Logical AND (&&)

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.

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

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Logical AND (&&)Chrome Full support 1Edge Full support 12Firefox Full support 1IE Full support 3Opera Full support 3Safari Full support 1WebView Android Full support 1Chrome Android Full support 18Firefox Android Full support 4Opera Android Full support 10.1Safari iOS Full support 1Samsung Internet Android Full support 1.0nodejs Full support 0.1.100

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See also