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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

Browser compatibilityUpdate compatibility data on GitHub

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

See also

© 2005–2018 Mozilla Developer Network and individual contributors.
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://wiki.developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Logical_nullish_assignment