三重叹号的使用

通过查看我们一个项目的源代码，我发现有些地方我们在条件语句中使用了三个感叹号，比如:

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

现在，我明白这不是某种很少使用的运算符，它只是连续的三个否定，就像 !(!(!someVar)))。我不明白它有什么用——在我看来，它可以安全地被单一叹号所取代。下面是我试图找到一个案例当 !!!a不等于 !a(采取直接从谷歌铬控制台) :

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

我是不是漏掉了一些罕见的(或显而易见的)案例？

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最佳答案

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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It is the same as one exclamation mark. The key idea behind it is to improve visibility for the programmer. Compiler will optimize it as single '!' anyway.

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Consider:

var x;
console.log(x == false);
console.log(!x, !x == true);
console.log(!!x, !!x == true, !!x == false);

... the console output is:

false
true true
false false true

notice how, even though x is "falsey" in the first use, it is not the same as false.

But the second use (!x) has an actual boolean - but it's the opposite value.

So the third use (!!x) turns the "falsey" value into a true boolean.

...with that in mind, the third exclamation point makes a TRUE negation of the original value (a negation of the "true boolean" value).

ETA:

OMG! I can't believe I didn't notice that this is a TRIPLE exclamation point question! Even after it was specifically pointed out to me.

So, while my answer is hopefully useful to someone, I have to agree with the others who have posted that a triple-exclamation is functionally the same as a single.

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In JavaScript, it's a way to show that what you are evaluating is not a boolean but a truthy or falsy value.

So for truthy/falsy values you either use !! or !!!.

And for boolean values you either use ! or nothing.

This is simply for readability.


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