The logical OR assignment (x ||= y
) operator only assigns if x
is falsy.
Syntax
expr1 ||= expr2
Description
Short-circuit evaluation
The logical OR operator works like this:
x || y; // returns x when x is truthy // returns y when x is not truthy
The logical OR operator short-circuits: the second operand is only evaluated if the first operand doesnβt already determine the result.
Logical OR assignment short-circuits as well, meaning it only performs an assignment if the logical operation would evaluate the right-hand side. In other words, x ||= y
is equivalent to:
x || (x = y);
And not equivalent to the following which would always perform an assignment:
x = x || y;
Note that this behavior is different to mathematical and bitwise assignment operators.
Examples
Setting default content
If the "lyrics" element is empty, set the innerHTML
to a default value:
document.getElementById('lyrics').innerHTML ||= '<i>No lyrics.</i>'
Here the short-circuit is especially beneficial, since the element will not be updated unnecessarily and won't cause unwanted side-effects such as additional parsing or rendering work, or loss of focus, etc.
Note: Pay attention to the value returned by the API you're checking against. If an empty string is returned (a falsy value), ||=
must be used, otherwise you want to use the ??=
operator (for null
or undefined
return values).
Specifications
Specification |
---|
Logical Assignment Operators The definition of 'Assignment operators' in that specification. |
Browser compatibility
Desktop | Mobile | Server | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Logical OR 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