三重感叹号的使用 [英] The use of the triple exclamation mark

问题描述

通过查看我们的一个项目的源代码，我发现在条件语句中使用三个感叹号的地方很多，例如:

Looking through the source code of one of our projects, I've found some amount of places where we're using three exclamation marks in conditional statements, like so:

if (!!!someVar) {
    // ...
}

现在，我知道这不是一种很少使用的运算符，它只是连续三个否定，例如!(!(!someVar))).我不知道它的用途-我认为可以安全地将其替换为单个感叹号.以下是我尝试查找!!!a不等于!a的情况的尝试(直接从Google chrome控制台获取):

Now, I understand that this isn't some kind of rarely used operator, it's just three negations in a row, like !(!(!someVar))). I don't understand what's the use of it - in my opinion it can safely be replaced with single exclamation mark. Following are my attempts to find a case when !!!a isn't equal to !a (taken straight from the google chrome console):

var a = ''
""
!!!a === !a
true
a = 'string'
"string"
!!!a === !a
true
a = null
null
!!!a === !a
true
a = 12
12
!!!a === !a
true
a = {b: 1}
Object {b: 1}
!!!a.c === !a.c // a.c is undefined here
true
a = []
[]
!!!a === !a
true
a = [1,2]
[1, 2]
!!!a === !a
true

我错过了一些罕见(或显而易见)的案件吗?

Am I missing some rare (or obvious) case?

推荐答案

!a和!!!a之间没有区别，因为!!!a只是!!(!a)并且因为!a是布尔值，所以只是它的双重否定，因此是相同的.

There is no difference between !a and !!!a, since !!!a is just !!(!a) and because !a is a boolean, !!(!a) is just its double negation, therefore the same.

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