为什么alert（!!" 0"）和alert（false ==" 0"）都在JavaScript中输出为true [英] Why do alert(!!"0") and alert(false == "0") both output true in JavaScript

问题描述

据我所知，在JavaScript！应该规范化一个布尔值，将其从某个其他类型转换为true或false。这意味着0转换为布尔值true。另一方面，如果我将它与false进行比较，事实证明它实际上是假的（因为比较的结果是真的）。我在这里错过了什么规则。我已经在IE和Opera中测试了它。

As far as I know in JavaScript !! is supposed to normalize a boolean value converting it to true or false from some other type. This would mean that the "0" converts to boolean true. On the other hand if I compare it with false it turns out that it is in fact false (as the result of the comparison is true). What rule am I missing here. I have tested it in IE and Opera.

推荐答案

== 运算符检查松散的相等性，这与真实性无关。

The == operator checks for loose equality, which has nothing to do with truthiness.

具体来说，它会将操作数转换为数字，然后比较数字。

包含数字的字符串将转换为它们包含的数字;布尔值转换为 0 和 1 。

通过调用 valueOf ，如果已定义。

Specifically, it will convert to operands to numbers, then compare the numbers.
Strings containing numbers convert to the numbers that they contain; booleans convert to 0 and 1.
Objects are converted by calling valueOf, if defined.

因此，以下所有条件均为真：

Thus, all of the following are true:

• 1== 1


• 0== false


• 1== true


• 2！= true


• 2！= false


• （{valueOf：function（）{return 2;}}）== 2


• （{valueOf：function（）{return 1;}}）== true

• "1" == 1
• "0" == false
• "1" == true
• "2" != true
• "2" != false
• ({ valueOf:function() { return 2; } }) == 2
• ({ valueOf:function() { return 1; } }) == true

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