Nullish coalescing assignment (??=)
The nullish coalescing assignment (??=
) operator, also known as the logical nullish assignment operator, only assigns if x
is nullish (null
or undefined
).
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Syntax
js
x ??= y
Description
Nullish coalescing assignment short-circuits, meaning that x ??= y
is equivalent to:
js
x ?? (x = y);
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
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
Specification |
---|
ECMAScript Language Specification # sec-assignment-operators |
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