Logical OR assignment (||=)

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 doesn’t 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, display a default value:

document.getElementById('lyrics').textContent ||= 'No lyrics.'

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

Browser compatibility

Desktop Mobile
Chrome Edge Firefox Internet Explorer Opera Safari WebView Android Chrome Android Firefox for Android Opera Android Safari on IOS Samsung Internet
Logical_OR_assignment
85
85
79
No
71
14
85
85
79
60
14
14.0

See also

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https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Logical_OR_assignment