ECMAScript® 2024 Language Specification

Draft ECMA-262 / February 15, 2024

27.3 GeneratorFunction Objects

GeneratorFunctions are functions that are usually created by evaluating GeneratorDeclarations, GeneratorExpressions, and GeneratorMethods. They may also be created by calling the %GeneratorFunction% intrinsic.

Figure 6 (Informative): Generator Objects Relationships
A staggering variety of boxes and arrows.

27.3.1 The GeneratorFunction Constructor

The GeneratorFunction constructor:

  • is %GeneratorFunction%.
  • is a subclass of Function.
  • creates and initializes a new GeneratorFunction when called as a function rather than as a constructor. Thus the function call GeneratorFunction (…) is equivalent to the object creation expression new GeneratorFunction (…) with the same arguments.
  • may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified GeneratorFunction behaviour must include a super call to the GeneratorFunction constructor to create and initialize subclass instances with the internal slots necessary for built-in GeneratorFunction behaviour. All ECMAScript syntactic forms for defining generator function objects create direct instances of GeneratorFunction. There is no syntactic means to create instances of GeneratorFunction subclasses.

27.3.1.1 GeneratorFunction ( ...parameterArgs, bodyArg )

The last argument (if any) specifies the body (executable code) of a generator function; any preceding arguments specify formal parameters.

This function performs the following steps when called:

  1. Let C be the active function object.
  2. If bodyArg is not present, set bodyArg to the empty String.
  3. Return ? CreateDynamicFunction(C, NewTarget, generator, parameterArgs, bodyArg).
 Note

See NOTE for 20.2.1.1.

27.3.2 Properties of the GeneratorFunction Constructor

The GeneratorFunction constructor:

  • is a standard built-in function object that inherits from the Function constructor.
  • has a [[Prototype]] internal slot whose value is %Function%.
  • has a "length" property whose value is 1𝔽.
  • has a "name" property whose value is "GeneratorFunction".
  • has the following properties:

27.3.2.1 GeneratorFunction.prototype

The initial value of GeneratorFunction.prototype is the GeneratorFunction prototype object.

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

27.3.3 Properties of the GeneratorFunction Prototype Object

The GeneratorFunction prototype object:

27.3.3.1 GeneratorFunction.prototype.constructor

The initial value of GeneratorFunction.prototype.constructor is %GeneratorFunction%.

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

27.3.3.2 GeneratorFunction.prototype.prototype

The initial value of GeneratorFunction.prototype.prototype is the Generator prototype object.

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

27.3.3.3 GeneratorFunction.prototype [ @@toStringTag ]

The initial value of the @@toStringTag property is the String value "GeneratorFunction".

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

27.3.4 GeneratorFunction Instances

Every GeneratorFunction instance is an ECMAScript function object and has the internal slots listed in Table 30. The value of the [[IsClassConstructor]] internal slot for all such instances is false.

Each GeneratorFunction instance has the following own properties:

27.3.4.1 length

The specification for the "length" property of Function instances given in 20.2.4.1 also applies to GeneratorFunction instances.

27.3.4.2 name

The specification for the "name" property of Function instances given in 20.2.4.2 also applies to GeneratorFunction instances.

27.3.4.3 prototype

Whenever a GeneratorFunction instance is created another ordinary object is also created and is the initial value of the generator function's "prototype" property. The value of the prototype property is used to initialize the [[Prototype]] internal slot of a newly created Generator when the generator function object is invoked using [[Call]].

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

Note

Unlike Function instances, the object that is the value of a GeneratorFunction's "prototype" property does not have a "constructor" property whose value is the GeneratorFunction instance.