Multipage preference

28 Reflection

28.1 The Reflect Object

The Reflect object:

28.1.1 Reflect.apply ( target, thisArg, args )

This function performs the following steps when called:

  1. If IsCallable(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 }.