blocks|key|229695|text|var+x+=+"somevalue"
var+isNotEmpty+=+!!x.length;|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|229696|让我们把它分成几部分：|unstyled|229697|x.length+++//+9
!x.length++//+false
!!x.length+//+true|229698|因此，它用于将"truethy“\"falsy”值转换为布尔值。|229699|229700|下列值等效于条件语句中的false|offset|length|style|BOLD|229701|空字符串编号(\+'')|CODE|229702|The+""+0|unordered-list-item|229703|
|229704|The+number+NaN|229705|229706|所有其他值都等于true。|229707|entityMap^0|0|0|0|0|0|6|4|0|9|2|0|4|2|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|16|8|@]|9|@]|A|$B|C]]|$1|D|3|E|5|F|7|17|8|@]|9|@]|A|$]]|$1|G|3|H|5|6|7|18|8|@]|9|@]|A|$B|C]]|$1|I|3|J|5|F|7|19|8|@]|9|@]|A|$]]|$1|K|3|-4|5|F|7|1A|8|@]|9|@]|A|$]]|$1|L|3|M|5|F|7|1B|8|@$N|1C|O|1D|P|Q]]|9|@]|A|$]]|$1|R|3|S|5|F|7|1E|8|@$N|1F|O|1G|P|T]]|9|@]|A|$]]|$1|U|3|V|5|W|7|1H|8|@$N|1I|O|1J|P|T]]|9|@]|A|$]]|$1|X|3|Y|5|F|7|1K|8|@]|9|@]|A|$]]|$1|Z|3|10|5|W|7|1L|8|@]|9|@]|A|$]]|$1|11|3|-4|5|F|7|1M|8|@]|9|@]|A|$]]|$1|12|3|13|5|F|7|1N|8|@]|9|@]|A|$]]|$1|14|3|-4|5|F|7|1O|8|@]|9|@]|A|$]]]|15|$]]

<pre><code>var x = "somevalue"
var isNotEmpty = !!x.length;
</code></pre>

Let's break it to pieces:

<pre><code>x.length // 9
!x.length // false
!!x.length // true
</code></pre>

So it's used convert a "truethy" \"falsy" value to a boolean.

<hr>

The following values are equivalent to false in conditional statements:

<ul>
<li>false</li>
<li>null</li>
<li>undefined</li>
<li>The empty string <code>""</code> (\ <code>''</code>)</li>
<li>The number 0</li>
<li>The number NaN</li>
</ul>

All other values are equivalent to true.

blocks|key|229709|text|它会强制转换为布尔值。第一个!对它求反一次，像这样转换值：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|229710|229711|undefined到true|unordered-list-item|229712|null到true|229713|%2B0到true|229714|-0到true|229715|''到true|229716|NaN到true|229717|false到true|229718|All其他表达式到false|229719|229720|然后另一个!再次否定它。一种简洁的布尔值转换，完全等同于ToBoolean，因为!是defined+as+its+negation。不过，这里没有必要这样做，因为它只用作条件运算符的条件，条件运算符将以同样的方式确定真实性。|229721|entityMap|0|LINK|mutability|MUTABLE|url|https://es5.github.io/#x9.2|1|https://es5.github.io/#x11.4.9^0|E|1|0|0|0|9|A|4|0|0|4|5|4|0|0|2|3|4|0|0|2|3|4|0|0|2|3|4|0|0|3|4|4|0|0|5|6|4|0|9|5|0|0|5|1|14|1|S|9|0|16|N|1|0^^$0|@$1|2|3|4|5|6|7|1A|8|@$9|1B|A|1C|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|1D|8|@]|D|@]|E|$]]|$1|G|3|H|5|I|7|1E|8|@$9|1F|A|1G|B|C]|$9|1H|A|1I|B|C]]|D|@]|E|$]]|$1|J|3|K|5|I|7|1J|8|@$9|1K|A|1L|B|C]|$9|1M|A|1N|B|C]]|D|@]|E|$]]|$1|L|3|M|5|I|7|1O|8|@$9|1P|A|1Q|B|C]|$9|1R|A|1S|B|C]]|D|@]|E|$]]|$1|N|3|O|5|I|7|1T|8|@$9|1U|A|1V|B|C]|$9|1W|A|1X|B|C]]|D|@]|E|$]]|$1|P|3|Q|5|I|7|1Y|8|@$9|1Z|A|20|B|C]|$9|21|A|22|B|C]]|D|@]|E|$]]|$1|R|3|S|5|I|7|23|8|@$9|24|A|25|B|C]|$9|26|A|27|B|C]]|D|@]|E|$]]|$1|T|3|U|5|I|7|28|8|@$9|29|A|2A|B|C]|$9|2B|A|2C|B|C]]|D|@]|E|$]]|$1|V|3|W|5|I|7|2D|8|@$9|2E|A|2F|B|C]]|D|@]|E|$]]|$1|X|3|-4|5|6|7|2G|8|@]|D|@]|E|$]]|$1|Y|3|Z|5|6|7|2H|8|@$9|2I|A|2J|B|C]|$9|2K|A|2L|B|C]]|D|@$9|2M|A|2N|1|2O]|$9|2P|A|2Q|1|2R]]|E|$]]|$1|10|3|-4|5|6|7|2S|8|@]|D|@]|E|$]]]|11|$12|$5|13|14|15|E|$16|17]]|18|$5|13|14|15|E|$16|19]]]]

It casts to boolean. The first <code>!</code> negates it once, converting values like so:

<ul>
<li><code>undefined</code> to <code>true</code></li>
<li><code>null</code> to <code>true</code></li>
<li><code>+0</code> to <code>true</code></li>
<li><code>-0</code> to <code>true</code></li>
<li><code>''</code> to <code>true</code></li>
<li><code>NaN</code> to <code>true</code></li>
<li><code>false</code> to <code>true</code></li>
<li>All other expressions to <code>false</code></li>
</ul>

Then the other <code>!</code> negates it again. A concise cast to boolean, exactly equivalent to <a href="https://es5.github.io/#x9.2" rel="noreferrer">ToBoolean</a> simply because <code>!</code> is <a href="https://es5.github.io/#x11.4.9" rel="noreferrer">defined as its negation</a>. It’s unnecessary here, though, because it’s only used as the condition of the conditional operator, which will determine truthiness in the same way.

blocks|key|2885833|text|双重否定将“真”或“假”值转换为布尔值，true或false。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|2885834|大多数人都熟悉使用真实性作为测试：|2885835|if+(options.guess)+{
++++//+runs+if+options.guess+is+truthy,+
}|code-block|syntax|javascript|2885836|但这并不一定意味着：|2885837|options.guess===true+++//+could+be,+could+be+not|2885838|如果您需要强制将“真实”值转换为真正的布尔值，!!是一种方便的方法：|2885839|!!options.guess===true+++//+always+true+if+options.guess+is+truthy|2885840|entityMap^0|K|4|P|5|0|0|0|0|0|N|2|0|0^^$0|@$1|2|3|4|5|6|7|W|8|@$9|X|A|Y|B|C]|$9|Z|A|10|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|11|8|@]|D|@]|E|$]]|$1|H|3|I|5|J|7|12|8|@]|D|@]|E|$K|L]]|$1|M|3|N|5|6|7|13|8|@]|D|@]|E|$]]|$1|O|3|P|5|J|7|14|8|@]|D|@]|E|$K|L]]|$1|Q|3|R|5|6|7|15|8|@$9|16|A|17|B|C]]|D|@]|E|$]]|$1|S|3|T|5|J|7|18|8|@]|D|@]|E|$K|L]]|$1|U|3|-4|5|6|7|19|8|@]|D|@]|E|$]]]|V|$]]

Double-negation turns a "truthy" or "falsy" value into a boolean value, <code>true</code> or <code>false</code>.

Most are familiar with using truthiness as a test:

<pre><code>if (options.guess) {
 // runs if options.guess is truthy, 
}
</code></pre>

But that does not necessarily mean:

<pre><code>options.guess===true // could be, could be not
</code></pre>

If you need to force a "truthy" value to a true boolean value, <code>!!</code> is a convenient way to do that:

<pre><code>!!options.guess===true // always true if options.guess is truthy
</code></pre>

<blockquote>
 Possible Duplicate: 
 <a href="https://stackoverflow.com/questions/784929/what-is-the-not-not-operator-in-javascript">What is the !! (not not) operator in JavaScript?</a> 
</blockquote>



I have encountered this piece of code 

<pre><code>function printStackTrace(options) {
 options = options || {guess: true};
 var ex = options.e || null, guess = !!options.guess;
 var p = new printStackTrace.implementation(), result = p.run(ex);
 return (guess) ? p.guessAnonymousFunctions(result) : result;
}
</code></pre>

And couldn't help to wonder why the double negation? And is there an alternative way to achieve the same effect?

(code is from <a href="https://github.com/eriwen/javascript-stacktrace/blob/master/stacktrace.js" rel="noreferrer">https://github.com/eriwen/javascript-stacktrace/blob/master/stacktrace.js</a>)

Double negation (!!) in javascript - what is the purpose?

 可能重复：我遇到过这段代码function printStackTrace(options) {    options = options || {guess: true};    var ex = options.e || null, guess = !!options.guess;    var p = new ...

问双重否定(!!)在javascript中-目的是什么？
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