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 {{Glossary("nullish")}} (null
or undefined
).
{{EmbedInteractiveExample("pages/js/expressions-nullish-coalescing-assignment.html")}}
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 }
Specifications
Browser compatibility
See also
- Nullish coalescing operator (
??
) - {{Glossary("Nullish")}}
- {{Glossary("Truthy")}}
- {{Glossary("Falsy")}}