Nullish coalescing assignment (??=)

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

undefined

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

See also