# Logical operators

## 說明

Operator Syntax Description

If a value can be converted to `true`, the value is so-called truthy (en-US). If a value can be converted to `false`, the value is so-called falsy (en-US).

Examples of expressions that can be converted to false are:

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

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

### Short-circuit evaluation

As logical expressions are evaluated left to right, they are tested for possible "short-circuit" evaluation using the following rules:

• `(some falsy expression) && expr` is short-circuit evaluated to the falsy expression;
• `(some truthy expression) || expr` is short-circuit evaluated to the truthy expression.

Short circuit means that the expr parts above are 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)

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``````

### Logical 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
``````

### Logical 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
``````

### Logical NOT (`!`)

The following code shows examples of the `!` (logical NOT) operator.

``````n1 = !true               // !t returns false
n2 = !false              // !f returns true
n3 = !''                 // !f returns true
n4 = !'Cat'              // !t returns false
``````

#### Double NOT (`!!`)

It is possible to use a couple of NOT operators in series to explicitly force the conversion of any value to the corresponding boolean primitive. The conversion is based on the "truthyness" or "falsyness" of the value (see truthy (en-US) and falsy (en-US)).

The same conversion can be done through the `Boolean` function.

``````n1 = !!true                   // !!truthy returns true
n2 = !!{}                     // !!truthy returns true: any object is truthy...
n3 = !!(new Boolean(false))   // ...even Boolean objects with a false .valueOf()!
n4 = !!false                  // !!falsy returns false
n5 = !!""                     // !!falsy returns false
n6 = !!Boolean(false)         // !!falsy 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)``

#### Converting between NOTs

The following operation involving booleans:

``!!bCondition``

is always equal to:

``bCondition``

### 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.

#### Removing nested AND

The following composite operation involving booleans:

``bCondition1 || (bCondition2 && bCondition3)``

is always equal to:

``bCondition1 || bCondition2 && bCondition3``

#### Removing nested OR

The following composite operation involving booleans:

``bCondition1 && (bCondition2 || bCondition3)``

is always equal to:

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

## Specifications

Specification Status Comment
ECMAScript 1st Edition (ECMA-262) Standard Initial definition.
ECMAScript 5.1 (ECMA-262) Standard Defined in several sections of the specification: Logical NOT Operator, Binary Logical Operators
ECMAScript 2015 (6th Edition, ECMA-262) Standard Defined in several sections of the specification: Logical NOT Operator, Binary Logical Operators
ECMAScript (ECMA-262) Living Standard Defined in several sections of the specification: Logical NOT Operator, Binary Logical Operators

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