Logical Assignment Operators

Javascript (ES2021) now supports the ability to combine logical operations and assignment with the new operators &&=, ||=, and ??=. Previously, compound assignment operators were only possible with mathematical and bitwise operations.

Logical AND assignment &&=

The logical AND assignment x &&= y operator performs assignment only if x is truthy. For example:

let a = 1;
let b = 0;

a &&= 2;
console.log(a); // output: 2

b &&= 2;
console.log(b); // output: 0

Short-circuit evaluation

If the left-hand expression is falsy, it is short-circuit evaluated and the right-hand expression is not evaluated, so the value of b is not reassigned because 0 is falsy.

a &&= 2 is equivalent to a && (a = 2), and the value of a is reassigned because it was originally truthy.

Logical OR assignment ||=

The logical OR assignment x ||= y operator only assigns if x is falsy.

const a = 50;
const b = '';

a ||= 10;
console.log(a);  // output: 50

b ||= 'string is empty.';
console.log(b); // output: "string is empty."

Short-circuit evaluation

From the above example, a ||= 10 is equivalent to a || (a = 10), but because the value of a is truthy, it is short-circuit evaluated and the right-hand assignment is not evaluated.

If you are checking against a falsy value such as an empty string or 0, ||= must be used, otherwise the logical nullish assignment operator ??= should be used to check for null or undefined values.

Logical nullish assignment ??=

The logical nullish assignment x ??= y operator only assigns if x is null or undefined.

const a = { limt: 50 };

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

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

Short-circuit evaluation

A logical nullish assignment expression is short-circuit evaluated to the left-hand side expression if the left-hand side is neither null nor undefined.

In the above example, the reassignment of a.limit is not performed because its value is defined as 50, which is not null or undefined.

a.speed would initially evaluate to undefined, so the right-hand expression is evaluated, which is equivalent to a.speed = 25, and now a = {limit: 50, speed: 25}.