ECMAScript® 2021 Language Specification

Draft ECMA-262 / February 25, 2021

10.3 Built-in Function Objects

The built-in function objects defined in this specification may be implemented as either ECMAScript function objects (10.2) whose behaviour is provided using ECMAScript code or as implementation provided function exotic objects whose behaviour is provided in some other manner. In either case, the effect of calling such functions must conform to their specifications. An implementation may also provide additional built-in function objects that are not defined in this specification.

If a built-in function object is implemented as an exotic object it must have the ordinary object behaviour specified in 10.1. All such function exotic objects also have [[Prototype]], [[Extensible]], and [[Realm]] internal slots.

Unless otherwise specified every built-in function object has the %Function.prototype% object as the initial value of its [[Prototype]] internal slot.

The behaviour specified for each built-in function via algorithm steps or other means is the specification of the function body behaviour for both [[Call]] and [[Construct]] invocations of the function. However, [[Construct]] invocation is not supported by all built-in functions. For each built-in function, when invoked with [[Call]], the [[Call]] thisArgument provides the this value, the [[Call]] argumentsList provides the named parameters, and the NewTarget value is undefined. When invoked with [[Construct]], the this value is uninitialized, the [[Construct]] argumentsList provides the named parameters, and the [[Construct]] newTarget parameter provides the NewTarget value. If the built-in function is implemented as an ECMAScript function object then this specified behaviour must be implemented by the ECMAScript code that is the body of the function. Built-in functions that are ECMAScript function objects must be strict functions. If a built-in constructor has any [[Call]] behaviour other than throwing a TypeError exception, an ECMAScript implementation of the function must be done in a manner that does not cause the function's [[IsClassConstructor]] internal slot to have the value true.

Built-in function objects that are not identified as constructors do not implement the [[Construct]] internal method unless otherwise specified in the description of a particular function. When a built-in constructor is called as part of a new expression the argumentsList parameter of the invoked [[Construct]] internal method provides the values for the built-in constructor's named parameters.

Built-in functions that are not constructors do not have a "prototype" property unless otherwise specified in the description of a particular function.

Built-in functions have an [[InitialName]] internal slot.

If a built-in function object is not implemented as an ECMAScript function it must provide [[Call]] and [[Construct]] internal methods that conform to the following definitions:

10.3.1 [[Call]] ( thisArgument, argumentsList )

The [[Call]] internal method of a built-in function object F takes arguments thisArgument (an ECMAScript language value) and argumentsList (a List of ECMAScript language values). It performs the following steps when called:

  1. Let callerContext be the running execution context.
  2. If callerContext is not already suspended, suspend callerContext.
  3. Let calleeContext be a new execution context.
  4. Set the Function of calleeContext to F.
  5. Let calleeRealm be F.[[Realm]].
  6. Set the Realm of calleeContext to calleeRealm.
  7. Set the ScriptOrModule of calleeContext to null.
  8. Perform any necessary implementation-defined initialization of calleeContext.
  9. Push calleeContext onto the execution context stack; calleeContext is now the running execution context.
  10. Let result be the Completion Record that is the result of evaluating F in a manner that conforms to the specification of F. thisArgument is the this value, argumentsList provides the named parameters, and the NewTarget value is undefined.
  11. Remove calleeContext from the execution context stack and restore callerContext as the running execution context.
  12. Return result.
 Note

When calleeContext is removed from the execution context stack it must not be destroyed if it has been suspended and retained by an accessible generator object for later resumption.

10.3.2 [[Construct]] ( argumentsList, newTarget )

The [[Construct]] internal method of a built-in function object F takes arguments argumentsList (a List of ECMAScript language values) and newTarget (a constructor). The steps performed are the same as [[Call]] (see 10.3.1) except that step 10 is replaced by:

  1. Let result be the Completion Record that is the result of evaluating F in a manner that conforms to the specification of F. The this value is uninitialized, argumentsList provides the named parameters, and newTarget provides the NewTarget value.

10.3.3 CreateBuiltinFunction ( steps, length, name, internalSlotsList [ , realm [ , prototype [ , prefix ] ] ] )

The abstract operation CreateBuiltinFunction takes arguments steps, length, name, and internalSlotsList (a List of names of internal slots) and optional arguments realm, prototype, and prefix. internalSlotsList contains the names of additional internal slots that must be defined as part of the object. This operation creates a built-in function object. It performs the following steps when called:

  1. Assert: steps is either a set of algorithm steps or other definition of a function's behaviour provided in this specification.
  2. If realm is not present, set realm to the current Realm Record.
  3. Assert: realm is a Realm Record.
  4. If prototype is not present, set prototype to realm.[[Intrinsics]].[[%Function.prototype%]].
  5. Let func be a new built-in function object that when called performs the action described by steps. The new function object has internal slots whose names are the elements of internalSlotsList, and an [[InitialName]] internal slot.
  6. Set func.[[Realm]] to realm.
  7. Set func.[[Prototype]] to prototype.
  8. Set func.[[Extensible]] to true.
  9. Set func.[[InitialName]] to null.
  10. Perform ! SetFunctionLength(func, length).
  11. If prefix is not present, then
    1. Perform ! SetFunctionName(func, name).
  12. Else,
    1. Perform ! SetFunctionName(func, name, prefix).
  13. Return func.

Each built-in function defined in this specification is created by calling the CreateBuiltinFunction abstract operation.