Web/JavaScript/Reference/Operators/Logical nullish assignment

The logical nullish assignment (x ??= y) operator only assigns if x is nullish (null or undefined).

Syntax

expr1 ??= expr2

Description

Short-circuit evaluation

The nullish coalescing operator is evaluated left to right, it is tested for possible short-circuit evaluation using the following rule:

(some expression that is neither null nor undefined) ?? expr is short-circuit evaluated to the left-hand side expression if the left-hand side proves to be neither null nor undefined.

Short circuit means that the expr part above is not evaluated, hence any side effects of doing so do not take effect (e.g., if expr is a function call, the calling never takes place).

Logical nullish assignment short-circuits as well meaning that x ??= y is equivalent to:

x ?? (x = y);

And not equivalent to the following which would always perform an assignment:

x = x ?? y;

Examples

Using logical nullish assignment

function config(options) {
  options.duration ??= 100;
  options.speed ??= 25;
  return options;
}

config({ duration: 125 }); // { duration: 125, speed: 25 }
config({}); // { duration: 100, speed: 25 }

Specifications

Specification

Logical Assignment OperatorsThe definition of 'Assignment operators' in that specification.

Browser compatibility

Update compatibility data on GitHub

	Desktop						Mobile						Server
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	Android webview	Chrome for Android	Firefox for Android	Opera for Android	Safari on iOS	Samsung Internet	Node.js
Logical nullish assignment (`x ??= y`)	Chrome Full support 85	Edge Full support 85	Firefox Full support 79	IE No support No	Opera No support No	Safari Full support 14	WebView Android Full support 85	Chrome Android Full support 85	Firefox Android Full support 79	Opera Android No support No	Safari iOS Full support 14	Samsung Internet Android No support No	nodejs No support No

Legend

Full support: Full support
No support: No support