Logical AND (&&)

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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.

句法

```expr1 && expr2
```

描述

``` expr 1 ``` can be converted to ``` true ``` , returns ``` expr 2 ``` ; else, returns ``` expr 1 ``` .

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 ``` Boolean ``` 构造函数。

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)
```

运算符优先级

The following expressions might seem equivalent, but they are not, because the ``` && ``` operator is executed before the ``` || ``` operator (see 运算符优先级 ).

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

范例

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
```

浏览器兼容性

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另请参阅

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