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
布尔
(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.
expr1 && expr2
若
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
;
""
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
或
布尔
构造函数。
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)
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
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
The following operation involving booleans :
bCondition1 && bCondition2
is always equal to:
!(!bCondition1 || !bCondition2)
The following operation involving booleans :
bCondition1 || bCondition2
is always equal to:
!(!bCondition1 && !bCondition2)
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
| 规范 |
|---|
|
ECMAScript (ECMA-262)
The definition of 'Logical AND expression' in that specification. |
| Desktop | Mobile | Server | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Logical AND (
&&
)
|
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完整支持