ECMAScript® 2024 Language Specification

Draft ECMA-262 / February 15, 2024

15.2 Function Definitions

Syntax

FunctionDeclaration[Yield, Await, Default] : function BindingIdentifier[?Yield, ?Await] ( FormalParameters[~Yield, ~Await] ) { FunctionBody[~Yield, ~Await] } [+Default] function ( FormalParameters[~Yield, ~Await] ) { FunctionBody[~Yield, ~Await] } FunctionExpression : function BindingIdentifier[~Yield, ~Await]opt ( FormalParameters[~Yield, ~Await] ) { FunctionBody[~Yield, ~Await] } FunctionBody[Yield, Await] : FunctionStatementList[?Yield, ?Await] FunctionStatementList[Yield, Await] : StatementList[?Yield, ?Await, +Return]opt

15.2.1 Static Semantics: Early Errors

FunctionDeclaration : function BindingIdentifier ( FormalParameters ) { FunctionBody } function ( FormalParameters ) { FunctionBody } FunctionExpression : function BindingIdentifieropt ( FormalParameters ) { FunctionBody } Note

The LexicallyDeclaredNames of a FunctionBody does not include identifiers bound using var or function declarations.

 FunctionBody : FunctionStatementList

15.2.2 Static Semantics: FunctionBodyContainsUseStrict

The syntax-directed operation FunctionBodyContainsUseStrict takes no arguments and returns a Boolean. It is defined piecewise over the following productions:

FunctionBody : FunctionStatementList
  1. If the Directive Prologue of FunctionBody contains a Use Strict Directive, return true; otherwise, return false.

15.2.3 Runtime Semantics: EvaluateFunctionBody

The syntax-directed operation EvaluateFunctionBody takes arguments functionObject (an ECMAScript function object) and argumentsList (a List of ECMAScript language values) and returns either a normal completion containing an ECMAScript language value or an abrupt completion. It is defined piecewise over the following productions:

FunctionBody : FunctionStatementList
  1. Perform ? FunctionDeclarationInstantiation(functionObject, argumentsList).
  2. Return ? Evaluation of FunctionStatementList.

15.2.4 Runtime Semantics: InstantiateOrdinaryFunctionObject

The syntax-directed operation InstantiateOrdinaryFunctionObject takes arguments env (an Environment Record) and privateEnv (a PrivateEnvironment Record or null) and returns an ECMAScript function object. It is defined piecewise over the following productions:

FunctionDeclaration : function BindingIdentifier ( FormalParameters ) { FunctionBody }
  1. Let name be StringValue of BindingIdentifier.
  2. Let sourceText be the source text matched by FunctionDeclaration.
  3. Let F be OrdinaryFunctionCreate(%Function.prototype%, sourceText, FormalParameters, FunctionBody, non-lexical-this, env, privateEnv).
  4. Perform SetFunctionName(F, name).
  5. Perform MakeConstructor(F).
  6. Return F.
 FunctionDeclaration : function ( FormalParameters ) { FunctionBody }
  1. Let sourceText be the source text matched by FunctionDeclaration.
  2. Let F be OrdinaryFunctionCreate(%Function.prototype%, sourceText, FormalParameters, FunctionBody, non-lexical-this, env, privateEnv).
  3. Perform SetFunctionName(F, "default").
  4. Perform MakeConstructor(F).
  5. Return F.
 Note

An anonymous FunctionDeclaration can only occur as part of an export default declaration, and its function code is therefore always strict mode code.

15.2.5 Runtime Semantics: InstantiateOrdinaryFunctionExpression

The syntax-directed operation InstantiateOrdinaryFunctionExpression takes optional argument name (a property key or a Private Name) and returns an ECMAScript function object. It is defined piecewise over the following productions:

FunctionExpression : function ( FormalParameters ) { FunctionBody }
  1. If name is not present, set name to "".
  2. Let env be the LexicalEnvironment of the running execution context.
  3. Let privateEnv be the running execution context's PrivateEnvironment.
  4. Let sourceText be the source text matched by FunctionExpression.
  5. Let closure be OrdinaryFunctionCreate(%Function.prototype%, sourceText, FormalParameters, FunctionBody, non-lexical-this, env, privateEnv).
  6. Perform SetFunctionName(closure, name).
  7. Perform MakeConstructor(closure).
  8. Return closure.
 FunctionExpression : function BindingIdentifier ( FormalParameters ) { FunctionBody }
  1. Assert: name is not present.
  2. Set name to StringValue of BindingIdentifier.
  3. Let outerEnv be the running execution context's LexicalEnvironment.
  4. Let funcEnv be NewDeclarativeEnvironment(outerEnv).
  5. Perform ! funcEnv.CreateImmutableBinding(name, false).
  6. Let privateEnv be the running execution context's PrivateEnvironment.
  7. Let sourceText be the source text matched by FunctionExpression.
  8. Let closure be OrdinaryFunctionCreate(%Function.prototype%, sourceText, FormalParameters, FunctionBody, non-lexical-this, funcEnv, privateEnv).
  9. Perform SetFunctionName(closure, name).
  10. Perform MakeConstructor(closure).
  11. Perform ! funcEnv.InitializeBinding(name, closure).
  12. Return closure.
 Note

The BindingIdentifier in a FunctionExpression can be referenced from inside the FunctionExpression's FunctionBody to allow the function to call itself recursively. However, unlike in a FunctionDeclaration, the BindingIdentifier in a FunctionExpression cannot be referenced from and does not affect the scope enclosing the FunctionExpression.

15.2.6 Runtime Semantics: Evaluation

FunctionDeclaration : function BindingIdentifier ( FormalParameters ) { FunctionBody }
  1. Return empty.
 Note 1

An alternative semantics is provided in B.3.2.

 FunctionDeclaration : function ( FormalParameters ) { FunctionBody }
  1. Return empty.
 FunctionExpression : function BindingIdentifieropt ( FormalParameters ) { FunctionBody }
  1. Return InstantiateOrdinaryFunctionExpression of FunctionExpression.
 Note 2

A "prototype" property is automatically created for every function defined using a FunctionDeclaration or FunctionExpression, to allow for the possibility that the function will be used as a constructor.

 FunctionStatementList : [empty]
  1. Return undefined.