Which operator will evaluate to TRUE if any of the two conditions are true?

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Syntax

Description

Logical AND (&&) evaluates operands from left to right, returning immediately with the value of the first falsy operand it encounters; if all values are truthy, the value of the last operand is returned.

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:

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

The AND operator preserves non-Boolean values and returns them as they are:

result = '' && 'foo'; // result is assigned "" (empty string) result = 2 && 0; // result is assigned 0 result = 'foo' && 4; // result is assigned 4

Even though the && operator can be used with non-Boolean operands, it is still 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 a short-circuit operator. As each operand is converted to a boolean, if the result of one conversion is found to be false, the AND operator stops and returns the original value of that falsy operand; it does not evaluate any of the remaining operands.

Consider the pseudocode below.

The expr part is never evaluated because the first operand (some falsy expression) is evaluated as falsy. If expr is a function, the function is never called. See the example below:

function A() { console.log('called A'); return false; } function B() { console.log('called B'); return true; } console.log(A() && B()); // logs "called A" to the console due to the call for function A, // && evaluates to false (function A returns false), then false is logged to the console; // the AND operator short-circuits here and ignores function B

Operator precedence

The AND operator has a higher precedence than the OR operator, meaning the && operator is executed before the || operator (see operator precedence).

true || false && false // returns true true && (false || false) // returns false (2 === 3) || (4 < 0) && (1 === 1) // returns false

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 provided that certain rules are followed.

The following composite operation involving booleans:

bCondition1 || (bCondition2 && bCondition3)

is always equal to:

bCondition1 || bCondition2 && bCondition3

See also