PR #2889

This document is a preview of the specification for PR #2889 commit 8025b209c9a44cd40099f3c3aefe00d2e41873b2, and should only be used as a historical reference. This commit may not have even been merged into the specification.

Do not attempt to implement this version of the specification. Do not reference this version as authoritative in any way. Instead, see https://github.com/tc39/ecma262 for the living specification.

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28 Reflection

28.1 The Reflect Object

The Reflect object:

  • is %Reflect%.
  • is the initial value of the "Reflect" property of the global object.
  • is an ordinary object.
  • has a [[Prototype]] internal slot whose value is %Object.prototype%.
  • is not a function object.
  • does not have a [[Construct]] internal method; it cannot be used as a constructor with the new operator.
  • does not have a [[Call]] internal method; it cannot be invoked as a function.

28.1.1 Reflect.apply ( target, thisArg, args )

This function performs the following steps when called:

  1. If IsFunction(target) is false, throw a TypeError exception.
  2. Let argList be ? CreateListFromArrayLike(args).
  3. Perform PrepareForTailCall().
  4. Return ? Call(target, thisArg, argList).

28.1.2 Reflect.construct ( target, args [ , newTarget ] )

This function performs the following steps when called:

  1. If IsConstructor(target) is false, throw a TypeError exception.
  2. If newTarget is not present, set newTarget to target.
  3. Else if IsConstructor(newTarget) is false, throw a TypeError exception.
  4. Let argList be ? CreateListFromArrayLike(args).
  5. Return ? Construct(target, argList, newTarget).

28.1.3 Reflect.defineProperty ( target, key, attrs )

This function performs the following steps when called:

  1. If target is not an Object, throw a TypeError exception.
  2. Let propertyKey be ? ToPropertyKey(key).
  3. Let propertyDesc be ? ToPropertyDescriptor(attrs).
  4. Return ? target.[[DefineOwnProperty]](propertyKey, propertyDesc).

28.1.4 Reflect.deleteProperty ( target, key )

This function performs the following steps when called:

  1. If target is not an Object, throw a TypeError exception.
  2. Let propertyKey be ? ToPropertyKey(key).
  3. Return ? target.[[Delete]](propertyKey).

28.1.5 Reflect.get ( target, key [ , receiver ] )

This function performs the following steps when called:

  1. If target is not an Object, throw a TypeError exception.
  2. Let propertyKey be ? ToPropertyKey(key).
  3. If receiver is not present, then
    1. Set receiver to target.
  4. Return ? target.[[Get]](propertyKey, receiver).

28.1.6 Reflect.getOwnPropertyDescriptor ( target, key )

This function performs the following steps when called:

  1. If target is not an Object, throw a TypeError exception.
  2. Let propertyKey be ? ToPropertyKey(key).
  3. Let propertyDesc be ? target.[[GetOwnProperty]](propertyKey).
  4. Return FromPropertyDescriptor(propertyDesc).

28.1.7 Reflect.getPrototypeOf ( target )

This function performs the following steps when called:

  1. If target is not an Object, throw a TypeError exception.
  2. Return ? target.[[GetPrototypeOf]]().

28.1.8 Reflect.has ( target, key )

This function performs the following steps when called:

  1. If target is not an Object, throw a TypeError exception.
  2. Let propertyKey be ? ToPropertyKey(key).
  3. Return ? target.[[HasProperty]](propertyKey).

28.1.9 Reflect.isExtensible ( target )

This function performs the following steps when called:

  1. If target is not an Object, throw a TypeError exception.
  2. Return ? target.[[IsExtensible]]().

28.1.10 Reflect.ownKeys ( target )

This function performs the following steps when called:

  1. If target is not an Object, throw a TypeError exception.
  2. Let keys be ? target.[[OwnPropertyKeys]]().
  3. Return CreateArrayFromList(keys).

28.1.11 Reflect.preventExtensions ( target )

This function performs the following steps when called:

  1. If target is not an Object, throw a TypeError exception.
  2. Return ? target.[[PreventExtensions]]().

28.1.12 Reflect.set ( target, key, value [ , receiver ] )

This function performs the following steps when called:

  1. If target is not an Object, throw a TypeError exception.
  2. Let propertyKey be ? ToPropertyKey(key).
  3. If receiver is not present, then
    1. Set receiver to target.
  4. Return ? target.[[Set]](propertyKey, value, receiver).

28.1.13 Reflect.setPrototypeOf ( target, proto )

This function performs the following steps when called:

  1. If target is not an Object, throw a TypeError exception.
  2. If proto is not an Object and proto is not null, throw a TypeError exception.
  3. Return ? target.[[SetPrototypeOf]](proto).

28.1.14 Reflect [ %Symbol.toStringTag% ]

The initial value of the %Symbol.toStringTag% property is the String value "Reflect".

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.

28.2 Proxy Objects

28.2.1 The Proxy Constructor

The Proxy constructor:

  • is %Proxy%.
  • is the initial value of the "Proxy" property of the global object.
  • creates and initializes a new Proxy object when called as a constructor.
  • is not intended to be called as a function and will throw an exception when called in that manner.

28.2.1.1 Proxy ( target, handler )

This function performs the following steps when called:

  1. If NewTarget is undefined, throw a TypeError exception.
  2. Return ? ProxyCreate(target, handler).

28.2.2 Properties of the Proxy Constructor

The Proxy constructor:

  • has a [[Prototype]] internal slot whose value is %Function.prototype%.
  • does not have a "prototype" property because Proxy objects do not have a [[Prototype]] internal slot that requires initialization.
  • has the following properties:

28.2.2.1 Proxy.revocable ( target, handler )

This function creates a revocable Proxy object.

It performs the following steps when called:

  1. Let proxy be ? ProxyCreate(target, handler).
  2. Let revokerClosure be a new Abstract Closure with no parameters that captures nothing and performs the following steps when called:
    1. Let activeFunc be the active function object.
    2. Let revocableProxy be activeFunc.[[RevocableProxy]].
    3. If revocableProxy is null, return NormalCompletion(undefined).
    4. Set activeFunc.[[RevocableProxy]] to null.
    5. Assert: revocableProxy is a Proxy exotic object.
    6. Set revocableProxy.[[ProxyTarget]] to null.
    7. Set revocableProxy.[[ProxyHandler]] to null.
    8. Return NormalCompletion(undefined).
  3. Let revoker be CreateBuiltinFunction(revokerClosure, 0, "", « [[RevocableProxy]] »).
  4. Set revoker.[[RevocableProxy]] to proxy.
  5. Let result be OrdinaryObjectCreate(%Object.prototype%).
  6. Perform ! CreateDataPropertyOrThrow(result, "proxy", proxy).
  7. Perform ! CreateDataPropertyOrThrow(result, "revoke", revoker).
  8. Return result.

28.3 Module Namespace Objects

A Module Namespace Object is a module namespace exotic object that provides runtime property-based access to a module's exported bindings. There is no constructor function for Module Namespace Objects. Instead, such an object is created for each module that is imported by an ImportDeclaration that contains a NameSpaceImport.

In addition to the properties specified in 10.4.6 each Module Namespace Object has the following own property:

28.3.1 %Symbol.toStringTag%

The initial value of the %Symbol.toStringTag% property is the String value "Module".

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.