javascript - 双感叹号？ [重复](Double exclamation points? [duplicate])

Question

Possible Duplicate:(可能重复：)

What is the !!(是什么 ！！) (not not) operator in JavaScript?(（不是）JavaScript中的运算符？)
What does the !!(什么!!) operator (double exclamation point) mean in JavaScript?(运算符（双惊叹号）在JavaScript中意味着什么？)

So I was debuging some code and ran across this:(所以我正在调试一些代码并遇到这个：)

var foo.bar = 0; // this is actually passed from another function, adding it for context function(foo) { var someVar = !!foo.bar; if (foo.bar) { // ..stuff happens } else { // .. something else happens } } Okay my questions is what is the point of !!(好的，我的问题是什么意思!!) ?(？) All that is doing is making the 0 === false .(所有这一切都是使0 === false 。) Is there any benefit to using that compared to boolean(foo.bar) ?(与boolean(foo.bar)相比，使用它有什么好处？) foo.bar can be evaluated in an if as is because 0 === false already, so why go through the conversion?(foo.bar可以在if as as中进行评估，因为0 === false ，所以为什么要进行转换？) (someVar is not reused anywhere else)(（someVar不会在其他任何地方重复使用）)   ask by jpalladino84 translate from so

Answer 1

This converts a value to a boolean and ensures a boolean type .(这会将值转换为布尔值并确保布尔类型 。)

"foo" // Evaluates to "foo". !"foo" // Evaluates to false. !!"foo" // Evaluates to true. If foo.bar is passed through, then it may not be 0 but some other falsy value.(如果foo.bar被传递，那么它可能不是0而是一些其他的虚假值。) See the following truth table:(请参阅以下真值表：) Truth Table for javascript(javascript的真值表) '' == '0' // false 0 == '' // true 0 == '0' // true false == 'false' // false false == '0' // true false == undefined // false false == null // false null == undefined // true " " == 0 // true Source: Doug Crockford(资料来源：Doug Crockford) Javascript also gets really weird when it comes to NaN values.(当谈到NaN值时，Javascript也变得非常奇怪。) And this is the only case I can think of off the top of my head where !!(这是我能想到的最好的情况！) would behave differently to ===.(会对===表现不同。) NaN === NaN //false !!NaN === !!NaN //true // !!NaN is false