Stage 1 Draft / October 9, 2018

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 the same precedence as ||.
  3. The right argument is selected if the left argument is null or undefined.

1Syntax

LogicalORExpression[In, Yield, Await]:LogicalANDExpression[?In, ?Yield, ?Await] LogicalORExpression[?In, ?Yield, ?Await]||LogicalANDExpression[?In, ?Yield, ?Await] LogicalORExpression[?In, ?Yield, ?Await]??LogicalANDExpression[?In, ?Yield, ?Await]

2Runtime Semantics: Evaluation

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

ACopyright & Software License

Copyright Notice

© 2018 Daniel Ehrenberg

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