Logical nullish assignment (??=)
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 |
---|
ECMAScript Language Specification # sec-assignment-operators |
Browser compatibility
BCD tables only load in the browser