ECMAScript® 2024 Language Specification

Draft ECMA-262 / February 15, 2024

14.3 Declarations and the Variable Statement

14.3.1 Let and Const Declarations

Note

let and const declarations define variables that are scoped to the running execution context's LexicalEnvironment. The variables are created when their containing Environment Record is instantiated but may not be accessed in any way until the variable's LexicalBinding is evaluated. A variable defined by a LexicalBinding with an Initializer is assigned the value of its Initializer's AssignmentExpression when the LexicalBinding is evaluated, not when the variable is created. If a LexicalBinding in a let declaration does not have an Initializer the variable is assigned the value undefined when the LexicalBinding is evaluated.

Syntax

LexicalDeclaration[In, Yield, Await] : LetOrConst BindingList[?In, ?Yield, ?Await] ; LetOrConst : let const BindingList[In, Yield, Await] : LexicalBinding[?In, ?Yield, ?Await] BindingList[?In, ?Yield, ?Await] , LexicalBinding[?In, ?Yield, ?Await] LexicalBinding[In, Yield, Await] : BindingIdentifier[?Yield, ?Await] Initializer[?In, ?Yield, ?Await]opt BindingPattern[?Yield, ?Await] Initializer[?In, ?Yield, ?Await]

14.3.1.1 Static Semantics: Early Errors

LexicalDeclaration : LetOrConst BindingList ; LexicalBinding : BindingIdentifier Initializeropt

14.3.1.2 Runtime Semantics: Evaluation

LexicalDeclaration : LetOrConst BindingList ;
  1. Perform ? Evaluation of BindingList.
  2. Return empty.
 BindingList : BindingList , LexicalBinding
  1. Perform ? Evaluation of BindingList.
  2. Return ? Evaluation of LexicalBinding.
 LexicalBinding : BindingIdentifier
  1. Let lhs be ! ResolveBinding(StringValue of BindingIdentifier).
  2. Perform ! InitializeReferencedBinding(lhs, undefined).
  3. Return empty.
 Note

A static semantics rule ensures that this form of LexicalBinding never occurs in a const declaration.

 LexicalBinding : BindingIdentifier Initializer
  1. Let bindingId be StringValue of BindingIdentifier.
  2. Let lhs be ! ResolveBinding(bindingId).
  3. If IsAnonymousFunctionDefinition(Initializer) is true, then
    1. Let value be ? NamedEvaluation of Initializer with argument bindingId.
  4. Else,
    1. Let rhs be ? Evaluation of Initializer.
    2. Let value be ? GetValue(rhs).
  5. Perform ! InitializeReferencedBinding(lhs, value).
  6. Return empty.
 LexicalBinding : BindingPattern Initializer
  1. Let rhs be ? Evaluation of Initializer.
  2. Let value be ? GetValue(rhs).
  3. Let env be the running execution context's LexicalEnvironment.
  4. Return ? BindingInitialization of BindingPattern with arguments value and env.

14.3.2 Variable Statement

Note

A var statement declares variables that are scoped to the running execution context's VariableEnvironment. Var variables are created when their containing Environment Record is instantiated and are initialized to undefined when created. Within the scope of any VariableEnvironment a common BindingIdentifier may appear in more than one VariableDeclaration but those declarations collectively define only one variable. A variable defined by a VariableDeclaration with an Initializer is assigned the value of its Initializer's AssignmentExpression when the VariableDeclaration is executed, not when the variable is created.

Syntax

VariableStatement[Yield, Await] : var VariableDeclarationList[+In, ?Yield, ?Await] ; VariableDeclarationList[In, Yield, Await] : VariableDeclaration[?In, ?Yield, ?Await] VariableDeclarationList[?In, ?Yield, ?Await] , VariableDeclaration[?In, ?Yield, ?Await] VariableDeclaration[In, Yield, Await] : BindingIdentifier[?Yield, ?Await] Initializer[?In, ?Yield, ?Await]opt BindingPattern[?Yield, ?Await] Initializer[?In, ?Yield, ?Await]

14.3.2.1 Runtime Semantics: Evaluation

VariableStatement : var VariableDeclarationList ;
  1. Perform ? Evaluation of VariableDeclarationList.
  2. Return empty.
 VariableDeclarationList : VariableDeclarationList , VariableDeclaration
  1. Perform ? Evaluation of VariableDeclarationList.
  2. Return ? Evaluation of VariableDeclaration.
 VariableDeclaration : BindingIdentifier
  1. Return empty.
 VariableDeclaration : BindingIdentifier Initializer
  1. Let bindingId be StringValue of BindingIdentifier.
  2. Let lhs be ? ResolveBinding(bindingId).
  3. If IsAnonymousFunctionDefinition(Initializer) is true, then
    1. Let value be ? NamedEvaluation of Initializer with argument bindingId.
  4. Else,
    1. Let rhs be ? Evaluation of Initializer.
    2. Let value be ? GetValue(rhs).
  5. Perform ? PutValue(lhs, value).
  6. Return empty.
 Note

If a VariableDeclaration is nested within a with statement and the BindingIdentifier in the VariableDeclaration is the same as a property name of the binding object of the with statement's Object Environment Record, then step 5 will assign value to the property instead of assigning to the VariableEnvironment binding of the Identifier.

 VariableDeclaration : BindingPattern Initializer
  1. Let rhs be ? Evaluation of Initializer.
  2. Let rval be ? GetValue(rhs).
  3. Return ? BindingInitialization of BindingPattern with arguments rval and undefined.

14.3.3 Destructuring Binding Patterns

Syntax

BindingPattern[Yield, Await] : ObjectBindingPattern[?Yield, ?Await] ArrayBindingPattern[?Yield, ?Await] ObjectBindingPattern[Yield, Await] : { } { BindingRestProperty[?Yield, ?Await] } { BindingPropertyList[?Yield, ?Await] } { BindingPropertyList[?Yield, ?Await] , BindingRestProperty[?Yield, ?Await]opt } ArrayBindingPattern[Yield, Await] : [ Elisionopt BindingRestElement[?Yield, ?Await]opt ] [ BindingElementList[?Yield, ?Await] ] [ BindingElementList[?Yield, ?Await] , Elisionopt BindingRestElement[?Yield, ?Await]opt ] BindingRestProperty[Yield, Await] : ... BindingIdentifier[?Yield, ?Await] BindingPropertyList[Yield, Await] : BindingProperty[?Yield, ?Await] BindingPropertyList[?Yield, ?Await] , BindingProperty[?Yield, ?Await] BindingElementList[Yield, Await] : BindingElisionElement[?Yield, ?Await] BindingElementList[?Yield, ?Await] , BindingElisionElement[?Yield, ?Await] BindingElisionElement[Yield, Await] : Elisionopt BindingElement[?Yield, ?Await] BindingProperty[Yield, Await] : SingleNameBinding[?Yield, ?Await] PropertyName[?Yield, ?Await] : BindingElement[?Yield, ?Await] BindingElement[Yield, Await] : SingleNameBinding[?Yield, ?Await] BindingPattern[?Yield, ?Await] Initializer[+In, ?Yield, ?Await]opt SingleNameBinding[Yield, Await] : BindingIdentifier[?Yield, ?Await] Initializer[+In, ?Yield, ?Await]opt BindingRestElement[Yield, Await] : ... BindingIdentifier[?Yield, ?Await] ... BindingPattern[?Yield, ?Await]

14.3.3.1 Runtime Semantics: PropertyBindingInitialization

The syntax-directed operation PropertyBindingInitialization takes arguments value (an ECMAScript language value) and environment (an Environment Record or undefined) and returns either a normal completion containing a List of property keys or an abrupt completion. It collects a list of all bound property names. It is defined piecewise over the following productions:

BindingPropertyList : BindingPropertyList , BindingProperty
  1. Let boundNames be ? PropertyBindingInitialization of BindingPropertyList with arguments value and environment.
  2. Let nextNames be ? PropertyBindingInitialization of BindingProperty with arguments value and environment.
  3. Return the list-concatenation of boundNames and nextNames.
 BindingProperty : SingleNameBinding
  1. Let name be the sole element of the BoundNames of SingleNameBinding.
  2. Perform ? KeyedBindingInitialization of SingleNameBinding with arguments value, environment, and name.
  3. Return « name ».
 BindingProperty : PropertyName : BindingElement
  1. Let P be ? Evaluation of PropertyName.
  2. Perform ? KeyedBindingInitialization of BindingElement with arguments value, environment, and P.
  3. Return « P ».

14.3.3.2 Runtime Semantics: RestBindingInitialization

The syntax-directed operation RestBindingInitialization takes arguments value (an ECMAScript language value), environment (an Environment Record or undefined), and excludedNames (a List of property keys) and returns either a normal completion containing unused or an abrupt completion. It is defined piecewise over the following productions:

BindingRestProperty : ... BindingIdentifier
  1. Let lhs be ? ResolveBinding(StringValue of BindingIdentifier, environment).
  2. Let restObj be OrdinaryObjectCreate(%Object.prototype%).
  3. Perform ? CopyDataProperties(restObj, value, excludedNames).
  4. If environment is undefined, return ? PutValue(lhs, restObj).
  5. Return ? InitializeReferencedBinding(lhs, restObj).

14.3.3.3 Runtime Semantics: KeyedBindingInitialization

The syntax-directed operation KeyedBindingInitialization takes arguments value (an ECMAScript language value), environment (an Environment Record or undefined), and propertyName (a property key) and returns either a normal completion containing unused or an abrupt completion.

Note

When undefined is passed for environment it indicates that a PutValue operation should be used to assign the initialization value. This is the case for formal parameter lists of non-strict functions. In that case the formal parameter bindings are preinitialized in order to deal with the possibility of multiple parameters with the same name.

It is defined piecewise over the following productions:

BindingElement : BindingPattern Initializeropt
  1. Let v be ? GetV(value, propertyName).
  2. If Initializer is present and v is undefined, then
    1. Let defaultValue be ? Evaluation of Initializer.
    2. Set v to ? GetValue(defaultValue).
  3. Return ? BindingInitialization of BindingPattern with arguments v and environment.
 SingleNameBinding : BindingIdentifier Initializeropt
  1. Let bindingId be StringValue of BindingIdentifier.
  2. Let lhs be ? ResolveBinding(bindingId, environment).
  3. Let v be ? GetV(value, propertyName).
  4. If Initializer is present and v is undefined, then
    1. If IsAnonymousFunctionDefinition(Initializer) is true, then
      1. Set v to ? NamedEvaluation of Initializer with argument bindingId.
    2. Else,
      1. Let defaultValue be ? Evaluation of Initializer.
      2. Set v to ? GetValue(defaultValue).
  5. If environment is undefined, return ? PutValue(lhs, v).
  6. Return ? InitializeReferencedBinding(lhs, v).