Web/JavaScript/Reference/Operators/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, 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 OperatorsThe definition of 'Assignment operators' in that specification.

Browser compatibility

Update compatibility data on GitHub

	Desktop						Mobile						Server
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	Android webview	Chrome for Android	Firefox for Android	Opera for Android	Safari on iOS	Samsung Internet	Node.js
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 Full support 79	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