Logical AND (&&) - JavaScript 编辑

The logical AND (&&) operator (logical conjunction) for a set of operands is true if and only if all of its operands are 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 expr2; else, returns expr1.

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

(some falsy expression) && expr is short-circuit evaluated to the falsy 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( A() && B() );
// logs "called A" due to the function call,
// then logs false (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 AND

The following code shows examples of the && (logical AND) operator.

a1 = true  && true       // t && t returns true
a2 = true  && false      // t && f returns false
a3 = false && true       // f && t returns false
a4 = false && (3 == 4)   // f && f returns false
a5 = 'Cat' && 'Dog'      // t && t returns "Dog"
a6 = false && 'Cat'      // f && t returns false
a7 = 'Cat' && false      // t && f returns false
a8 = ''    && false      // f && f returns ""
a9 = false && ''         // f && f returns false

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 AND expression' in that specification.

Browser compatibility

BCD tables only load in the browser

See also

• Boolean
• Truthy
• Falsy