Logical OR assignment (||=) - JavaScript 编辑

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

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The source for this interactive example is stored in a GitHub repository. If you'd like to contribute to the interactive examples project, please clone https://github.com/mdn/interactive-examples and send us a pull request.

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

Specification
Logical Assignment Operators The definition of 'Assignment operators' in that specification.

Browser compatibility

BCD tables only load in the browser

See also

• Logical OR (||)
• The nullish coalescing operator (??)
• Bitwise OR assignment (|=)
• Boolean
• Truthy
• Falsy