javascript operator
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).
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
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:
const x = 1;
x ??= 2;
Neither would the following trigger the setter:
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.
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.
function config(options) {
options.duration ??= 100;
options.speed ??= 25;
return options;
}
config({ duration: 125 }); // { duration: 125, speed: 25 }
config({}); // { duration: 100, speed: 25 }