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|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|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|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|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|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。

文章

问答

视频

学习中心

腾讯云实验室

直播

竞赛

腾讯云代码分析专区

腾讯iOA零信任安全管理系统专区

腾讯云架构师技术同盟交流圈

腾讯云数据库专区

腾讯云顾问专区

腾讯云原生专区

腾讯混元专区

腾讯云TCE专区

腾讯云Lighthouse专区

腾讯云HAI专区

腾讯云Edgeone专区

腾讯云存储专区

腾讯云智能专区

腾讯轻联专区 

腾讯云开发专区

TAPD专区

腾讯轻量云游戏服专区

腾讯云最具价值专家

腾讯云架构师技术同盟

腾讯云创作之星

腾讯云开发者先锋

腾讯云代码助手

云原生构建

TAPD 敏捷项目管理

Cloud Studio

SDK中心

API中心

命令行工具

涵盖代码开发、场景应用、自动测试全流程，助你从零构建专属AI助手

一站式MCP教程库，解锁AI应用新玩法

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);  //T...

问布尔值上的Javascript if条件
EN

Stack Overflow用户

社区

活动

圈层

关于

腾讯云开发者

热门产品

热门推荐

更多推荐

问布尔值上的Javascript if条件EN