Logical OR assignment (||=)

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The logical OR assignment (x ||= y) operator only assigns if x is falsy.

 语法

expr1 ||= expr2

描述

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.

例子

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).

规范

Specification
Logical Assignment Operators
Assignment operators

浏览器兼容性

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ChromeEdgeFirefoxInternet ExplorerOperaSafariAndroid webviewChrome for AndroidFirefox for AndroidOpera for AndroidSafari on iOSSamsung InternetNode.js
Logical OR assignment (x ||= y)Chrome Full support 85Edge Full support 85Firefox Full support 79IE No support NoOpera No support NoSafari Full support 14WebView Android Full support 85Chrome Android Full support 85Firefox Android Full support 79Opera Android No support NoSafari iOS Full support 14Samsung Internet Android No support Nonodejs No support No

Legend

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参见