The logical nullish assignment (x ??= y
) operator only assigns if x
is nullish (null
or undefined
).
Syntax
expr1 ??= expr2
Description
Short-circuit evaluation
The nullish coalescing operator is evaluated left to right, it is tested for possible short-circuit evaluation using the following rule:
(some expression that is neither null nor undefined) ?? expr
is short-circuit evaluated to the left-hand side expression if the left-hand side proves to be neither null
nor undefined
.
Short circuit means that the expr
part above is not evaluated, hence any side effects of doing so do not take effect (e.g., if expr
is a function call, the calling never takes place).
Logical nullish assignment short-circuits as well meaning that x ??= y
is equivalent to:
x ?? (x = y);
And not equivalent to the following which would always perform an assignment:
x = x ?? y;
Examples
Using logical nullish assignment
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 |
---|
Logical Assignment Operators The definition of 'Assignment operators' in that specification. |
Browser compatibility
Desktop | Mobile | Server | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Logical nullish assignment (x ??= y ) | Chrome Full support 85 | Edge Full support 85 | Firefox Full support 79 | IE No support No | Opera No support No | Safari Full support 14 | WebView Android Full support 85 | Chrome Android Full support 85 | Firefox Android No support No | Opera Android No support No | Safari iOS Full support 14 | Samsung Internet Android No support No | nodejs No support No |
Legend
- Full support
- Full support
- No support
- No support