blocks|key|5217085|text|在Javascript中，以下值被视为条件条件的false：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|5217086|空字符串''|5217087|The+number+|unordered-list-item|5217088|
|5217089|+|5217090|false|5217091|null+|5217092|undefined|5217093|The+number+NaN|5217094|+0+|5217095|5217096|5217097|5217098|其他所有内容都被视为true。|5217099|'false'不是上面的任何一个，所以它是true。|5217100|entityMap^0|O|5|0|4|2|0|0|0|0|0|5|0|0|4|0|0|9|0|B|3|0|1|1|0|0|0|0|A|4|0|0|7|L|4|0^^$0|@$1|2|3|4|5|6|7|17|8|@$9|18|A|19|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|1A|8|@$9|1B|A|1C|B|C]]|D|@]|E|$]]|$1|H|3|I|5|J|7|1D|8|@]|D|@]|E|$]]|$1|K|3|L|5|6|7|1E|8|@]|D|@]|E|$]]|$1|M|3|N|5|J|7|1F|8|@]|D|@]|E|$]]|$1|O|3|P|5|J|7|1G|8|@$9|1H|A|1I|B|C]]|D|@]|E|$]]|$1|Q|3|R|5|J|7|1J|8|@$9|1K|A|1L|B|C]]|D|@]|E|$]]|$1|S|3|T|5|J|7|1M|8|@$9|1N|A|1O|B|C]]|D|@]|E|$]]|$1|U|3|V|5|J|7|1P|8|@$9|1Q|A|1R|B|C]]|D|@]|E|$]]|$1|W|3|X|5|6|7|1S|8|@$9|1T|A|1U|B|C]]|D|@]|E|$]]|$1|Y|3|L|5|6|7|1V|8|@]|D|@]|E|$]]|$1|Z|3|-4|5|J|7|1W|8|@]|D|@]|E|$]]|$1|10|3|-4|5|6|7|1X|8|@]|D|@]|E|$]]|$1|11|3|12|5|6|7|1Y|8|@$9|1Z|A|20|B|C]]|D|@]|E|$]]|$1|13|3|14|5|6|7|21|8|@$9|22|A|23|B|C]|$9|24|A|25|B|C]]|D|@]|E|$]]|$1|15|3|-4|5|6|7|26|8|@]|D|@]|E|$]]]|16|$]]

In Javascript the following values are treated as <code>false</code> for conditionals:

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

Everything else is treated as <code>true</code>.

<code>'false'</code> is none of the above, so it's <code>true</code>.

blocks|key|5216916|text|字符串'false'的计算结果为布尔型true|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|5216917|entityMap^0|3|7|J|4|0^^$0|@$1|2|3|4|5|6|7|H|8|@$9|I|A|J|B|C]|$9|K|A|L|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|M|8|@]|D|@]|E|$]]]|G|$]]

The string <code>'false'</code> evaluates to the boolean <code>true</code>

blocks|key|5216953|text|这是因为它实际上不是一个布尔值，而是一个字符串'false'。当您将字符串转换为布尔值时，''为false，其他任何值都为true。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|5216954|您可以检查它是否等于字符串'false'+(或'true')。|5216955|var+myBoolean+=+'false';+//+(string)
myBoolean+=+myBoolean+!==+'false';+//false+(boolean)|code-block|syntax|javascript|5216956|entityMap^0|N|7|19|2|1C|5|1P|4|0|D|7|N|6|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@$9|P|A|Q|B|C]|$9|R|A|S|B|C]|$9|T|A|U|B|C]|$9|V|A|W|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|X|8|@$9|Y|A|Z|B|C]|$9|10|A|11|B|C]]|D|@]|E|$]]|$1|H|3|I|5|J|7|12|8|@]|D|@]|E|$K|L]]|$1|M|3|-4|5|6|7|13|8|@]|D|@]|E|$]]]|N|$]]

This is because it's not actually a boolean, it's a the string <code>'false'</code>. When you convert a string to a boolean, <code>''</code> is <code>false</code> and anything else is <code>true</code>.

You check if it's equal to the string <code>'false'</code> (or <code>'true'</code>) or not.

<pre><code>var myBoolean = 'false'; // (string)
myBoolean = myBoolean !== 'false'; //false (boolean)
</code></pre>

blocks|key|13375|text|‘'false’==是真的，这是因为JavaScript的隐式类型强制。请查看来自Crockford's+The+Elements+of+JavaScript+Style的其他示例。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|13376|13377|+'‘==+'0’//+false|blockquote|13378|13379|0+==+'‘//+true|13380|0+==+'0‘//+true|13381|false+==+'+false‘//+false|13382|false+==+'0‘//+true|13383|false+==未定义//+false|13384|false+==+null+//+false|13385|null+==未定义//+true|13386|‘\t\r\n’==+0+//+true|13387|13388|您可以通过将代码更改为如下所示来解决此特定问题|13389|var+myBoolean+=+document.getElementById("someBoolean").value+===+"true"|code-block|syntax|javascript|13390|而且，使用它几乎总是更好!！和===而不是！和==|13391|entityMap|0|LINK|mutability|MUTABLE|url|http://javascript.crockford.com/style2.html^0|15|18|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|1G|8|@]|9|@$A|1H|B|1I|1|1J]]|C|$]]|$1|D|3|-4|5|6|7|1K|8|@]|9|@]|C|$]]|$1|E|3|F|5|G|7|1L|8|@]|9|@]|C|$]]|$1|H|3|-4|5|6|7|1M|8|@]|9|@]|C|$]]|$1|I|3|J|5|6|7|1N|8|@]|9|@]|C|$]]|$1|K|3|L|5|6|7|1O|8|@]|9|@]|C|$]]|$1|M|3|N|5|6|7|1P|8|@]|9|@]|C|$]]|$1|O|3|P|5|6|7|1Q|8|@]|9|@]|C|$]]|$1|Q|3|R|5|6|7|1R|8|@]|9|@]|C|$]]|$1|S|3|T|5|6|7|1S|8|@]|9|@]|C|$]]|$1|U|3|V|5|6|7|1T|8|@]|9|@]|C|$]]|$1|W|3|X|5|6|7|1U|8|@]|9|@]|C|$]]|$1|Y|3|-4|5|6|7|1V|8|@]|9|@]|C|$]]|$1|Z|3|10|5|6|7|1W|8|@]|9|@]|C|$]]|$1|11|3|12|5|13|7|1X|8|@]|9|@]|C|$14|15]]|$1|16|3|17|5|6|7|1Y|8|@]|9|@]|C|$]]|$1|18|3|-4|5|6|7|1Z|8|@]|9|@]|C|$]]]|19|$1A|$5|1B|1C|1D|C|$1E|1F]]]]

'false' == true, crazily enough because of JavaScript's implicit type coercion. Check out these other examples from <a href="http://javascript.crockford.com/style2.html" rel="nofollow noreferrer">Crockford's The Elements of JavaScript Style</a>.
<blockquote>
'' == '0' // false 
0 == '' // true
0 == '0' // true
false == 'false' // false 
false == '0' // true
false == undefined // false 
false == null // false
null == undefined
// true
' \t\r\n ' == 0 // true
</blockquote>
You could solve this particular problem by changing your code to something like
<pre><code>var myBoolean = document.getElementById(&quot;someBoolean&quot;).value === &quot;true&quot;
</code></pre>
Also, it is almost always better to use !! and === rather than ! and ==

blocks|key|3603335|text|document.getElementById("someBoolean")不返回布尔值true/false，它返回一个元素或未定义/+null|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3603336|你可以颠倒你的逻辑，得到预期的结果：|3603337|++++if(!myBoolean)
++++{+
++++++alert('This+element+does+not+exist');
++++}

++++if(!eval(myBoolean))
++++{
+++++alert("Do+not+know+why+you+would+ever+want+to+do+this");
//+you+could+do+typeof()+
++++}|code-block|syntax|javascript|3603338|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|L|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|M|8|@]|9|@]|A|$G|H]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

document.getElementById("someBoolean") does not return a boolean true/false it returns an element or undefined / null

you could reverse your logic and get the expected result:

<pre><code> if(!myBoolean)
 { 
 alert('This element does not exist');
 }

 if(!eval(myBoolean))
 {
 alert("Do not know why you would ever want to do this");
// you could do typeof() 
 }
</code></pre>

blocks|key|5217051|text|尝试：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|5217052|var+myBoolean=+document.getElementById("someBoolean").value;++//This+is+a+'false'

if(myBoolean+!=+"false")
{+
++alert(Your+boolean+is+True);++//This+condition+always+getting+executed+even+though+myBoolean+is+false;
}|code-block|syntax|javascript|5217053|就像其他人所说的那样，字符串不是布尔值，所以把它当做布尔值使用会给你一个逻辑错误。|5217054|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|L|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|M|8|@]|9|@]|A|$]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

Try:

<pre><code>var myBoolean= document.getElementById("someBoolean").value; //This is a 'false'

if(myBoolean != "false")
{ 
 alert(Your boolean is True); //This condition always getting executed even though myBoolean is false;
}
</code></pre>

Like others have said, a string isn't a boolean value, so using it as though it was will give you a logical error.

blocks|key|5217116|text|根据ECMA，字符串是布尔型真值|type|unstyled|depth|inlineStyleRanges|entityRanges|data|5217117|var+a+=+"" 
a&&false+//->+false 
a%7C%7Cfalse+//->+""+(truthy) |code-block|syntax|javascript|5217118|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

A string IS a boolean truth value according to ECMA

<pre><code>var a = ""&lt;br/&gt;
a&amp;&amp;false //-&gt; false&lt;br/&gt;
a||false //-&gt; "" (truthy)&lt;br/&gt;
</code></pre>

Can you explain why the if condition doesn't work without the eval function:

<pre><code>var myBoolean= document.getElementById("someBoolean").value; //This is a 'false'

if(myBoolean)
{ 
 alert(Your boolean is True); //This condition always getting executed even though myBoolean is false;
}

if(eval(myBoolean))
{
 alert("You will never see this alert bcoz boolean is false");
}
</code></pre>

Javascript if condition on boolean

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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你能解释为什么if条件在没有eval函数的情况下不起作用：var myBoolean= document.getElementById("someBoolean").value;  //This is a 'false'if(myBoolean){   alert(Your boolean is True);  //This condition always getting executed ev

问布尔值上的Javascript if条件
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