JavaScript's ??= Operator

December 22, 2024

Definition

The nullish coalescing assignment operator ??= is relatively new to JavaScript. It was officially added in ECMAScript 2021 (ES12) as part of the “Logical Assignment Operators” proposal.

The ??= operator only assigns a new value if the current one is null or undefined.

// Old way (pre-2021)
if (user.name === null || user.name === undefined) {
  user.name = 'Anonymous';
}

// Or using the nullish coalescing operator (??)
user.name = user.name ?? 'Anonymous';

// New way (ES2021 and later)
user.name ??= 'Anonymous';

Edge cases

let score = 0;
score ??= 100;    // Keeps 0


let tag = '';
tag ??= 'default'; // Keeps empty string


let active = false;
active ??= true;   // Keeps false

Ref

https://www.trevorlasn.com/blog/javascript-nullish-coalescing-assignment-operator

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