Stage 1 Draft / October 13, 2025

Declarations in Conditionals

Introduction

This is the formal specification for a proposed extension of the if and while JavaScript control flow statements. It modifies the original ECMAScript specification with several new or revised clauses. See the proposal's explainer for the proposal's background, motivation, and usage examples.

14 ECMAScript Language: Statements and Declarations

14.6 The if Statement

Syntax

IfStatement[Yield, Await, Return] : if ( Expression[+In, ?Yield, ?Await] IfCondition[+In, ?Yield, ?Await] ) Statement[?Yield, ?Await, ?Return] else Statement[?Yield, ?Await, ?Return] if ( Expression[+In, ?Yield, ?Await] IfCondition[+In, ?Yield, ?Await] ) Statement[?Yield, ?Await, ?Return] [lookahead ≠ else] IfCondition[In, Yield, Await] : Expression[?In, ?Yield, ?Await] LetOrConst BindingIdentifier[?Yield, ?Await] Initializer[?In, ?Yield, ?Await] LexicalDeclaration[?In, ?Yield, ?Await] ; Expression[?In, ?Yield, ?Await] Note
The lookahead-restriction [lookahead ≠ else] resolves the classic "dangling else" problem in the usual way. That is, when the choice of associated if is otherwise ambiguous, the else is associated with the nearest (innermost) of the candidate ifs

14.6.1 Static Semantics: Early Errors

IfStatement : if ( Expression IfCondition ) Statement else Statement IfStatement : if ( Expression IfCondition ) Statement Note

It is only necessary to apply this rule if the extension specified in B.3.1 is implemented.

14.6.2 Runtime Semantics: Evaluation

IfStatement : if ( Expression IfCondition ) Statement else Statement
  1. Let exprRef be ? Evaluation of ExpressionIfCondition.
  2. Let exprValue be ToBoolean(? GetValue(exprRef)).
  3. If exprValue is true, then
    1. Let stmtCompletion be Completion(Evaluation of the first Statement).
  4. Else,
    1. Let stmtCompletion be Completion(Evaluation of the second Statement).
  5. Return ? UpdateEmpty(stmtCompletion, undefined).
 IfStatement : if ( Expression IfCondition ) Statement
  1. Let exprRef be ? Evaluation of ExpressionIfCondition.
  2. Let exprValue be ToBoolean(? GetValue(exprRef)).
  3. If exprValue is false, then
    1. Return undefined.
  4. Else,
    1. Let stmtCompletion be Completion(Evaluation of Statement).
    2. Return ? UpdateEmpty(stmtCompletion, undefined).

14.7 Iteration Statements

14.7.3 The while Statement

Syntax

WhileStatement[Yield, Await, Return] : while ( Expression[+In, ?Yield, ?Await] WhileCondition[+In, ?Yield, ?Await] ) Statement[?Yield, ?Await, ?Return] WhileCondition[In, Yield, Await] : Expression[?In, ?Yield, ?Await] LetOrConst BindingIdentifier[?Yield, ?Await] Initializer[?In, ?Yield, ?Await] LexicalDeclaration[?In, ?Yield, ?Await] ; Expression[?In, ?Yield, ?Await]

14.7.3.1 Static Semantics: Early Errors

WhileStatement : while ( Expression WhileCondition ) Statement Note

It is only necessary to apply this rule if the extension specified in B.3.1 is implemented.

14.7.3.2 Runtime Semantics: WhileLoopEvaluation

The syntax-directed operation WhileLoopEvaluation takes argument labelSet (a List of Strings) and returns either a normal completion containing an ECMAScript language value or an abrupt completion. It is defined piecewise over the following productions:

WhileStatement : while ( Expression WhileCondition ) Statement
  1. Let V be undefined.
  2. Repeat,
    1. Let exprRef be ? Evaluation of ExpressionWhileCondition.
    2. Let exprValue be ? GetValue(exprRef).
    3. If ToBoolean(exprValue) is false, return V.
    4. Let stmtResult be Completion(Evaluation of Statement).
    5. If LoopContinues(stmtResult, labelSet) is false, return ? UpdateEmpty(stmtResult, V).
    6. If stmtResult.[[Value]] is not empty, set V to stmtResult.[[Value]].

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