Logical OR (||) - JavaScript 编辑

The logical OR (||) operator (logical disjunction) for a set of operands is true if and only if one or more of its operands is true. It is typically used with Boolean (logical) values. When it is, it returns a Boolean value. However, the || operator actually returns the value of one of the specified operands, so if this operator is used with non-Boolean values, it will return a non-Boolean value.

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.

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

If expr1 can be converted to true, returns expr1; else, returns expr2.

If a value can be converted to true, the value is so-called truthy. If a value can be converted to false, the value is so-called falsy.

Examples of expressions that can be converted to false are:

• null;
• NaN;
• 0;
• empty string ("" or '' or ``);
• undefined.

Even though the || operator can be used with operands that are not Boolean values, it can still be considered a boolean operator since its return value can always be converted to a boolean primitive. To explicitly convert its return value (or any expression in general) to the corresponding boolean value, use a double NOT operator or the Boolean constructor.

Short-circuit evaluation

The logical OR expression is evaluated left to right, it is tested for possible "short-circuit" evaluation using the following rule:

(some truthy expression) || expr is short-circuit evaluated to the truthy expression.

Short circuit means that the expr part above is not evaluated, hence any side effects of doing so do not take effect (e.g., if expr is a function call, the calling never takes place). This happens because the value of the operator is already determined after the evaluation of the first operand. See example:

function A(){ console.log('called A'); return false; }
function B(){ console.log('called B'); return true; }

console.log( B() || A() );
// logs "called B" due to the function call,
// then logs true (which is the resulting value of the operator)

Operator precedence

The following expressions might seem equivalent, but they are not, because the && operator is executed before the || operator (see operator precedence).

true || false && false      // returns true, because && is executed first
(true || false) && false    // returns false, because operator precedence cannot apply

Examples

Using OR

The following code shows examples of the || (logical OR) operator.

o1 = true  || true       // t || t returns true
o2 = false || true       // f || t returns true
o3 = true  || false      // t || f returns true
o4 = false || (3 == 4)   // f || f returns false
o5 = 'Cat' || 'Dog'      // t || t returns "Cat"
o6 = false || 'Cat'      // f || t returns "Cat"
o7 = 'Cat' || false      // t || f returns "Cat"
o8 = ''    || false      // f || f returns false
o9 = false || ''         // f || f returns ""
o10 = false || varObject // f || object returns varObject

Note: If you use this operator to provide a default value to some variable, be aware that any falsy value will not be used. If you only need to filter out null or undefined, consider using the nullish coalescing operator.

Conversion rules for booleans

Converting AND to OR

The following operation involving booleans:

bCondition1 && bCondition2

is always equal to:

!(!bCondition1 || !bCondition2)

Converting OR to AND

The following operation involving booleans:

bCondition1 || bCondition2

is always equal to:

!(!bCondition1 && !bCondition2)

Removing nested parentheses

As logical expressions are evaluated left to right, it is always possible to remove parentheses from a complex expression following some rules.

The following composite operation involving booleans:

bCondition1 && (bCondition2 || bCondition3)

is always equal to:

!(!bCondition1 || !bCondition2 && !bCondition3)

Specifications

Specification
ECMAScript (ECMA-262) The definition of 'Logical OR expression' in that specification.

Browser compatibility

BCD tables only load in the browser

See also

• The nullish coalescing operator (??)
• Boolean
• Truthy
• Falsy