Stage 2 Draft / August 1, 2019

Nullish Coalescing Operator

Introduction

This document specifies the nullish coalescing operator ??. See the explainer for an introduction.

The main design decisions made in this specification are:

  1. The right argument of ?? is evaluated only if needed ("short circuiting").
  2. ?? has lower precedence than ||.
  3. ?? cannot immediately contain, or be contained within, an && or || operation.
  4. The right argument is selected if the left argument is null or undefined.

1Binary Logical Operators

Syntax

LogicalExpression[In, Yield, Await]:LogicalORExpression[?In, ?Yield, ?Await] CoalesceExpression[?In, ?Yield, ?Await] CoalesceExpression[In, Yield, Await]:CoalesceExpressionHead[?In, ?Yield, ?Await]??BitwiseORExpression[?In, ?Yield, ?Await] CoalesceExpressionHead[In, Yield, Await]:CoalesceExpression[?In, ?Yield, ?Await] BitwiseORExpression[?In, ?Yield, ?Await]

1.1Static Semantics: IsFunctionDefinition

LogicalANDExpression:LogicalANDExpression&&BitwiseORExpression LogicalORExpression:LogicalORExpression||LogicalANDExpression CoalesceExpression:CoalesceExpressionHead??BitwiseORExpression
  1. Return false.

1.2Static Semantics: IsSimpleAssignmentTarget

LogicalANDExpression:LogicalANDExpression&&BitwiseORExpression LogicalORExpression:LogicalORExpression||LogicalANDExpression CoalesceExpression:CoalesceExpressionHead??BitwiseORExpression
  1. Return false.

1.3Runtime Semantics: Evaluation

CoalesceExpression:CoalesceExpressionHead??BitwiseORExpression
  1. Let lref be the result of evaluating CoalesceExpressionHead.
  2. Let lval be ? GetValue(lref).
  3. If lval is undefined or null,
    1. Let rref be the result of evaluating BitwiseORExpression.
    2. Return ? GetValue(rref).
  4. Otherwise, return lval.

2Conditional Operator ( ? : )

Syntax

ConditionalExpression[In, Yield, Await]:LogicalORExpression[?In, ?Yield, ?Await] LogicalORExpression[?In, ?Yield, ?Await]?AssignmentExpression[+In, ?Yield, ?Await]:AssignmentExpression[?In, ?Yield, ?Await] LogicalExpression[?In, ?Yield, ?Await] LogicalExpression[?In, ?Yield, ?Await]?AssignmentExpression[+In, ?Yield, ?Await]:AssignmentExpression[+In, ?Yield, ?Await]

2.1Static Semantics: IsFunctionDefinition

ConditionalExpression:LogicalORExpression?AssignmentExpression:AssignmentExpression ConditionalExpression:LogicalExpression?AssignmentExpression:AssignmentExpression
  1. Return false.

2.2Static Semantics: IsSimpleAssignmentTarget

ConditionalExpression:LogicalORExpression?AssignmentExpression:AssignmentExpression ConditionalExpression:LogicalExpression?AssignmentExpression:AssignmentExpression
  1. Return false.

2.3Runtime Semantics: Evaluation

ConditionalExpression:LogicalORExpression?AssignmentExpression:AssignmentExpression ConditionalExpression:LogicalExpression?AssignmentExpression:AssignmentExpression
  1. Let lref be the result of evaluating LogicalORExpression.
  2. Let lref be the result of evaluating LogicalExpression.
  3. Let lval be ToBoolean(? GetValue(lref)).
  4. If lval is true, then
    1. Let trueRef be the result of evaluating the first AssignmentExpression.
    2. Return ? GetValue(trueRef).
  5. Else,
    1. Let falseRef be the result of evaluating the second AssignmentExpression.
    2. Return ? GetValue(falseRef).

3Tail Position Calls

3.1Static Semantics: HasCallInTailPosition

With parameter call.

3.1.1Expression Rules

ConditionalExpression:LogicalORExpression?AssignmentExpression:AssignmentExpression ConditionalExpression:LogicalExpression?AssignmentExpression:AssignmentExpression
  1. Let has be HasCallInTailPosition of the first AssignmentExpression with argument call.
  2. If has is true, return true.
  3. Return HasCallInTailPosition of the second AssignmentExpression with argument call.
LogicalANDExpression:LogicalANDExpression&&BitwiseORExpression
  1. Return HasCallInTailPosition of BitwiseORExpression with argument call.
LogicalORExpression:LogicalORExpression||LogicalANDExpression
  1. Return HasCallInTailPosition of LogicalANDExpression with argument call.
CoalesceExpression:CoalesceExpressionHead??BitwiseORExpression

1. Return HasCallInTailPosition of BitwiseORExpression with argument call.

ACopyright & Software License

Copyright Notice

© 2019 Daniel Ehrenberg, Daniel Rosenwasser

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