JavaScript `??=`: Nullish Coalescing Assignment

The nullish coalescing assignment operator, ??=, assigns a value only when the current left-hand value is null or undefined.

let name = null;
name ??= 'Guest';

console.log(name); // Guest

Function Defaults

function greet(user) {
  user ??= 'Stranger';
  console.log(`Hello, ${user}!`);
}

A default parameter may be even clearer when only undefined needs a fallback:

function greet(user = 'Stranger') {
  console.log(`Hello, ${user}!`);
}

Object Properties

const settings = { theme: undefined, fontSize: 16 };
settings.theme ??= 'light';

??= vs. ||=

||= treats every falsy value as missing. ??= preserves 0, false, and '':

let count = 0;
count ??= 10;
console.log(count); // 0

Conclusion

Use ??= when only null or undefined should trigger a default assignment. It is a compact way to initialize optional values without overwriting meaningful falsy data.