompiler;
 
    Local<String> source_string;
 
    // Origin information
    Local<Value> resource_name;
    Local<Integer> resource_line_offset;
    Local<Integer> resource_column_offset;
    ScriptOriginOptions resource_options;
    Local<Value> source_map_url;
    Local<PrimitiveArray> host_defined_options;
 
    // Cached data from previous compilation (if a kConsume*Cache flag is
    // set), or hold newly generated cache data (kProduce*Cache flags) are
    // set when calling a compile method.
    CachedData* cached_data;
  };
 
  /**
   * For streaming incomplete script data to V8. The embedder should implement a
   * subclass of this class.
   */
  class V8_EXPORT ExternalSourceStream {
   public:
    virtual ~ExternalSourceStream() = default;
 
    /**
     * V8 calls this to request the next chunk of data from the embedder. This
     * function will be called on a background thread, so it's OK to block and
     * wait for the data, if the embedder doesn't have data yet. Returns the
     * length of the data returned. When the data ends, GetMoreData should
     * return 0. Caller takes ownership of the data.
     *
     * When streaming UTF-8 data, V8 handles multi-byte characters split between
     * two data chunks, but doesn't handle multi-byte characters split between
     * more than two data chunks. The embedder can avoid this problem by always
     * returning at least 2 bytes of data.
     *
     * When streaming UTF-16 data, V8 does not handle characters split between
     * two data chunks. The embedder has to make sure that chunks have an even
     * length.
     *
     * If the embedder wants to cancel the streaming, they should make the next
     * GetMoreData call return 0. V8 will interpret it as end of data (and most
     * probably, parsing will fail). The streaming task will return as soon as
     * V8 has parsed the data it received so far.
     */
    virtual size_t GetMoreData(const uint8_t** src) = 0;
 
    /**
     * V8 calls this method to set a 'bookmark' at the current position in
     * the source stream, for the purpose of (maybe) later calling
     * ResetToBookmark. If ResetToBookmark is called later, then subsequent
     * calls to GetMoreData should return the same data as they did when
     * SetBookmark was called earlier.
     *
     * The embedder may return 'false' to indicate it cannot provide this
     * functionality.
     */
    virtual bool SetBookmark();
 
    /**
     * V8 calls this to return to a previously set bookmark.
     */
    virtual void ResetToBookmark();
  };
 
  /**
   * Source code which can be streamed into V8 in pieces. It will be parsed
   * while streaming and compiled after parsing has completed. StreamedSource
   * must be kept alive while the streaming task is run (see ScriptStreamingTask
   * below).
   */
  class V8_EXPORT StreamedSource {
   public:
    enum Encoding { ONE_BYTE, TWO_BYTE, UTF8 };
 
    V8_DEPRECATE_SOON(
        "This class takes ownership of source_stream, so use the constructor "
        "taking a unique_ptr to make these semantics clearer")
    StreamedSource(ExternalSourceStream* source_stream, Encoding encoding);
    StreamedSource(std::unique_ptr<ExternalSourceStream> source_stream,
                   Encoding encoding);
    ~StreamedSource();
 
    internal::ScriptStreamingData* impl() const { return impl_.get(); }
 
    // Prevent copying.
    StreamedSource(const StreamedSource&) = delete;
    StreamedSource& operator=(const StreamedSource&) = delete;
 
   private:
    std::unique_ptr<internal::ScriptStreamingData> impl_;
  };
 
  /**
   * A streaming task which the embedder must run on a background thread to
   * stream scripts into V8. Returned by ScriptCompiler::StartStreamingScript.
   */
  class V8_EXPORT ScriptStreamingTask final {
   public:
    void Run();
 
   private:
    friend class ScriptCompiler;
 
    explicit ScriptStreamingTask(internal::ScriptStreamingData* data)
        : data_(data) {}
 
    internal::ScriptStreamingData* data_;
  };
 
  enum CompileOptions {
    kNoCompileOptions = 0,
    kConsumeCodeCache,
    kEagerCompile
  };
 
  /**
   * The reason for which we are not requesting or providing a code cache.
   */
  enum NoCacheReason {
    kNoCacheNoReason = 0,
    kNoCacheBecauseCachingDisabled,
    kNoCacheBecauseNoResource,
    kNoCacheBecauseInlineScript,
    kNoCacheBecauseModule,
    kNoCacheBecauseStreamingSource,
    kNoCacheBecauseInspector,
    kNoCacheBecauseScriptTooSmall,
    kNoCacheBecauseCacheTooCold,
    kNoCacheBecauseV8Extension,
    kNoCacheBecauseExtensionModule,
    kNoCacheBecausePacScript,
    kNoCacheBecauseInDocumentWrite,
    kNoCacheBecauseResourceWithNoCacheHandler,
    kNoCacheBecauseDeferredProduceCodeCache
  };
 
  /**
   * Compiles the specified script (context-independent).
   * Cached data as part of the source object can be optionally produced to be
   * consumed later to speed up compilation of identical source scripts.
   *
   * Note that when producing cached data, the source must point to NULL for
   * cached data. When consuming cached data, the cached data must have been
   * produced by the same version of V8.
   *
   * \param source Script source code.
   * \return Compiled script object (context independent; for running it must be
   *   bound to a context).
   */
  static V8_WARN_UNUSED_RESULT MaybeLocal<UnboundScript> CompileUnboundScript(
      Isolate* isolate, Source* source,
      CompileOptions options = kNoCompileOptions,
      NoCacheReason no_cache_reason = kNoCacheNoReason);
 
  /**
   * Compiles the specified script (bound to current context).
   *
   * \param source Script source code.
   * \param pre_data Pre-parsing data, as obtained by ScriptData::PreCompile()
   *   using pre_data speeds compilation if it's done multiple times.
   *   Owned by caller, no references are kept when this function returns.
   * \return Compiled script object, bound to the context that was active
   *   when this function was called. When run it will always use this
   *   context.
   */
  static V8_WARN_UNUSED_RESULT MaybeLocal<Script> Compile(
      Local<Context> context, Source* source,
      CompileOptions options = kNoCompileOptions,
      NoCacheReason no_cache_reason = kNoCacheNoReason);
 
  /**
   * Returns a task which streams script data into V8, or NULL if the script
   * cannot be streamed. The user is responsible for running the task on a
   * background thread and deleting it. When ran, the task starts parsing the
   * script, and it will request data from the StreamedSource as needed. When
   * ScriptStreamingTask::Run exits, all data has been streamed and the script
   * can be compiled (see Compile below).
   *
   * This API allows to start the streaming with as little data as possible, and
   * the remaining data (for example, the ScriptOrigin) is passed to Compile.
   */
  static ScriptStreamingTask* StartStreamingScript(
      Isolate* isolate, StreamedSource* source,
      CompileOptions options = kNoCompileOptions);
 
  /**
   * Compiles a streamed script (bound to current context).
   *
   * This can only be called after the streaming has finished
   * (ScriptStreamingTask has been run). V8 doesn't construct the source string
   * during streaming, so the embedder needs to pass the full source here.
   */
  static V8_WARN_UNUSED_RESULT MaybeLocal<Script> Compile(
      Local<Context> context, StreamedSource* source,
      Local<String> full_source_string, const ScriptOrigin& origin);
 
  /**
   * Return a version tag for CachedData for the current V8 version & flags.
   *
   * This value is meant only for determining whether a previously generated
   * CachedData instance is still valid; the tag has no other meaing.
   *
   * Background: The data carried by CachedData may depend on the exact
   *   V8 version number or current compiler flags. This means that when
   *   persisting CachedData, the embedder must take care to not pass in
   *   data from another V8 version, or the same version with different
   *   features enabled.
   *
   *   The easiest way to do so is to clear the embedder's cache on any
   *   such change.
   *
   *   Alternatively, this tag can be stored alongside the cached data and
   *   compared when it is being used.
   */
  static uint32_t CachedDataVersionTag();
 
  /**
   * Compile an ES module, returning a Module that encapsulates
   * the compiled code.
   *
   * Corresponds to the ParseModule abstract operation in the
   * ECMAScript specification.
   */
  static V8_WARN_UNUSED_RESULT MaybeLocal<Module> CompileModule(
      Isolate* isolate, Source* source,
      CompileOptions options = kNoCompileOptions,
      NoCacheReason no_cache_reason = kNoCacheNoReason);
 
  /**
   * Compile a function for a given context. This is equivalent to running
   *
   * with (obj) {
   *   return function(args) { ... }
   * }
   *
   * It is possible to specify multiple context extensions (obj in the above
   * example).
   */
  static V8_WARN_UNUSED_RESULT MaybeLocal<Function> CompileFunctionInContext(
      Local<Context> context, Source* source, size_t arguments_count,
      Local<String> arguments[], size_t context_extension_count,
      Local<Object> context_extensions[],
      CompileOptions options = kNoCompileOptions,
      NoCacheReason no_cache_reason = kNoCacheNoReason,
      Local<ScriptOrModule>* script_or_module_out = nullptr);
 
  /**
   * Creates and returns code cache for the specified unbound_script.
   * This will return nullptr if the script cannot be serialized. The
   * CachedData returned by this function should be owned by the caller.
   */
  static CachedData* CreateCodeCache(Local<UnboundScript> unbound_script);
 
  /**
   * Creates and returns code cache for the specified unbound_module_script.
   * This will return nullptr if the script cannot be serialized. The
   * CachedData returned by this function should be owned by the caller.
   */
  static CachedData* CreateCodeCache(
      Local<UnboundModuleScript> unbound_module_script);
 
  /**
   * Creates and returns code cache for the specified function that was
   * previously produced by CompileFunctionInContext.
   * This will return nullptr if the script cannot be serialized. The
   * CachedData returned by this function should be owned by the caller.
   */
  static CachedData* CreateCodeCacheForFunction(Local<Function> function);
 
 private:
  static V8_WARN_UNUSED_RESULT MaybeLocal<UnboundScript> CompileUnboundInternal(
      Isolate* isolate, Source* source, CompileOptions options,
      NoCacheReason no_cache_reason);
};
 
 
/**
 * An error message.
 */
class V8_EXPORT Message {
 public:
  Local<String> Get() const;
 
  /**
   * Return the isolate to which the Message belongs.
   */
  Isolate* GetIsolate() const;
 
  V8_WARN_UNUSED_RESULT MaybeLocal<String> GetSourceLine(
      Local<Context> context) const;
 
  /**
   * Returns the origin for the script from where the function causing the
   * error originates.
   */
  ScriptOrigin GetScriptOrigin() const;
 
  /**
   * Returns the resource name for the script from where the function causing
   * the error originates.
   */
  Local<Value> GetScriptResourceName() const;
 
  /**
   * Exception stack trace. By default stack traces are not captured for
   * uncaught exceptions. SetCaptureStackTraceForUncaughtExceptions allows
   * to change this option.
   */
  Local<StackTrace> GetStackTrace() const;
 
  /**
   * Returns the number, 1-based, of the line where the error occurred.
   */
  V8_WARN_UNUSED_RESULT Maybe<int> GetLineNumber(Local<Context> context) const;
 
  /**
   * Returns the index within the script of the first character where
   * the error occurred.
   */
  int GetStartPosition() const;
 
  /**
   * Returns the index within the script of the last character where
   * the error occurred.
   */
  int GetEndPosition() const;
 
  /**
   * Returns the Wasm function index where the error occurred. Returns -1 if
   * message is not from a Wasm script.
   */
  int GetWasmFunctionIndex() const;
 
  /**
   * Returns the error level of the message.
   */
  int ErrorLevel() const;
 
  /**
   * Returns the index within the line of the first character where
   * the error occurred.
   */
  int GetStartColumn() const;
  V8_WARN_UNUSED_RESULT Maybe<int> GetStartColumn(Local<Context> context) const;
 
  /**
   * Returns the index within the line of the last character where
   * the error occurred.
   */
  int GetEndColumn() const;
  V8_WARN_UNUSED_RESULT Maybe<int> GetEndColumn(Local<Context> context) const;
 
  /**
   * Passes on the value set by the embedder when it fed the script from which
   * this Message was generated to V8.
   */
  bool IsSharedCrossOrigin() const;
  bool IsOpaque() const;
 
  // TODO(1245381): Print to a string instead of on a FILE.
  static void PrintCurrentStackTrace(Isolate* isolate, FILE* out);
 
  static const int kNoLineNumberInfo = 0;
  static const int kNoColumnInfo = 0;
  static const int kNoScriptIdInfo = 0;
  static const int kNoWasmFunctionIndexInfo = -1;
};
 
 
/**
 * Representation of a JavaScript stack trace. The information collected is a
 * snapshot of the execution stack and the information remains valid after
 * execution continues.
 */
class V8_EXPORT StackTrace {
 public:
  /**
   * Flags that determine what information is placed captured for each
   * StackFrame when grabbing the current stack trace.
   * Note: these options are deprecated and we always collect all available
   * information (kDetailed).
   */
  enum StackTraceOptions {
    kLineNumber = 1,
    kColumnOffset = 1 << 1 | kLineNumber,
    kScriptName = 1 << 2,
    kFunctionName = 1 << 3,
    kIsEval = 1 << 4,
    kIsConstructor = 1 << 5,
    kScriptNameOrSourceURL = 1 << 6,
    kScriptId = 1 << 7,
    kExposeFramesAcrossSecurityOrigins = 1 << 8,
    kOverview = kLineNumber | kColumnOffset | kScriptName | kFunctionName,
    kDetailed = kOverview | kIsEval | kIsConstructor | kScriptNameOrSourceURL
  };
 
  /**
   * Returns a StackFrame at a particular index.
   */
  Local<StackFrame> GetFrame(Isolate* isolate, uint32_t index) const;
 
  /**
   * Returns the number of StackFrames.
   */
  int GetFrameCount() const;
 
  /**
   * Grab a snapshot of the current JavaScript execution stack.
   *
   * \param frame_limit The maximum number of stack frames we want to capture.
   * \param options Enumerates the set of things we will capture for each
   *   StackFrame.
   */
  static Local<StackTrace> CurrentStackTrace(
      Isolate* isolate, int frame_limit, StackTraceOptions options = kDetailed);
};
 
 
/**
 * A single JavaScript stack frame.
 */
class V8_EXPORT StackFrame {
 public:
  /**
   * Returns the number, 1-based, of the line for the associate function call.
   * This method will return Message::kNoLineNumberInfo if it is unable to
   * retrieve the line number, or if kLineNumber was not passed as an option
   * when capturing the StackTrace.
   */
  int GetLineNumber() const;
 
  /**
   * Returns the 1-based column offset on the line for the associated function
   * call.
   * This method will return Message::kNoColumnInfo if it is unable to retrieve
   * the column number, or if kColumnOffset was not passed as an option when
   * capturing the StackTrace.
   */
  int GetColumn() const;
 
  /**
   * Returns the id of the script for the function for this StackFrame.
   * This method will return Message::kNoScriptIdInfo if it is unable to
   * retrieve the script id, or if kScriptId was not passed as an option when
   * capturing the StackTrace.
   */
  int GetScriptId() const;
 
  /**
   * Returns the name of the resource that contains the script for the
   * function for this StackFrame.
   */
  Local<String> GetScriptName() const;
 
  /**
   * Returns the name of the resource that contains the script for the
   * function for this StackFrame or sourceURL value if the script name
   * is undefined and its source ends with //# sourceURL=... string or
   * deprecated //@ sourceURL=... string.
   */
  Local<String> GetScriptNameOrSourceURL() const;
 
  /**
   * Returns the name of the function associated with this stack frame.
   */
  Local<String> GetFunctionName() const;
 
  /**
   * Returns whether or not the associated function is compiled via a call to
   * eval().
   */
  bool IsEval() const;
 
  /**
   * Returns whether or not the associated function is called as a
   * constructor via "new".
   */
  bool IsConstructor() const;
 
  /**
   * Returns whether or not the associated functions is defined in wasm.
   */
  bool IsWasm() const;
 
  /**
   * Returns whether or not the associated function is defined by the user.
   */
  bool IsUserJavaScript() const;
};
 
 
// A StateTag represents a possible state of the VM.
enum StateTag {
  JS,
  GC,
  PARSER,
  BYTECODE_COMPILER,
  COMPILER,
  OTHER,
  EXTERNAL,
  ATOMICS_WAIT,
  IDLE
};
 
// A RegisterState represents the current state of registers used
// by the sampling profiler API.
struct RegisterState {
  RegisterState() : pc(nullptr), sp(nullptr), fp(nullptr), lr(nullptr) {}
  void* pc;  // Instruction pointer.
  void* sp;  // Stack pointer.
  void* fp;  // Frame pointer.
  void* lr;  // Link register (or nullptr on platforms without a link register).
};
 
// The output structure filled up by GetStackSample API function.
struct SampleInfo {
  size_t frames_count;            // Number of frames collected.
  StateTag vm_state;              // Current VM state.
  void* external_callback_entry;  // External callback address if VM is
                                  // executing an external callback.
  void* top_context;              // Incumbent native context address.
};
 
struct MemoryRange {
  const void* start = nullptr;
  size_t length_in_bytes = 0;
};
 
struct JSEntryStub {
  MemoryRange code;
};
 
struct UnwindState {
  MemoryRange code_range;
  MemoryRange embedded_code_range;
  JSEntryStub js_entry_stub;
  JSEntryStub js_construct_entry_stub;
  JSEntryStub js_run_microtasks_entry_stub;
};
 
struct JSEntryStubs {
  JSEntryStub js_entry_stub;
  JSEntryStub js_construct_entry_stub;
  JSEntryStub js_run_microtasks_entry_stub;
};
 
/**
 * A JSON Parser and Stringifier.
 */
class V8_EXPORT JSON {
 public:
  /**
   * Tries to parse the string |json_string| and returns it as value if
   * successful.
   *
   * \param the context in which to parse and create the value.
   * \param json_string The string to parse.
   * \return The corresponding value if successfully parsed.
   */
  static V8_WARN_UNUSED_RESULT MaybeLocal<Value> Parse(
      Local<Context> context, Local<String> json_string);
 
  /**
   * Tries to stringify the JSON-serializable object |json_object| and returns
   * it as string if successful.
   *
   * \param json_object The JSON-serializable object to stringify.
   * \return The corresponding string if successfully stringified.
   */
  static V8_WARN_UNUSED_RESULT MaybeLocal<String> Stringify(
      Local<Context> context, Local<Value> json_object,
      Local<String> gap = Local<String>());
};
 
/**
 * Value serialization compatible with the HTML structured clone algorithm.
 * The format is backward-compatible (i.e. safe to store to disk).
 */
class V8_EXPORT ValueSerializer {
 public:
  class V8_EXPORT Delegate {
   public:
    virtual ~Delegate() = default;
 
    /**
     * Handles the case where a DataCloneError would be thrown in the structured
     * clone spec. Other V8 embedders may throw some other appropriate exception
     * type.
     */
    virtual void ThrowDataCloneError(Local<String> message) = 0;
 
    /**
     * The embedder overrides this method to write some kind of host object, if
     * possible. If not, a suitable exception should be thrown and
     * Nothing<bool>() returned.
     */
    virtual Maybe<bool> WriteHostObject(Isolate* isolate, Local<Object> object);
 
    /**
     * Called when the ValueSerializer is going to serialize a
     * SharedArrayBuffer object. The embedder must return an ID for the
     * object, using the same ID if this SharedArrayBuffer has already been
     * serialized in this buffer. When deserializing, this ID will be passed to
     * ValueDeserializer::GetSharedArrayBufferFromId as |clone_id|.
     *
     * If the object cannot be serialized, an
     * exception should be thrown and Nothing<uint32_t>() returned.
     */
    virtual Maybe<uint32_t> GetSharedArrayBufferId(
        Isolate* isolate, Local<SharedArrayBuffer> shared_array_buffer);
 
    virtual Maybe<uint32_t> GetWasmModuleTransferId(
        Isolate* isolate, Local<WasmModuleObject> module);
    /**
     * Allocates memory for the buffer of at least the size provided. The actual
     * size (which may be greater or equal) is written to |actual_size|. If no
     * buffer has been allocated yet, nullptr will be provided.
     *
     * If the memory cannot be allocated, nullptr should be returned.
     * |actual_size| will be ignored. It is assumed that |old_buffer| is still
     * valid in this case and has not been modified.
     *
     * The default implementation uses the stdlib's `realloc()` function.
     */
    virtual void* ReallocateBufferMemory(void* old_buffer, size_t size,
                                         size_t* actual_size);
 
    /**
     * Frees a buffer allocated with |ReallocateBufferMemory|.
     *
     * The default implementation uses the stdlib's `free()` function.
     */
    virtual void FreeBufferMemory(void* buffer);
  };
 
  explicit ValueSerializer(Isolate* isolate);
  ValueSerializer(Isolate* isolate, Delegate* delegate);
  ~ValueSerializer();
 
  /**
   * Writes out a header, which includes the format version.
   */
  void WriteHeader();
 
  /**
   * Serializes a JavaScript value into the buffer.
   */
  V8_WARN_UNUSED_RESULT Maybe<bool> WriteValue(Local<Context> context,
                                               Local<Value> value);
 
  /**
   * Returns the stored data (allocated using the delegate's
   * ReallocateBufferMemory) and its size. This serializer should not be used
   * once the buffer is released. The contents are undefined if a previous write
   * has failed. Ownership of the buffer is transferred to the caller.
   */
  V8_WARN_UNUSED_RESULT std::pair<uint8_t*, size_t> Release();
 
  /**
   * Marks an ArrayBuffer as havings its contents transferred out of band.
   * Pass the corresponding ArrayBuffer in the deserializing context to
   * ValueDeserializer::TransferArrayBuffer.
   */
  void TransferArrayBuffer(uint32_t transfer_id,
                           Local<ArrayBuffer> array_buffer);
 
 
  /**
   * Indicate whether to treat ArrayBufferView objects as host objects,
   * i.e. pass them to Delegate::WriteHostObject. This should not be
   * called when no Delegate was passed.
   *
   * The default is not to treat ArrayBufferViews as host objects.
   */
  void SetTreatArrayBufferViewsAsHostObjects(bool mode);
 
  /**
   * Write raw data in various common formats to the buffer.
   * Note that integer types are written in base-128 varint format, not with a
   * binary copy. For use during an override of Delegate::WriteHostObject.
   */
  void WriteUint32(uint32_t value);
  void WriteUint64(uint64_t value);
  void WriteDouble(double value);
  void WriteRawBytes(const void* source, size_t length);
 
  ValueSerializer(const ValueSerializer&) = delete;
  void operator=(const ValueSerializer&) = delete;
 
 private:
  struct PrivateData;
  PrivateData* private_;
};
 
/**
 * Deserializes values from data written with ValueSerializer, or a compatible
 * implementation.
 */
class V8_EXPORT ValueDeserializer {
 public:
  class V8_EXPORT Delegate {
   public:
    virtual ~Delegate() = default;
 
    /**
     * The embedder overrides this method to read some kind of host object, if
     * possible. If not, a suitable exception should be thrown and
     * MaybeLocal<Object>() returned.
     */
    virtual MaybeLocal<Object> ReadHostObject(Isolate* isolate);
 
    /**
     * Get a WasmModuleObject given a transfer_id previously provided
     * by ValueSerializer::GetWasmModuleTransferId
     */
    virtual MaybeLocal<WasmModuleObject> GetWasmModuleFromId(
        Isolate* isolate, uint32_t transfer_id);
 
    /**
     * Get a SharedArrayBuffer given a clone_id previously provided
     * by ValueSerializer::GetSharedArrayBufferId
     */
    virtual MaybeLocal<SharedArrayBuffer> GetSharedArrayBufferFromId(
        Isolate* isolate, uint32_t clone_id);
  };
 
  ValueDeserializer(Isolate* isolate, const uint8_t* data, size_t size);
  ValueDeserializer(Isolate* isolate, const uint8_t* data, size_t size,
                    Delegate* delegate);
  ~ValueDeserializer();
 
  /**
   * Reads and validates a header (including the format version).
   * May, for example, reject an invalid or unsupported wire format.
   */
  V8_WARN_UNUSED_RESULT Maybe<bool> ReadHeader(Local<Context> context);
 
  /**
   * Deserializes a JavaScript value from the buffer.
   */
  V8_WARN_UNUSED_RESULT MaybeLocal<Value> ReadValue(Local<Context> context);
 
  /**
   * Accepts the array buffer corresponding to the one passed previously to
   * ValueSerializer::TransferArrayBuffer.
   */
  void TransferArrayBuffer(uint32_t transfer_id,
                           Local<ArrayBuffer> array_buffer);
 
  /**
   * Similar to TransferArrayBuffer, but for SharedArrayBuffer.
   * The id is not necessarily in the same namespace as unshared ArrayBuffer
   * objects.
   */
  void TransferSharedArrayBuffer(uint32_t id,
                                 Local<SharedArrayBuffer> shared_array_buffer);
 
  /**
   * Must be called before ReadHeader to enable support for reading the legacy
   * wire format (i.e., which predates this being shipped).
   *
   * Don't use this unless you need to read data written by previous versions of
   * blink::ScriptValueSerializer.
   */
  void SetSupportsLegacyWireFormat(bool supports_legacy_wire_format);
 
  /**
   * Reads the underlying wire format version. Likely mostly to be useful to
   * legacy code reading old wire format versions. Must be called after
   * ReadHeader.
   */
  uint32_t GetWireFormatVersion() const;
 
  /**
   * Reads raw data in various common formats to the buffer.
   * Note that integer types are read in base-128 varint format, not with a
   * binary copy. For use during an override of Delegate::ReadHostObject.
   */
  V8_WARN_UNUSED_RESULT bool ReadUint32(uint32_t* value);
  V8_WARN_UNUSED_RESULT bool ReadUint64(uint64_t* value);
  V8_WARN_UNUSED_RESULT bool ReadDouble(double* value);
  V8_WARN_UNUSED_RESULT bool ReadRawBytes(size_t length, const void** data);
 
  ValueDeserializer(const ValueDeserializer&) = delete;
  void operator=(const ValueDeserializer&) = delete;
 
 private:
  struct PrivateData;
  PrivateData* private_;
};
 
 
// --- Value ---
 
 
/**
 * The superclass of all JavaScript values and objects.
 */
class V8_EXPORT Value : public Data {
 public:
  /**
   * Returns true if this value is the undefined value.  See ECMA-262
   * 4.3.10.
   *
   * This is equivalent to `value === undefined` in JS.
   */
  V8_INLINE bool IsUndefined() const;
 
  /**
   * Returns true if this value is the null value.  See ECMA-262
   * 4.3.11.
   *
   * This is equivalent to `value === null` in JS.
   */
  V8_INLINE bool IsNull() const;
 
  /**
   * Returns true if this value is either the null or the undefined value.
   * See ECMA-262
   * 4.3.11. and 4.3.12
   *
   * This is equivalent to `value == null` in JS.
   */
  V8_INLINE bool IsNullOrUndefined() const;
 
  /**
   * Returns true if this value is true.
   *
   * This is not the same as `BooleanValue()`. The latter performs a
   * conversion to boolean, i.e. the result of `Boolean(value)` in JS, whereas
   * this checks `value === true`.
   */
  bool IsTrue() const;
 
  /**
   * Returns true if this value is false.
   *
   * This is not the same as `!BooleanValue()`. The latter performs a
   * conversion to boolean, i.e. the result of `!Boolean(value)` in JS, whereas
   * this checks `value === false`.
   */
  bool IsFalse() const;
 
  /**
   * Returns true if this value is a symbol or a string.
   *
   * This is equivalent to
   * `typeof value === 'string' || typeof value === 'symbol'` in JS.
   */
  bool IsName() const;
 
  /**
   * Returns true if this value is an instance of the String type.
   * See ECMA-262 8.4.
   *
   * This is equivalent to `typeof value === 'string'` in JS.
   */
  V8_INLINE bool IsString() const;
 
  /**
   * Returns true if this value is a symbol.
   *
   * This is equivalent to `typeof value === 'symbol'` in JS.
   */
  bool IsSymbol() const;
 
  /**
   * Returns true if this value is a function.
   *
   * This is equivalent to `typeof value === 'function'` in JS.
   */
  bool IsFunction() const;
 
  /**
   * Returns true if this value is an array. Note that it will return false for
   * an Proxy for an array.
   */
  bool IsArray() const;
 
  /**
   * Returns true if this value is an object.
   */
  bool IsObject() const;
 
  /**
   * Returns true if this value is a bigint.
   *
   * This is equivalent to `typeof value === 'bigint'` in JS.
   */
  bool IsBigInt() const;
 
  /**
   * Returns true if this value is boolean.
   *
   * This is equivalent to `typeof value === 'boolean'` in JS.
   */
  bool IsBoolean() const;
 
  /**
   * Returns true if this value is a number.
   *
   * This is equivalent to `typeof value === 'number'` in JS.
   */
  bool IsNumber() const;
 
  /**
   * Returns true if this value is an `External` object.
   */
  bool IsExternal() const;
 
  /**
   * Returns true if this value is a 32-bit signed integer.
   */
  bool IsInt32() const;
 
  /**
   * Returns true if this value is a 32-bit unsigned integer.
   */
  bool IsUint32() const;
 
  /**
   * Returns true if this value is a Date.
   */
  bool IsDate() const;
 
  /**
   * Returns true if this value is an Arguments object.
   */
  bool IsArgumentsObject() const;
 
  /**
   * Returns true if this value is a BigInt object.
   */
  bool IsBigIntObject() const;
 
  /**
   * Returns true if this value is a Boolean object.
   */
  bool IsBooleanObject() const;
 
  /**
   * Returns true if this value is a Number object.
   */
  bool IsNumberObject() const;
 
  /**
   * Returns true if this value is a String object.
   */
  bool IsStringObject() const;
 
  /**
   * Returns true if this value is a Symbol object.
   */
  bool IsSymbolObject() const;
 
  /**
   * Returns true if this value is a NativeError.
   */
  bool IsNativeError() const;
 
  /**
   * Returns true if this value is a RegExp.
   */
  bool IsRegExp() const;
 
  /**
   * Returns true if this value is an async function.
   */
  bool IsAsyncFunction() const;
 
  /**
   * Returns true if this value is a Generator function.
   */
  bool IsGeneratorFunction() const;
 
  /**
   * Returns true if this value is a Generator object (iterator).
   */
  bool IsGeneratorObject() const;
 
  /**
   * Returns true if this value is a Promise.
   */
  bool IsPromise() const;
 
  /**
   * Returns true if this value is a Map.
   */
  bool IsMap() const;
 
  /**
   * Returns true if this value is a Set.
   */
  bool IsSet() const;
 
  /**
   * Returns true if this value is a Map Iterator.
   */
  bool IsMapIterator() const;
 
  /**
   * Returns true if this value is a Set Iterator.
   */
  bool IsSetIterator() const;
 
  /**
   * Returns true if this value is a WeakMap.
   */
  bool IsWeakMap() const;
 
  /**
   * Returns true if this value is a WeakSet.
   */
  bool IsWeakSet() const;
 
  /**
   * Returns true if this value is an ArrayBuffer.
   */
  bool IsArrayBuffer() const;
 
  /**
   * Returns true if this value is an ArrayBufferView.
   */
  bool IsArrayBufferView() const;
 
  /**
   * Returns true if this value is one of TypedArrays.
   */
  bool IsTypedArray() const;
 
  /**
   * Returns true if this value is an Uint8Array.
   */
  bool IsUint8Array() const;
 
  /**
   * Returns true if this value is an Uint8ClampedArray.
   */
  bool IsUint8ClampedArray() const;
 
  /**
   * Returns true if this value is an Int8Array.
   */
  bool IsInt8Array() const;
 
  /**
   * Returns true if this value is an Uint16Array.
   */
  bool IsUint16Array() const;
 
  /**
   * Returns true if this value is an Int16Array.
   */
  bool IsInt16Array() const;
 
  /**
   * Returns true if this value is an Uint32Array.
   */
  bool IsUint32Array() const;
 
  /**
   * Returns true if this value is an Int32Array.
   */
  bool IsInt32Array() const;
 
  /**
   * Returns true if this value is a Float32Array.
   */
  bool IsFloat32Array() const;
 
  /**
   * Returns true if this value is a Float64Array.
   */
  bool IsFloat64Array() const;
 
  /**
   * Returns true if this value is a BigInt64Array.
   */
  bool IsBigInt64Array() const;
 
  /**
   * Returns true if this value is a BigUint64Array.
   */
  bool IsBigUint64Array() const;
 
  /**
   * Returns true if this value is a DataView.
   */
  bool IsDataView() const;
 
  /**
   * Returns true if this value is a SharedArrayBuffer.
   */
  bool IsSharedArrayBuffer() const;
 
  /**
   * Returns true if this value is a JavaScript Proxy.
   */
  bool IsProxy() const;
 
  /**
   * Returns true if this value is a WasmModuleObject.
   */
  bool IsWasmModuleObject() const;
 
  /**
   * Returns true if the value is a Module Namespace Object.
   */
  bool IsModuleNamespaceObject() const;
 
  /**
   * Perform the equivalent of `BigInt(value)` in JS.
   */
  V8_WARN_UNUSED_RESULT MaybeLocal<BigInt> ToBigInt(
      Local<Context> context) const;
  /**
   * Perform the equivalent of `Number(value)` in JS.
   */
  V8_WARN_UNUSED_RESULT MaybeLocal<Number> ToNumber(
      Local<Context> context) const;
  /**
   * Perform the equivalent of `String(value)` in JS.
   */
  V8_WARN_UNUSED_RESULT MaybeLocal<String> ToString(
      Local<Context> context) const;
  /**
   * Provide a string representation of this value usable for debugging.
   * This operation has no observable side effects and will succeed
   * unless e.g. execution is being terminated.
   */
  V8_WARN_UNUSED_RESULT MaybeLocal<String> ToDetailString(
      Local<Context> context) const;
  /**
   * Perform the equivalent of `Object(value)` in JS.
   */
  V8_WARN_UNUSED_RESULT MaybeLocal<Object> ToObject(
      Local<Context> context) const;
  /**
   * Perform the equivalent of `Number(value)` in JS and convert the result
   * to an integer. Negative values are rounded up, positive values are rounded
   * down. NaN is converted to 0. Infinite values yield undefined results.
   */
  V8_WARN_UNUSED_RESULT MaybeLocal<Integer> ToInteger(
      Local<Context> context) const;
  /**
   * Perform the equivalent of `Number(value)` in JS and convert the result
   * to an unsigned 32-bit integer by performing the steps in
   * https://tc39.es/ecma262/#sec-touint32.
   */
  V8_WARN_UNUSED_RESULT MaybeLocal<Uint32> ToUint32(
      Local<Context> context) const;
  /**
   * Perform the equivalent of `Number(value)` in JS and convert the result
   * to a signed 32-bit integer by performing the steps in
   * https://tc39.es/ecma262/#sec-toint32.
   */
  V8_WARN_UNUSED_RESULT MaybeLocal<Int32> ToInt32(Local<Context> context) const;
 
  /**
   * Perform the equivalent of `Boolean(value)` in JS. This can never fail.
   */
  Local<Boolean> ToBoolean(Isolate* isolate) const;
 
  /**
   * Attempts to convert a string to an array index.
   * Returns an empty handle if the conversion fails.
   */
  V8_WARN_UNUSED_RESULT MaybeLocal<Uint32> ToArrayIndex(
      Local<Context> context) const;
 
  /** Returns the equivalent of `ToBoolean()->Value()`. */
  bool BooleanValue(Isolate* isolate) const;
 
  /** Returns the equivalent of `ToNumber()->Value()`. */
  V8_WARN_UNUSED_RESULT Maybe<double> NumberValue(Local<Context> context) const;
  /** Returns the equivalent of `ToInteger()->Value()`. */
  V8_WARN_UNUSED_RESULT Maybe<int64_t> IntegerValue(
      Local<Context> context) const;
  /** Returns the equivalent of `ToUint32()->Value()`. */
  V8_WARN_UNUSED_RESULT Maybe<uint32_t> Uint32Value(
      Local<Context> context) const;
  /** Returns the equivalent of `ToInt32()->Value()`. */
  V8_WARN_UNUSED_RESULT Maybe<int32_t> Int32Value(Local<Context> context) const;
 
  /** JS == */
  V8_WARN_UNUSED_RESULT Maybe<bool> Equals(Local<Context> context,
                                           Local<Value> that) const;
  bool StrictEquals(Local<Value> that) const;
  bool SameValue(Local<Value> that) const;
 
  template <class T> V8_INLINE static Value* Cast(T* value);
 
  Local<String> TypeOf(Isolate*);
 
  Maybe<bool> InstanceOf(Local<Context> context, Local<Object> object);
 
 private:
  V8_INLINE bool QuickIsUndefined() const;
  V8_INLINE bool QuickIsNull() const;
  V8_INLINE bool QuickIsNullOrUndefined() const;
  V8_INLINE bool QuickIsString() const;
  bool FullIsUndefined() const;
  bool FullIsNull() const;
  bool FullIsString() const;
};
 
 
/**
 * The superclass of primitive values.  See ECMA-262 4.3.2.
 */
class V8_EXPORT Primitive : public Value { };
 
 
/**
 * A primitive boolean value (ECMA-262, 4.3.14).  Either the true
 * or false value.
 */
class V8_EXPORT Boolean : public Primitive {
 public:
  bool Value() const;
  V8_INLINE static Boolean* Cast(v8::Value* obj);
  V8_INLINE static Local<Boolean> New(Isolate* isolate, bool value);
 
 private:
  static void CheckCast(v8::Value* obj);
};
 
 
/**
 * A superclass for symbols and strings.
 */
class V8_EXPORT Name : public Primitive {
 public:
  /**
   * Returns the identity hash for this object. The current implementation
   * uses an inline property on the object to store the identity hash.
   *
   * The return value will never be 0. Also, it is not guaranteed to be
   * unique.
   */
  int GetIdentityHash();
 
  V8_INLINE static Name* Cast(Value* obj);
 
 private:
  static void CheckCast(Value* obj);
};
 
/**
 * A flag describing different modes of string creation.
 *
 * Aside from performance implications there are no differences between the two
 * creation modes.
 */
enum class NewStringType {
  /**
   * Create a new string, always allocating new storage memory.
   */
  kNormal,
 
  /**
   * Acts as a hint that the string should be created in the
   * old generation heap space and be deduplicated if an identical string
   * already exists.
   */
  kInternalized
};
 
/**
 * A JavaScript string value (ECMA-262, 4.3.17).
 */
class V8_EXPORT String : public Name {
 public:
  static constexpr int kMaxLength =
      internal::kApiSystemPointerSize == 4 ? (1 << 28) - 16 : (1 << 29) - 24;
 
  enum Encoding {
    UNKNOWN_ENCODING = 0x1,
    TWO_BYTE_ENCODING = 0x0,
    ONE_BYTE_ENCODING = 0x8
  };
  /**
   * Returns the number of characters (UTF-16 code units) in this string.
   */
  int Length() const;
 
  /**
   * Returns the number of bytes in the UTF-8 encoded
   * representation of this string.
   */
  int Utf8Length(Isolate* isolate) const;
 
  /**
   * Returns whether this string is known to contain only one byte data,
   * i.e. ISO-8859-1 code points.
   * Does not read the string.
   * False negatives are possible.
   */
  bool IsOneByte() const;
 
  /**
   * Returns whether this string contain only one byte data,
   * i.e. ISO-8859-1 code points.
   * Will read the entire string in some cases.
   */
  bool ContainsOnlyOneByte() const;
 
  /**
   * Write the contents of the string to an external buffer.
   * If no arguments are given, expects the buffer to be large
   * enough to hold the entire string and NULL terminator. Copies
   * the contents of the string and the NULL terminator into the
   * buffer.
   *
   * WriteUtf8 will not write partial UTF-8 sequences, preferring to stop
   * before the end of the buffer.
   *
   * Copies up to length characters into the output buffer.
   * Only null-terminates if there is enough space in the buffer.
   *
   * \param buffer The buffer into which the string will be copied.
   * \param start The starting position within the string at which
   * copying begins.
   * \param length The number of characters to copy from the string.  For
   *    WriteUtf8 the number of bytes in the buffer.
   * \param nchars_ref The number of characters written, can be NULL.
   * \param options Various options that might affect performance of this or
   *    subsequent operations.
   * \return The number of characters copied to the buffer excluding the null
   *    terminator.  For WriteUtf8: The number of bytes copied to the buffer
   *    including the null terminator (if written).
   */
  enum WriteOptions {
    NO_OPTIONS = 0,
    HINT_MANY_WRITES_EXPECTED = 1,
    NO_NULL_TERMINATION = 2,
    PRESERVE_ONE_BYTE_NULL = 4,
    // Used by WriteUtf8 to replace orphan surrogate code units with the
    // unicode replacement character. Needs to be set to guarantee valid UTF-8
    // output.
    REPLACE_INVALID_UTF8 = 8
  };
 
  // 16-bit character codes.
  int Write(Isolate* isolate, uint16_t* buffer, int start = 0, int length = -1,
            int options = NO_OPTIONS) const;
  // One byte characters.
  int WriteOneByte(Isolate* isolate, uint8_t* buffer, int start = 0,
                   int length = -1, int options = NO_OPTIONS) const;
  // UTF-8 encoded characters.
  int WriteUtf8(Isolate* isolate, char* buffer, int length = -1,
                int* nchars_ref = nullptr, int options = NO_OPTIONS) const;
 
  /**
   * A zero length string.
   */
  V8_INLINE static Local<String> Empty(Isolate* isolate);
 
  /**
   * Returns true if the string is external
   */
  bool IsExternal() const;
 
  /**
   * Returns true if the string is both external and one-byte.
   */
  bool IsExternalOneByte() const;
 
  class V8_EXPORT ExternalStringResourceBase {  // NOLINT
   public:
    virtual ~ExternalStringResourceBase() = default;
 
    /**
     * If a string is cacheable, the value returned by
     * ExternalStringResource::data() may be cached, otherwise it is not
     * expected to be stable beyond the current top-level task.
     */
    virtual bool IsCacheable() const { return true; }
 
    // Disallow copying and assigning.
    ExternalStringResourceBase(const ExternalStringResourceBase&) = delete;
    void operator=(const ExternalStringResourceBase&) = delete;
 
   protected:
    ExternalStringResourceBase() = default;
 
    /**
     * Internally V8 will call this Dispose method when the external string
     * resource is no longer needed. The default implementation will use the
     * delete operator. This method can be overridden in subclasses to
     * control how allocated external string resources are disposed.
     */
    virtual void Dispose() { delete this; }
 
    /**
     * For a non-cacheable string, the value returned by
     * |ExternalStringResource::data()| has to be stable between |Lock()| and
     * |Unlock()|, that is the string must behave as is |IsCacheable()| returned
     * true.
     *
     * These two functions must be thread-safe, and can be called from anywhere.
     * They also must handle lock depth, in the sense that each can be called
     * several times, from different threads, and unlocking should only happen
     * when the balance of Lock() and Unlock() calls is 0.
     */
    virtual void Lock() const {}
 
    /**
     * Unlocks the string.
     */
    virtual void Unlock() const {}
 
   private:
    friend class internal::ExternalString;
    friend class v8::String;
    friend class internal::ScopedExternalStringLock;
  };
 
  /**
   * An ExternalStringResource is a wrapper around a two-byte string
   * buffer that resides outside V8's heap. Implement an
   * ExternalStringResource to manage the life cycle of the underlying
   * buffer.  Note that the string data must be immutable.
   */
  class V8_EXPORT ExternalStringResource
      : public ExternalStringResourceBase {
   public:
    /**
     * Override the destructor to manage the life cycle of the underlying
     * buffer.
     */
    ~ExternalStringResource() override = default;
 
    /**
     * The string data from the underlying buffer.
     */
    virtual const uint16_t* data() const = 0;
 
    /**
     * The length of the string. That is, the number of two-byte characters.
     */
    virtual size_t length() const = 0;
 
   protected:
    ExternalStringResource() = default;
  };
 
  /**
   * An ExternalOneByteStringResource is a wrapper around an one-byte
   * string buffer that resides outside V8's heap. Implement an
   * ExternalOneByteStringResource to manage the life cycle of the
   * underlying buffer.  Note that the string data must be immutable
   * and that the data must be Latin-1 and not UTF-8, which would require
   * special treatment internally in the engine and do not allow efficient
   * indexing.  Use String::New or convert to 16 bit data for non-Latin1.
   */
 
  class V8_EXPORT ExternalOneByteStringResource
      : public ExternalStringResourceBase {
   public:
    /**
     * Override the destructor to manage the life cycle of the underlying
     * buffer.
     */
    ~ExternalOneByteStringResource() override = default;
    /** The string data from the underlying buffer.*/
    virtual const char* data() const = 0;
    /** The number of Latin-1 characters in the string.*/
    virtual size_t length() const = 0;
   protected:
    ExternalOneByteStringResource() = default;
  };
 
  /**
   * If the string is an external string, return the ExternalStringResourceBase
   * regardless of the encoding, otherwise return NULL.  The encoding of the
   * string is returned in encoding_out.
   */
  V8_INLINE ExternalStringResourceBase* GetExternalStringResourceBase(
      Encoding* encoding_out) const;
 
  /**
   * Get the ExternalStringResource for an external string.  Returns
   * NULL if IsExternal() doesn't return true.
   */
  V8_INLINE ExternalStringResource* GetExternalStringResource() const;
 
  /**
   * Get the ExternalOneByteStringResource for an external one-byte string.
   * Returns NULL if IsExternalOneByte() doesn't return true.
   */
  const ExternalOneByteStringResource* GetExternalOneByteStringResource() const;
 
  V8_INLINE static String* Cast(v8::Value* obj);
 
  /**
   * Allocates a new string from a UTF-8 literal. This is equivalent to calling
   * String::NewFromUtf(isolate, "...").ToLocalChecked(), but without the check
   * overhead.
   *
   * When called on a string literal containing '\0', the inferred length is the
   * length of the input array minus 1 (for the final '\0') and not the value
   * returned by strlen.
   **/
  template <int N>
  static V8_WARN_UNUSED_RESULT Local<String> NewFromUtf8Literal(
      Isolate* isolate, const char (&literal)[N],
      NewStringType type = NewStringType::kNormal) {
    static_assert(N <= kMaxLength, "String is too long");
    return NewFromUtf8Literal(isolate, literal, type, N - 1);
  }
 
  /** Allocates a new string from UTF-8 data. Only returns an empty value when
   * length > kMaxLength. **/
  static V8_WARN_UNUSED_RESULT MaybeLocal<String> NewFromUtf8(
      Isolate* isolate, const char* data,
      NewStringType type = NewStringType::kNormal, int length = -1);
 
  /** Allocates a new string from Latin-1 data.  Only returns an empty value
   * when length > kMaxLength. **/
  static V8_WARN_UNUSED_RESULT MaybeLocal<String> NewFromOneByte(
      Isolate* isolate, const uint8_t* data,
      NewStringType type = NewStringType::kNormal, int length = -1);
 
  /** Allocates a new string from UTF-16 data. Only returns an empty value when
   * length > kMaxLength. **/
  static V8_WARN_UNUSED_RESULT MaybeLocal<String> NewFromTwoByte(
      Isolate* isolate, const uint16_t* data,
      NewStringType type = NewStringType::kNormal, int length = -1);
 
  /**
   * Creates a new string by concatenating the left and the right strings
   * passed in as parameters.
   */
  static Local<String> Concat(Isolate* isolate, Local<String> left,
                              Local<String> right);
 
  /**
   * Creates a new external string using the data defined in the given
   * resource. When the external string is no longer live on V8's heap the
   * resource will be disposed by calling its Dispose method. The caller of
   * this function should not otherwise delete or modify the resource. Neither
   * should the underlying buffer be deallocated or modified except through the
   * destructor of the external string resource.
   */
  static V8_WARN_UNUSED_RESULT MaybeLocal<String> NewExternalTwoByte(
      Isolate* isolate, ExternalStringResource* resource);
 
  /**
   * Associate an external string resource with this string by transforming it
   * in place so that existing references to this string in the JavaScript heap
   * will use the external string resource. The external string resource's
   * character contents need to be equivalent to this string.
   * Returns true if the string has been changed to be an external string.
   * The string is not modified if the operation fails. See NewExternal for
   * information on the lifetime of the resource.
   */
  bool MakeExternal(ExternalStringResource* resource);
 
  /**
   * Creates a new external string using the one-byte data defined in the given
   * resource. When the external string is no longer live on V8's heap the
   * resource will be disposed by calling its Dispose method. The caller of
   * this function should not otherwise delete or modify the resource. Neither
   * should the underlying buffer be deallocated or modified except through the
   * destructor of the external string resource.
   */
  static V8_WARN_UNUSED_RESULT MaybeLocal<String> NewExternalOneByte(
      Isolate* isolate, ExternalOneByteStringResource* resource);
 
  /**
   * Associate an external string resource with this string by transforming it
   * in place so that existing references to this string in the JavaScript heap
   * will use the external string resource. The external string resource's
   * character contents need to be equivalent to this string.
   * Returns true if the string has been changed to be an external string.
   * The string is not modified if the operation fails. See NewExternal for
   * information on the lifetime of the resource.
   */
  bool MakeExternal(ExternalOneByteStringResource* resource);
 
  /**
   * Returns true if this string can be made external.
   */
  bool CanMakeExternal();
 
  /**
   * Returns true if the strings values are equal. Same as JS ==/===.
   */
  bool StringEquals(Local<String> str);
 
  /**
   * Converts an object to a UTF-8-encoded character array.  Useful if
   * you want to print the object.  If conversion to a string fails
   * (e.g. due to an exception in the toString() method of the object)
   * then the length() method returns 0 and the * operator returns
   * NULL.
   */
  class V8_EXPORT Utf8Value {
   public:
    Utf8Value(Isolate* isolate, Local<v8::Value> obj);
    ~Utf8Value();
    char* operator*() { return str_; }
    const char* operator*() const { return str_; }
    int length() const { return length_; }
 
    // Disallow copying and assigning.
    Utf8Value(const Utf8Value&) = delete;
    void operator=(const Utf8Value&) = delete;
 
   private:
    char* str_;
    int length_;
  };
 
  /**
   * Converts an object to a two-byte (UTF-16-encoded) string.
   * If conversion to a string fails (eg. due to an exception in the toString()
   * method of the object) then the length() method returns 0 and the * operator
   * returns NULL.
   */
  class V8_EXPORT Value {
   public:
    Value(Isolate* isolate, Local<v8::Value> obj);
    ~Value();
    uint16_t* operator*() { return str_; }
    const uint16_t* operator*() const { return str_; }
    int length() const { return length_; }
 
    // Disallow copying and assigning.
    Value(const Value&) = delete;
    void operator=(const Value&) = delete;
 
   private:
    uint16_t* str_;
    int length_;
  };
 
 private:
  void VerifyExternalStringResourceBase(ExternalStringResourceBase* v,
                                        Encoding encoding) const;
  void VerifyExternalStringResource(ExternalStringResource* val) const;
  ExternalStringResource* GetExternalStringResourceSlow() const;
  ExternalStringResourceBase* GetExternalStringResourceBaseSlow(
      String::Encoding* encoding_out) const;
 
  static Local<v8::String> NewFromUtf8Literal(Isolate* isolate,
                                              const char* literal,
                                              NewStringType type, int length);
 
  static void CheckCast(v8::Value* obj);
};
 
// Zero-length string specialization (templated string size includes
// terminator).
template <>
inline V8_WARN_UNUSED_RESULT Local<String> String::NewFromUtf8Literal(
    Isolate* isolate, const char (&literal)[1], NewStringType type) {
  return String::Empty(isolate);
}
 
/**
 * A JavaScript symbol (ECMA-262 edition 6)
 */
class V8_EXPORT Symbol : public Name {
 public:
  /**
   * Returns the description string of the symbol, or undefined if none.
   */
  Local<Value> Description() const;
 
  V8_DEPRECATE_SOON("Use Symbol::Description()")
  Local<Value> Name() const { return Description(); }
 
  /**
   * Create a symbol. If description is not empty, it will be used as the
   * description.
   */
  static Local<Symbol> New(Isolate* isolate,
                           Local<String> description = Local<String>());
 
  /**
   * Access global symbol registry.
   * Note that symbols created this way are never collected, so
   * they should only be used for statically fixed properties.
   * Also, there is only one global name space for the descriptions used as
   * keys.
   * To minimize the potential for clashes, use qualified names as keys.
   */
  static Local<Symbol> For(Isolate* isolate, Local<String> description);
 
  /**
   * Retrieve a global symbol. Similar to |For|, but using a separate
   * registry that is not accessible by (and cannot clash with) JavaScript code.
   */
  static Local<Symbol> ForApi(Isolate* isolate, Local<String> description);
 
  // Well-known symbols
  static Local<Symbol> GetAsyncIterator(Isolate* isolate);
  static Local<Symbol> GetHasInstance(Isolate* isolate);
  static Local<Symbol> GetIsConcatSpreadable(Isolate* isolate);
  static Local<Symbol> GetIterator(Isolate* isolate);
  static Local<Symbol> GetMatch(Isolate* isolate);
  static Local<Symbol> GetReplace(Isolate* isolate);
  static Local<Symbol> GetSearch(Isolate* isolate);
  static Local<Symbol> GetSplit(Isolate* isolate);
  static Local<Symbol> GetToPrimitive(Isolate* isolate);
  static Local<Symbol> GetToStringTag(Isolate* isolate);
  static Local<Symbol> GetUnscopables(Isolate* isolate);
 
  V8_INLINE static Symbol* Cast(Value* obj);
 
 private:
  Symbol();
  static void CheckCast(Value* obj);
};
 
 
/**
 * A private symbol
 *
 * This is an experimental feature. Use at your own risk.
 */
class V8_EXPORT Private : public Data {
 public:
  /**
   * Returns the print name string of the private symbol, or undefined if none.
   */
  Local<Value> Name() const;
 
  /**
   * Create a private symbol. If name is not empty, it will be the description.
   */
  static Local<Private> New(Isolate* isolate,
                            Local<String> name = Local<String>());
 
  /**
   * Retrieve a global private symbol. If a symbol with this name has not
   * been retrieved in the same isolate before, it is created.
   * Note that private symbols created this way are never collected, so
   * they should only be used for statically fixed properties.
   * Also, there is only one global name space for the names used as keys.
   * To minimize the potential for clashes, use qualified names as keys,
   * e.g., "Class#property".
   */
  static Local<Private> ForApi(Isolate* isolate, Local<String> name);
 
  V8_INLINE static Private* Cast(Data* data);
 
 private:
  Private();
 
  static void CheckCast(Data* that);
};
 
 
/**
 * A JavaScript number value (ECMA-262, 4.3.20)
 */
class V8_EXPORT Number : public Primitive {
 public:
  double Value() const;
  static Local<Number> New(Isolate* isolate, double value);
  V8_INLINE static Number* Cast(v8::Value* obj);
 private:
  Number();
  static void CheckCast(v8::Value* obj);
};
 
 
/**
 * A JavaScript value representing a signed integer.
 */
class V8_EXPORT Integer : public Number {
 public:
  static Local<Integer> New(Isolate* isolate, int32_t value);
  static Local<Integer> NewFromUnsigned(Isolate* isolate, uint32_t value);
  int64_t Value() const;
  V8_INLINE static Integer* Cast(v8::Value* obj);
 private:
  Integer();
  static void CheckCast(v8::Value* obj);
};
 
 
/**
 * A JavaScript value representing a 32-bit signed integer.
 */
class V8_EXPORT Int32 : public Integer {
 public:
  int32_t Value() const;
  V8_INLINE static Int32* Cast(v8::Value* obj);
 
 private:
  Int32();
  static void CheckCast(v8::Value* obj);
};
 
 
/**
 * A JavaScript value representing a 32-bit unsigned integer.
 */
class V8_EXPORT Uint32 : public Integer {
 public:
  uint32_t Value() const;
  V8_INLINE static Uint32* Cast(v8::Value* obj);
 
 private:
  Uint32();
  static void CheckCast(v8::Value* obj);
};
 
/**
 * A JavaScript BigInt value (https://tc39.github.io/proposal-bigint)
 */
class V8_EXPORT BigInt : public Primitive {
 public:
  static Local<BigInt> New(Isolate* isolate, int64_t value);
  static Local<BigInt> NewFromUnsigned(Isolate* isolate, uint64_t value);
  /**
   * Creates a new BigInt object using a specified sign bit and a
   * specified list of digits/words.
   * The resulting number is calculated as:
   *
   * (-1)^sign_bit * (words[0] * (2^64)^0 + words[1] * (2^64)^1 + ...)
   */
  static MaybeLocal<BigInt> NewFromWords(Local<Context> context, int sign_bit,
                                         int word_count, const uint64_t* words);
 
  /**
   * Returns the value of this BigInt as an unsigned 64-bit integer.
   * If `lossless` is provided, it will reflect whether the return value was
   * truncated or wrapped around. In particular, it is set to `false` if this
   * BigInt is negative.
   */
  uint64_t Uint64Value(bool* lossless = nullptr) const;
 
  /**
   * Returns the value of this BigInt as a signed 64-bit integer.
   * If `lossless` is provided, it will reflect whether this BigInt was
   * truncated or not.
   */
  int64_t Int64Value(bool* lossless = nullptr) const;
 
  /**
   * Returns the number of 64-bit words needed to store the result of
   * ToWordsArray().
   */
  int WordCount() const;
 
  /**
   * Writes the contents of this BigInt to a specified memory location.
   * `sign_bit` must be provided and will be set to 1 if this BigInt is
   * negative.
   * `*word_count` has to be initialized to the length of the `words` array.
   * Upon return, it will be set to the actual number of words that would
   * be needed to store this BigInt (i.e. the return value of `WordCount()`).
   */
  void ToWordsArray(int* sign_bit, int* word_count, uint64_t* words) const;
 
  V8_INLINE static BigInt* Cast(v8::Value* obj);
 
 private:
  BigInt();
  static void CheckCast(v8::Value* obj);
};
 
/**
 * PropertyAttribute.
 */
enum PropertyAttribute {
  /** None. **/
  None = 0,
  /** ReadOnly, i.e., not writable. **/
  ReadOnly = 1 << 0,
  /** DontEnum, i.e., not enumerable. **/
  DontEnum = 1 << 1,
  /** DontDelete, i.e., not configurable. **/
  DontDelete = 1 << 2
};
 
/**
 * Accessor[Getter|Setter] are used as callback functions when
 * setting|getting a particular property. See Object and ObjectTemplate's
 * method SetAccessor.
 */
typedef void (*AccessorGetterCallback)(
    Local<String> property,
    const PropertyCallbackInfo<Value>& info);
typedef void (*AccessorNameGetterCallback)(
    Local<Name> property,
    const PropertyCallbackInfo<Value>& info);
 
 
typedef void (*AccessorSetterCallback)(
    Local<String> property,
    Local<Value> value,
    const PropertyCallbackInfo<void>& info);
typedef void (*AccessorNameSetterCallback)(
    Local<Name> property,
    Local<Value> value,
    const PropertyCallbackInfo<void>& info);
 
 
/**
 * Access control specifications.
 *
 * Some accessors should be accessible across contexts.  These
 * accessors have an explicit access control parameter which specifies
 * the kind of cross-context access that should be allowed.
 *
 * TODO(dcarney): Remove PROHIBITS_OVERWRITING as it is now unused.
 */
enum AccessControl {
  DEFAULT               = 0,
  ALL_CAN_READ          = 1,
  ALL_CAN_WRITE         = 1 << 1,
  PROHIBITS_OVERWRITING = 1 << 2
};
 
/**
 * Property filter bits. They can be or'ed to build a composite filter.
 */
enum PropertyFilter {
  ALL_PROPERTIES = 0,
  ONLY_WRITABLE = 1,
  ONLY_ENUMERABLE = 2,
  ONLY_CONFIGURABLE = 4,
  SKIP_STRINGS = 8,
  SKIP_SYMBOLS = 16
};
 
/**
 * Options for marking whether callbacks may trigger JS-observable side effects.
 * Side-effect-free callbacks are allowlisted during debug evaluation with
 * throwOnSideEffect. It applies when calling a Function, FunctionTemplate,
 * or an Accessor callback. For Interceptors, please see
 * PropertyHandlerFlags's kHasNoSideEffect.
 * Callbacks that only cause side effects to the receiver are allowlisted if
 * invoked on receiver objects that are created within the same debug-evaluate
 * call, as these objects are temporary and the side effect does not escape.
 */
enum class SideEffectType {
  kHasSideEffect,
  kHasNoSideEffect,
  kHasSideEffectToReceiver
};
 
/**
 * Keys/Properties filter enums:
 *
 * KeyCollectionMode limits the range of collected properties. kOwnOnly limits
 * the collected properties to the given Object only. kIncludesPrototypes will
 * include all keys of the objects's prototype chain as well.
 */
enum class KeyCollectionMode { kOwnOnly, kIncludePrototypes };
 
/**
 * kIncludesIndices allows for integer indices to be collected, while
 * kSkipIndices will exclude integer indices from being collected.
 */
enum class IndexFilter { kIncludeIndices, kSkipIndices };
 
/**
 * kConvertToString will convert integer indices to strings.
 * kKeepNumbers will return numbers for integer indices.
 */
enum class KeyConversionMode { kConvertToString, kKeepNumbers, kNoNumbers };
 
/**
 * Integrity level for objects.
 */
enum class IntegrityLevel { kFrozen, kSealed };
 
/**
 * A JavaScript object (ECMA-262, 4.3.3)
 */
class V8_EXPORT Object : public Value {
 public:
  /**
   * Set only return Just(true) or Empty(), so if it should never fail, use
   * result.Check().
   */
  V8_WARN_UNUSED_RESULT Maybe<bool> Set(Local<Context> context,
                                        Local<Value> key, Local<Value> value);
 
  V8_WARN_UNUSED_RESULT Maybe<bool> Set(Local<Context> context, uint32_t index,
                                        Local<Value> value);
 
  // Implements CreateDataProperty (ECMA-262, 7.3.4).
  //
  // Defines a configurable, writable, enumerable property with the given value
  // on the object unless the property already exists and is not configurable
  // or the object is not extensible.
  //
  // Returns true on success.
  V8_WARN_UNUSED_RESULT Maybe<bool> CreateDataProperty(Local<Context> context,
                                                       Local<Name> key,
                                                       Local<Value> value);
  V8_WARN_UNUSED_RESULT Maybe<bool> CreateDataProperty(Local<Context> context,
                                                       uint32_t index,
                                                       Local<Value> value);
 
  // Implements DefineOwnProperty.
  //
  // In general, CreateDataProperty will be faster, however, does not allow
  // for specifying attributes.
  //
  // Returns true on success.
  V8_WARN_UNUSED_RESULT Maybe<bool> DefineOwnProperty(
      Local<Context> context, Local<Name> key, Local<Value> value,
      PropertyAttribute attributes = None);
 
  // Implements Object.DefineProperty(O, P, Attributes), see Ecma-262 19.1.2.4.
  //
  // The defineProperty function is used to add an own property or
  // update the attributes of an existing own property of an object.
  //
  // Both data and accessor descriptors can be used.
  //
  // In general, CreateDataProperty is faster, however, does not allow
  // for specifying attributes or an accessor descriptor.
  //
  // The PropertyDescriptor can change when redefining a property.
  //
  // Returns true on success.
  V8_WARN_UNUSED_RESULT Maybe<bool> DefineProperty(
      Local<Context> context, Local<Name> key,
      PropertyDescriptor& descriptor);  // NOLINT(runtime/references)
 
  V8_WARN_UNUSED_RESULT MaybeLocal<Value> Get(Local<Context> context,
                                              Local<Value> key);
 
  V8_WARN_UNUSED_RESULT MaybeLocal<Value> Get(Local<Context> context,
                                              uint32_t index);
 
  /**
   * Gets the property attributes of a property which can be None or
   * any combination of ReadOnly, DontEnum and DontDelete. Returns
   * None when the property doesn't exist.
   */
  V8_WARN_UNUSED_RESULT Maybe<PropertyAttribute> GetPropertyAttributes(
      Local<Context> context, Local<Value> key);
 
  /**
   * Returns Object.getOwnPropertyDescriptor as per ES2016 section 19.1.2.6.
   */
  V8_WARN_UNUSED_RESULT MaybeLocal<Value> GetOwnPropertyDescriptor(
      Local<Context> context, Local<Name> key);
 
  /**
   * Object::Has() calls the abstract operation HasProperty(O, P) described
   * in ECMA-262, 7.3.10. Has() returns
   * true, if the object has the property, either own or on the prototype chain.
   * Interceptors, i.e., PropertyQueryCallbacks, are called if present.
   *
   * Has() has the same side effects as JavaScript's `variable in object`.
   * For example, calling Has() on a revoked proxy will throw an exception.
   *
   * \note Has() converts the key to a name, which possibly calls back into
   * JavaScript.
   *
   * See also v8::Object::HasOwnProperty() and
   * v8::Object::HasRealNamedProperty().
   */
  V8_WARN_UNUSED_RESULT Maybe<bool> Has(Local<Context> context,
                                        Local<Value> key);
 
  V8_WARN_UNUSED_RESULT Maybe<bool> Delete(Local<Context> context,
                                           Local<Value> key);
 
  V8_WARN_UNUSED_RESULT Maybe<bool> Has(Local<Context> context, uint32_t index);
 
  V8_WARN_UNUSED_RESULT Maybe<bool> Delete(Local<Context> context,
                                           uint32_t index);
 
  /**
   * Note: SideEffectType affects the getter only, not the setter.
   */
  V8_WARN_UNUSED_RESULT Maybe<bool> SetAccessor(
      Local<Context> context, Local<Name> name,
      AccessorNameGetterCallback getter,
      AccessorNameSetterCallback setter = nullptr,
      MaybeLocal<Value> data = MaybeLocal<Value>(),
      AccessControl settings = DEFAULT, PropertyAttribute attribute = None,
      SideEffectType getter_side_effect_type = SideEffectType::kHasSideEffect,
      SideEffectType setter_side_effect_type = SideEffectType::kHasSideEffect);
 
  void SetAccessorProperty(Local<Name> name, Local<Function> getter,
                           Local<Function> setter = Local<Function>(),
                           PropertyAttribute attribute = None,
                           AccessControl settings = DEFAULT);
 
  /**
   * Sets a native data property like Template::SetNativeDataProperty, but
   * this method sets on this object directly.
   */
  V8_WARN_UNUSED_RESULT Maybe<bool> SetNativeDataProperty(
      Local<Context> context, Local<Name> name,
      AccessorNameGetterCallback getter,
      AccessorNameSetterCallback setter = nullptr,
      Local<Value> data = Local<Value>(), PropertyAttribute attributes = None,
      SideEffectType getter_side_effect_type = SideEffectType::kHasSideEffect,
      SideEffectType setter_side_effect_type = SideEffectType::kHasSideEffect);
 
  /**
   * Attempts to create a property with the given name which behaves like a data
   * property, except that the provided getter is invoked (and provided with the
   * data value) to supply its value the first time it is read. After the
   * property is accessed once, it is replaced with an ordinary data property.
   *
   * Analogous to Template::SetLazyDataProperty.
   */
  V8_WARN_UNUSED_RESULT Maybe<bool> SetLazyDataProperty(
      Local<Context> context, Local<Name> name,
      AccessorNameGetterCallback getter, Local<Value> data = Local<Value>(),
      PropertyAttribute attributes = None,
      SideEffectType getter_side_effect_type = SideEffectType::kHasSideEffect,
      SideEffectType setter_side_effect_type = SideEffectType::kHasSideEffect);
 
  /**
   * Functionality for private properties.
   * This is an experimental feature, use at your own risk.
   * Note: Private properties are not inherited. Do not rely on this, since it
   * may change.
   */
  Maybe<bool> HasPrivate(Local<Context> context, Local<Private> key);
  Maybe<bool> SetPrivate(Local<Context> context, Local<Private> key,
                         Local<Value> value);
  Maybe<bool> DeletePrivate(Local<Context> context, Local<Private> key);
  MaybeLocal<Value> GetPrivate(Local<Context> context, Local<Private> key);
 
  /**
   * Returns an array containing the names of the enumerable properties
   * of this object, including properties from prototype objects.  The
   * array returned by this method contains the same values as would
   * be enumerated by a for-in statement over this object.
   */
  V8_WARN_UNUSED_RESULT MaybeLocal<Array> GetPropertyNames(
      Local<Context> context);
  V8_WARN_UNUSED_RESULT MaybeLocal<Array> GetPropertyNames(
      Local<Context> context, KeyCollectionMode mode,
      PropertyFilter property_filter, IndexFilter index_filter,
      KeyConversionMode key_conversion = KeyConversionMode::kKeepNumbers);
 
  /**
   * This function has the same functionality as GetPropertyNames but
   * the returned array doesn't contain the names of properties from
   * prototype objects.
   */
  V8_WARN_UNUSED_RESULT MaybeLocal<Array> GetOwnPropertyNames(
      Local<Context> context);
 
  /**
   * Returns an array containing the names of the filtered properties
   * of this object, including properties from prototype objects.  The
   * array returned by this method contains the same values as would
   * be enumerated by a for-in statement over this object.
   */
  V8_WARN_UNUSED_RESULT MaybeLocal<Array> GetOwnPropertyNames(
      Local<Context> context, PropertyFilter filter,
      KeyConversionMode key_conversion = KeyConversionMode::kKeepNumbers);
 
  /**
   * Get the prototype object.  This does not skip objects marked to
   * be skipped by __proto__ and it does not consult the security
   * handler.
   */
  Local<Value> GetPrototype();
 
  /**
   * Set the prototype object.  This does not skip objects marked to
   * be skipped by __proto__ and it does not consult the security
   * handler.
   */
  V8_WARN_UNUSED_RESULT Maybe<bool> SetPrototype(Local<Context> context,
                                                 Local<Value> prototype);
 
  /**
   * Finds an instance of the given function template in the prototype
   * chain.
   */
  Local<Object> FindInstanceInPrototypeChain(Local<FunctionTemplate> tmpl);
 
  /**
   * Call builtin Object.prototype.toString on this object.
   * This is different from Value::ToString() that may call
   * user-defined toString function. This one does not.
   */
  V8_WARN_UNUSED_RESULT MaybeLocal<String> ObjectProtoToString(
      Local<Context> context);
 
  /**
   * Returns the name of the function invoked as a constructor for this object.
   */
  Local<String> GetConstructorName();
 
  /**
   * Sets the integrity level of the object.
   */
  Maybe<bool> SetIntegrityLevel(Local<Context> context, IntegrityLevel level);
 
  /** Gets the number of internal fields for this Object. */
  int InternalFieldCount();
 
  /** Same as above, but works for PersistentBase. */
  V8_INLINE static int InternalFieldCount(
      const PersistentBase<Object>& object) {
    return object.val_->InternalFieldCount();
  }
 
  /** Same as above, but works for TracedReferenceBase. */
  V8_INLINE static int InternalFieldCount(
      const TracedReferenceBase<Object>& object) {
    return object.val_->InternalFieldCount();
  }
 
  /** Gets the value from an internal field. */
  V8_INLINE Local<Value> GetInternalField(int index);
 
  /** Sets the value in an internal field. */
  void SetInternalField(int index, Local<Value> value);
 
  /**
   * Gets a 2-byte-aligned native pointer from an internal field. This field
   * must have been set by SetAlignedPointerInInternalField, everything else
   * leads to undefined behavior.
   */
  V8_INLINE void* GetAlignedPointerFromInternalField(int index);
 
  /** Same as above, but works for PersistentBase. */
  V8_INLINE static void* GetAlignedPointerFromInternalField(
      const PersistentBase<Object>& object, int index) {
    return object.val_->GetAlignedPointerFromInternalField(index);
  }
 
  /** Same as above, but works for TracedGlobal. */
  V8_INLINE static void* GetAlignedPointerFromInternalField(
      const TracedReferenceBase<Object>& object, int index) {
    return object.val_->GetAlignedPointerFromInternalField(index);
  }
 
  /**
   * Sets a 2-byte-aligned native pointer in an internal field. To retrieve such
   * a field, GetAlignedPointerFromInternalField must be used, everything else
   * leads to undefined behavior.
   */
  void SetAlignedPointerInInternalField(int index, void* value);
  void SetAlignedPointerInInternalFields(int argc, int indices[],
                                         void* values[]);
 
  /**
   * HasOwnProperty() is like JavaScript's Object.prototype.hasOwnProperty().
   *
   * See also v8::Object::Has() and v8::Object::HasRealNamedProperty().
   */
  V8_WARN_UNUSED_RESULT Maybe<bool> HasOwnProperty(Local<Context> context,
                                                   Local<Name> key);
  V8_WARN_UNUSED_RESULT Maybe<bool> HasOwnProperty(Local<Context> context,
                                                   uint32_t index);
  /**
   * Use HasRealNamedProperty() if you want to check if an object has an own
   * property without causing side effects, i.e., without calling interceptors.
   *
   * This function is similar to v8::Object::HasOwnProperty(), but it does not
   * call interceptors.
   *
   * \note Consider using non-masking interceptors, i.e., the interceptors are
   * not called if the receiver has the real named property. See
   * `v8::PropertyHandlerFlags::kNonMasking`.
   *
   * See also v8::Object::Has().
   */
  V8_WARN_UNUSED_RESULT Maybe<bool> HasRealNamedProperty(Local<Context> context,
                                                         Local<Name> key);
  V8_WARN_UNUSED_RESULT Maybe<bool> HasRealIndexedProperty(
      Local<Context> context, uint32_t index);
  V8_WARN_UNUSED_RESULT Maybe<bool> HasRealNamedCallbackProperty(
      Local<Context> context, Local<Name> key);
 
  /**
   * If result.IsEmpty() no real property was located in the prototype chain.
   * This means interceptors in the prototype chain are not called.
   */
  V8_WARN_UNUSED_RESULT MaybeLocal<Value> GetRealNamedPropertyInPrototypeChain(
      Local<Context> context, Local<Name> key);
 
  /**
   * Gets the property attributes of a real property in the prototype chain,
   * which can be None or any combination of ReadOnly, DontEnum and DontDelete.
   * Interceptors in the prototype chain are not called.
   */
  V8_WARN_UNUSED_RESULT Maybe<PropertyAttribute>
  GetRealNamedPropertyAttributesInPrototypeChain(Local<Context> context,
                                                 Local<Name> key);
 
  /**
   * If result.IsEmpty() no real property was located on the object or
   * in the prototype chain.
   * This means interceptors in the prototype chain are not called.
   */
  V8_WARN_UNUSED_RESULT MaybeLocal<Value> GetRealNamedProperty(
      Local<Context> context, Local<Name> key);
 
  /**
   * Gets the property attributes of a real property which can be
   * None or any combination of ReadOnly, DontEnum and DontDelete.
   * Interceptors in the prototype chain are not called.
   */
  V8_WARN_UNUSED_RESULT Maybe<PropertyAttribute> GetRealNamedPropertyAttributes(
      Local<Context> context, Local<Name> key);
 
  /** Tests for a named lookup interceptor.*/
  bool HasNamedLookupInterceptor();
 
  /** Tests for an index lookup interceptor.*/
  bool HasIndexedLookupInterceptor();
 
  /**
   * Returns the identity hash for this object. The current implementation
   * uses a hidden property on the object to store the identity hash.
   *
   * The return value will never be 0. Also, it is not guaranteed to be
   * unique.
   */
  int GetIdentityHash();
 
  /**
   * Clone this object with a fast but shallow copy.  Values will point
   * to the same values as the original object.
   */
  // TODO(dcarney): take an isolate and optionally bail out?
  Local<Object> Clone();
 
  /**
   * Returns the context in which the object was created.
   */
  Local<Context> CreationContext();
 
  /** Same as above, but works for Persistents */
  V8_INLINE static Local<Context> CreationContext(
      const PersistentBase<Object>& object) {
    return object.val_->CreationContext();
  }
 
  /**
   * Checks whether a callback is set by the
   * ObjectTemplate::SetCallAsFunctionHandler method.
   * When an Object is callable this method returns true.
   */
  bool IsCallable();
 
  /**
   * True if this object is a constructor.
   */
  bool IsConstructor();
 
  /**
   * True if this object can carry information relevant to the embedder in its
   * embedder fields, false otherwise. This is generally true for objects
   * constructed through function templates but also holds for other types where
   * V8 automatically adds internal fields at compile time, such as e.g.
   * v8::ArrayBuffer.
   */
  bool IsApiWrapper();
 
  /**
   * True if this object was created from an object template which was marked
   * as undetectable. See v8::ObjectTemplate::MarkAsUndetectable for more
   * information.
   */
  bool IsUndetectable();
 
  /**
   * Call an Object as a function if a callback is set by the
   * ObjectTemplate::SetCallAsFunctionHandler method.
   */
  V8_WARN_UNUSED_RESULT MaybeLocal<Value> CallAsFunction(Local<Context> context,
                                                         Local<Value> recv,
                                                         int argc,
                                                         Local<Value> argv[]);
 
  /**
   * Call an Object as a constructor if a callback is set by the
   * ObjectTemplate::SetCallAsFunctionHandler method.
   * Note: This method behaves like the Function::NewInstance method.
   */
  V8_WARN_UNUSED_RESULT MaybeLocal<Value> CallAsConstructor(
      Local<Context> context, int argc, Local<Value> argv[]);
 
  /**
   * Return the isolate to which the Object belongs to.
   */
  Isolate* GetIsolate();
 
  /**
   * If this object is a Set, Map, WeakSet or WeakMap, this returns a
   * representation of the elements of this object as an array.
   * If this object is a SetIterator or MapIterator, this returns all
   * elements of the underlying collection, starting at the iterator's current
   * position.
   * For other types, this will return an empty MaybeLocal<Array> (without
   * scheduling an exception).
   */
  MaybeLocal<Array> PreviewEntries(bool* is_key_value);
 
  static Local<Object> New(Isolate* isolate);
 
  /**
   * Creates a JavaScript object with the given properties, and
   * a the given prototype_or_null (which can be any JavaScript
   * value, and if it's null, the newly created object won't have
   * a prototype at all). This is similar to Object.create().
   * All properties will be created as enumerable, configurable
   * and writable properties.
   */
  static Local<Object> New(Isolate* isolate, Local<Value> prototype_or_null,
                           Local<Name>* names, Local<Value>* values,
                           size_t length);
 
  V8_INLINE static Object* Cast(Value* obj);
 
 private:
  Object();
  static void CheckCast(Value* obj);
  Local<Value> SlowGetInternalField(int index);
  void* SlowGetAlignedPointerFromInternalField(int index);
};
 
 
/**
 * An instance of the built-in array constructor (ECMA-262, 15.4.2).
 */
class V8_EXPORT Array : public Object {
 public:
  uint32_t Length() const;
 
  /**
   * Creates a JavaScript array with the given length. If the length
   * is negative the returned array will have length 0.
   */
  static Local<Array> New(Isolate* isolate, int length = 0);
 
  /**
   * Creates a JavaScript array out of a Local<Value> array in C++
   * with a known length.
   */
  static Local<Array> New(Isolate* isolate, Local<Value>* elements,
                          size_t length);
  V8_INLINE static Array* Cast(Value* obj);
 private:
  Array();
  static void CheckCast(Value* obj);
};
 
 
/**
 * An instance of the built-in Map constructor (ECMA-262, 6th Edition, 23.1.1).
 */
class V8_EXPORT Map : public Object {
 public:
  size_t Size() const;
  void Clear();
  V8_WARN_UNUSED_RESULT MaybeLocal<Value> Get(Local<Context> context,
                                              Local<Value> key);
  V8_WARN_UNUSED_RESULT MaybeLocal<Map> Set(Local<Context> context,
                                            Local<Value> key,
                                            Local<Value> value);
  V8_WARN_UNUSED_RESULT Maybe<bool> Has(Local<Context> context,
                                        Local<Value> key);
  V8_WARN_UNUSED_RESULT Maybe<bool> Delete(Local<Context> context,
                                           Local<Value> key);
 
  /**
   * Returns an array of length Size() * 2, where index N is the Nth key and
   * index N + 1 is the Nth value.
   */
  Local<Array> AsArray() const;
 
  /**
   * Creates a new empty Map.
   */
  static Local<Map> New(Isolate* isolate);
 
  V8_INLINE static Map* Cast(Value* obj);
 
 private:
  Map();
  static void CheckCast(Value* obj);
};
 
 
/**
 * An instance of the built-in Set constructor (ECMA-262, 6th Edition, 23.2.1).
 */
class V8_EXPORT Set : public Object {
 public:
  size_t Size() const;
  void Clear();
  V8_WARN_UNUSED_RESULT MaybeLocal<Set> Add(Local<Context> context,
                                            Local<Value> key);
  V8_WARN_UNUSED_RESULT Maybe<bool> Has(Local<Context> context,
                                        Local<Value> key);
  V8_WARN_UNUSED_RESULT Maybe<bool> Delete(Local<Context> context,
                                           Local<Value> key);
 
  /**
   * Returns an array of the keys in this Set.
   */
  Local<Array> AsArray() const;
 
  /**
   * Creates a new empty Set.
   */
  static Local<Set> New(Isolate* isolate);
 
  V8_INLINE static Set* Cast(Value* obj);
 
 private:
  Set();
  static void CheckCast(Value* obj);
};
 
 
template<typename T>
class ReturnValue {
 public:
  template <class S> V8_INLINE ReturnValue(const ReturnValue<S>& that)
      : value_(that.value_) {
    static_assert(std::is_base_of<T, S>::value, "type check");
  }
  // Local setters
  template <typename S>
  V8_INLINE void Set(const Global<S>& handle);
  template <typename S>
  V8_INLINE void Set(const TracedReferenceBase<S>& handle);
  template <typename S>
  V8_INLINE void Set(const Local<S> handle);
  // Fast primitive setters
  V8_INLINE void Set(bool value);
  V8_INLINE void Set(double i);
  V8_INLINE void Set(int32_t i);
  V8_INLINE void Set(uint32_t i);
  // Fast JS primitive setters
  V8_INLINE void SetNull();
  V8_INLINE void SetUndefined();
  V8_INLINE void SetEmptyString();
  // Convenience getter for Isolate
  V8_INLINE Isolate* GetIsolate() const;
 
  // Pointer setter: Uncompilable to prevent inadvertent misuse.
  template <typename S>
  V8_INLINE void Set(S* whatever);
 
  // Getter. Creates a new Local<> so it comes with a certain performance
  // hit. If the ReturnValue was not yet set, this will return the undefined
  // value.
  V8_INLINE Local<Value> Get() const;
 
 private:
  template<class F> friend class ReturnValue;
  template<class F> friend class FunctionCallbackInfo;
  template<class F> friend class PropertyCallbackInfo;
  template <class F, class G, class H>
  friend class PersistentValueMapBase;
  V8_INLINE void SetInternal(internal::Address value) { *value_ = value; }
  V8_INLINE internal::Address GetDefaultValue();
  V8_INLINE explicit ReturnValue(internal::Address* slot);
  internal::Address* value_;
};
 
 
/**
 * The argument information given to function call callbacks.  This
 * class provides access to information about the context of the call,
 * including the receiver, the number and values of arguments, and
 * the holder of the function.
 */
template<typename T>
class FunctionCallbackInfo {
 public:
  /** The number of available arguments. */
  V8_INLINE int Length() const;
  /**
   * Accessor for the available arguments. Returns `undefined` if the index
   * is out of bounds.
   */
  V8_INLINE Local<Value> operator[](int i) const;
  /** Returns the receiver. This corresponds to the "this" value. */
  V8_INLINE Local<Object> This() const;
  /**
   * If the callback was created without a Signature, this is the same
   * value as This(). If there is a signature, and the signature didn't match
   * This() but one of its hidden prototypes, this will be the respective
   * hidden prototype.
   *
   * Note that this is not the prototype of This() on which the accessor
   * referencing this callback was found (which in V8 internally is often
   * referred to as holder [sic]).
   */
  V8_INLINE Local<Object> Holder() const;
  /** For construct calls, this returns the "new.target" value. */
  V8_INLINE Local<Value> NewTarget() const;
  /** Indicates whether this is a regular call or a construct call. */
  V8_INLINE bool IsConstructCall() const;
  /** The data argument specified when creating the callback. */
  V8_INLINE Local<Value> Data() const;
  /** The current Isolate. */
  V8_INLINE Isolate* GetIsolate() const;
  /** The ReturnValue for the call. */
  V8_INLINE ReturnValue<T> GetReturnValue() const;
  // This shouldn't be public, but the arm compiler needs it.
  static const int kArgsLength = 6;
 
 protected:
  friend class internal::FunctionCallbackArguments;
  friend class internal::CustomArguments<FunctionCallbackInfo>;
  friend class debug::ConsoleCallArguments;
  static const int kHolderIndex = 0;
  static const int kIsolateIndex = 1;
  static const int kReturnValueDefaultValueIndex = 2;
  static const int kReturnValueIndex = 3;
  static const int kDataIndex = 4;
  static const int kNewTargetIndex = 5;
 
  V8_INLINE FunctionCallbackInfo(internal::Address* implicit_args,
                                 internal::Address* values, int length);
  internal::Address* implicit_args_;
  internal::Address* values_;
  int length_;
};
 
 
/**
 * The information passed to a property callback about the context
 * of the property access.
 */
template<typename T>
class PropertyCallbackInfo {
 public:
  /**
   * \return The isolate of the property access.
   */
  V8_INLINE Isolate* GetIsolate() const;
 
  /**
   * \return The data set in the configuration, i.e., in
   * `NamedPropertyHandlerConfiguration` or
   * `IndexedPropertyHandlerConfiguration.`
   */
  V8_INLINE Local<Value> Data() const;
 
  /**
   * \return The receiver. In many cases, this is the object on which the
   * property access was intercepted. When using
   * `Reflect.get`, `Function.prototype.call`, or similar functions, it is the
   * object passed in as receiver or thisArg.
   *
   * \code
   *  void GetterCallback(Local<Name> name,
   *                      const v8::PropertyCallbackInfo<v8::Value>& info) {
   *     auto context = info.GetIsolate()->GetCurrentContext();
   *
   *     v8::Local<v8::Value> a_this =
   *         info.This()
   *             ->GetRealNamedProperty(context, v8_str("a"))
   *             .ToLocalChecked();
   *     v8::Local<v8::Value> a_holder =
   *         info.Holder()
   *             ->GetRealNamedProperty(context, v8_str("a"))
   *             .ToLocalChecked();
   *
   *    CHECK(v8_str("r")->Equals(context, a_this).FromJust());
   *    CHECK(v8_str("obj")->Equals(context, a_holder).FromJust());
   *
   *    info.GetReturnValue().Set(name);
   *  }
   *
   *  v8::Local<v8::FunctionTemplate> templ =
   *  v8::FunctionTemplate::New(isolate);
   *  templ->InstanceTemplate()->SetHandler(
   *      v8::NamedPropertyHandlerConfiguration(GetterCallback));
   *  LocalContext env;
   *  env->Global()
   *      ->Set(env.local(), v8_str("obj"), templ->GetFunction(env.local())
   *                                           .ToLocalChecked()
   *                                           ->NewInstance(env.local())
   *                                           .ToLocalChecked())
   *      .FromJust();
   *
   *  CompileRun("obj.a = 'obj'; var r = {a: 'r'}; Reflect.get(obj, 'x', r)");
   * \endcode
   */
  V8_INLINE Local<Object> This() const;
 
  /**
   * \return The object in the prototype chain of the receiver that has the
   * interceptor. Suppose you have `x` and its prototype is `y`, and `y`
   * has an interceptor. Then `info.This()` is `x` and `info.Holder()` is `y`.
   * The Holder() could be a hidden object (the global object, rather
   * than the global proxy).
   *
   * \note For security reasons, do not pass the object back into the runtime.
   */
  V8_INLINE Local<Object> Holder() const;
 
  /**
   * \return The return value of the callback.
   * Can be changed by calling Set().
   * \code
   * info.GetReturnValue().Set(...)
   * \endcode
   *
   */
  V8_INLINE ReturnValue<T> GetReturnValue() const;
 
  /**
   * \return True if the intercepted function should throw if an error occurs.
   * Usually, `true` corresponds to `'use strict'`.
   *
   * \note Always `false` when intercepting `Reflect.set()`
   * independent of the language mode.
   */
  V8_INLINE bool ShouldThrowOnError() const;
 
  // This shouldn't be public, but the arm compiler needs it.
  static const int kArgsLength = 7;
 
 protected:
  friend class MacroAssembler;
  friend class internal::PropertyCallbackArguments;
  friend class internal::CustomArguments<PropertyCallbackInfo>;
  static const int kShouldThrowOnErrorIndex = 0;
  static const int kHolderIndex = 1;
  static const int kIsolateIndex = 2;
  static const int kReturnValueDefaultValueIndex = 3;
  static const int kReturnValueIndex = 4;
  static const int kDataIndex = 5;
  static const int kThisIndex = 6;
 
  V8_INLINE PropertyCallbackInfo(internal::Address* args) : args_(args) {}
  internal::Address* args_;
};
 
 
typedef void (*FunctionCallback)(const FunctionCallbackInfo<Value>& info);
 
enum class ConstructorBehavior { kThrow, kAllow };
 
/**
 * A JavaScript function object (ECMA-262, 15.3).
 */
class V8_EXPORT Function : public Object {
 public:
  /**
   * Create a function in the current execution context
   * for a given FunctionCallback.
   */
  static MaybeLocal<Function> New(
      Local<Context> context, FunctionCallback callback,
      Local<Value> data = Local<Value>(), int length = 0,
      ConstructorBehavior behavior = ConstructorBehavior::kAllow,
      SideEffectType side_effect_type = SideEffectType::kHasSideEffect);
 
  V8_WARN_UNUSED_RESULT MaybeLocal<Object> NewInstance(
      Local<Context> context, int argc, Local<Value> argv[]) const;
 
  V8_WARN_UNUSED_RESULT MaybeLocal<Object> NewInstance(
      Local<Context> context) const {
    return NewInstance(context, 0, nullptr);
  }
 
  /**
   * When side effect checks are enabled, passing kHasNoSideEffect allows the
   * constructor to be invoked without throwing. Calls made within the
   * constructor are still checked.
   */
  V8_WARN_UNUSED_RESULT MaybeLocal<Object> NewInstanceWithSideEffectType(
      Local<Context> context, int argc, Local<Value> argv[],
      SideEffectType side_effect_type = SideEffectType::kHasSideEffect) const;
 
  V8_WARN_UNUSED_RESULT MaybeLocal<Value> Call(Local<Context> context,
                                               Local<Value> recv, int argc,
                                               Local<Value> argv[]);
 
  void SetName(Local<String> name);
  Local<Value> GetName() const;
 
  /**
   * Name inferred from variable or property assignment of this function.
   * Used to facilitate debugging and profiling of JavaScript code written
   * in an OO style, where many functions are anonymous but are assigned
   * to object properties.
   */
  Local<Value> GetInferredName() const;
 
  /**
   * displayName if it is set, otherwise name if it is configured, otherwise
   * function name, otherwise inferred name.
   */
  Local<Value> GetDebugName() const;
 
  /**
   * User-defined name assigned to the "displayName" property of this function.
   * Used to facilitate debugging and profiling of JavaScript code.
   */
  Local<Value> GetDisplayName() const;
 
  /**
   * Returns zero based line number of function body and
   * kLineOffsetNotFound if no information available.
   */
  int GetScriptLineNumber() const;
  /**
   * Returns zero based column number of function body and
   * kLineOffsetNotFound if no information available.
   */
  int GetScriptColumnNumber() const;
 
  /**
   * Returns scriptId.
   */
  int ScriptId() const;
 
  /**
   * Returns the original function if this function is bound, else returns
   * v8::Undefined.
   */
  Local<Value> GetBoundFunction() const;
 
  ScriptOrigin GetScriptOrigin() const;
  V8_INLINE static Function* Cast(Value* obj);
  static const int kLineOffsetNotFound;
 
 private:
  Function();
  static void CheckCast(Value* obj);
};
 
#ifndef V8_PROMISE_INTERNAL_FIELD_COUNT
// The number of required internal fields can be defined by embedder.
#define V8_PROMISE_INTERNAL_FIELD_COUNT 0
#endif
 
/**
 * An instance of the built-in Promise constructor (ES6 draft).
 */
class V8_EXPORT Promise : public Object {
 public:
  /**
   * State of the promise. Each value corresponds to one of the possible values
   * of the [[PromiseState]] field.
   */
  enum PromiseState { kPending, kFulfilled, kRejected };
 
  class V8_EXPORT Resolver : public Object {
   public:
    /**
     * Create a new resolver, along with an associated promise in pending state.
     */
    static V8_WARN_UNUSED_RESULT MaybeLocal<Resolver> New(
        Local<Context> context);
 
    /**
     * Extract the associated promise.
     */
    Local<Promise> GetPromise();
 
    /**
     * Resolve/reject the associated promise with a given value.
     * Ignored if the promise is no longer pending.
     */
    V8_WARN_UNUSED_RESULT Maybe<bool> Resolve(Local<Context> context,
                                              Local<Value> value);
 
    V8_WARN_UNUSED_RESULT Maybe<bool> Reject(Local<Context> context,
                                             Local<Value> value);
 
    V8_INLINE static Resolver* Cast(Value* obj);
 
   private:
    Resolver();
    static void CheckCast(Value* obj);
  };
 
  /**
   * Register a resolution/rejection handler with a promise.
   * The handler is given the respective resolution/rejection value as
   * an argument. If the promise is already resolved/rejected, the handler is
   * invoked at the end of turn.
   */
  V8_WARN_UNUSED_RESULT MaybeLocal<Promise> Catch(Local<Context> context,
                                                  Local<Function> handler);
 
  V8_WARN_UNUSED_RESULT MaybeLocal<Promise> Then(Local<Context> context,
                                                 Local<Function> handler);
 
  V8_WARN_UNUSED_RESULT MaybeLocal<Promise> Then(Local<Context> context,
                                                 Local<Function> on_fulfilled,
                                                 Local<Function> on_rejected);
 
  /**
   * Returns true if the promise has at least one derived promise, and
   * therefore resolve/reject handlers (including default handler).
   */
  bool HasHandler();
 
  /**
   * Returns the content of the [[PromiseResult]] field. The Promise must not
   * be pending.
   */
  Local<Value> Result();
 
  /**
   * Returns the value of the [[PromiseState]] field.
   */
  PromiseState State();
 
  /**
   * Marks this promise as handled to avoid reporting unhandled rejections.
   */
  void MarkAsHandled();
 
  V8_INLINE static Promise* Cast(Value* obj);
 
  static const int kEmbedderFieldCount = V8_PROMISE_INTERNAL_FIELD_COUNT;
 
 private:
  Promise();
  static void CheckCast(Value* obj);
};
 
/**
 * An instance of a Property Descriptor, see Ecma-262 6.2.4.
 *
 * Properties in a descriptor are present or absent. If you do not set
 * `enumerable`, `configurable`, and `writable`, they are absent. If `value`,
 * `get`, or `set` are absent, but you must specify them in the constructor, use
 * empty handles.
 *
 * Accessors `get` and `set` must be callable or undefined if they are present.
 *
 * \note Only query properties if they are present, i.e., call `x()` only if
 * `has_x()` returns true.
 *
 * \code
 * // var desc = {writable: false}
 * v8::PropertyDescriptor d(Local<Value>()), false);
 * d.value(); // error, value not set
 * if (d.has_writable()) {
 *   d.writable(); // false
 * }
 *
 * // var desc = {value: undefined}
 * v8::PropertyDescriptor d(v8::Undefined(isolate));
 *
 * // var desc = {get: undefined}
 * v8::PropertyDescriptor d(v8::Undefined(isolate), Local<Value>()));
 * \endcode
 */
class V8_EXPORT PropertyDescriptor {
 public:
  // GenericDescriptor
  PropertyDescriptor();
 
  // DataDescriptor
  explicit PropertyDescriptor(Local<Value> value);