Nullish coalescing assignment (??=)

Baseline Widely available

This feature is well established and works across many devices and browser versions. It’s been available across browsers since September 2020.

The nullish coalescing assignment (??=) operator, also known as the logical nullish assignment operator, only evaluates the right operand and assigns to the left if the left operand is nullish (null or undefined).

Try it

const a = { duration: 50 };

a.speed ??= 25;
console.log(a.speed);
// Expected output: 25

a.duration ??= 10;
console.log(a.duration);
// Expected output: 50

Syntax

js
x ??= y

Description

Nullish coalescing assignment short-circuits, meaning that x ??= y is equivalent to x ?? (x = y), except that the expression x is only evaluated once.

No assignment is performed if the left-hand side is not nullish, due to short-circuiting of the nullish coalescing operator. For example, the following does not throw an error, despite x being const:

js
const x = 1;
x ??= 2;

Neither would the following trigger the setter:

js
const x = {
  get value() {
    return 1;
  },
  set value(v) {
    console.log("Setter called");
  },
};

x.value ??= 2;

In fact, if x is not nullish, y is not evaluated at all.

js
const x = 1;
x ??= console.log("y evaluated");
// Logs nothing

Examples

Using nullish coalescing assignment

You can use the nullish coalescing assignment operator to apply default values to object properties. Compared to using destructuring and default values, ??= also applies the default value if the property has value null.

js
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
ECMAScript® 2025 Language Specification
# sec-assignment-operators

Browser compatibility

Report problems with this compatibility data on GitHub
desktopmobileserver
Chrome
Edge
Firefox
Opera
Safari
Chrome Android
Firefox for Android
Opera Android
Safari on iOS
Samsung Internet
WebView Android
WebView on iOS
Deno
Node.js
Nullish coalescing assignment (x ??= y)

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Full support
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See also