blocks|key|124875|text|据我所知，你能做到|type|unstyled|depth|inlineStyleRanges|entityRanges|data|124876|if(typeof+X+==+'undefined')|code-block|syntax|javascript|124877|但是有一些资源加载器你可能想看一看。摆在我面前的答案也是正确的。|124878|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|$]]

As far as I know you can do

<pre><code>if(typeof X == 'undefined')
</code></pre>

But there are resources loader you might want to take a look at. And the answer given before me is also correct.

blocks|key|125742|text|if(jQuery)应该足够了，不是吗？|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|125743|entityMap^0|0|A|0^^$0|@$1|2|3|4|5|6|7|H|8|@$9|I|A|J|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|K|8|@]|D|@]|E|$]]]|G|$]]

<code>if(jQuery)</code> should be enough, shouldn't it?

blocks|key|5442168|text|来自here|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|5442169|if(typeof+jQuery+==+"undefined")+{+
++document.write("undefined");+
}else{+
+++document.write("Exists");+
}+|code-block|syntax|javascript|5442170|entityMap|0|LINK|mutability|MUTABLE|url|http://www.satyakomatineni.com/akc/display?url=displaynoteimpurl&ownerUserId=satya&reportId=2129^0|2|4|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@$A|R|B|S|1|T]]|C|$]]|$1|D|3|E|5|F|7|U|8|@]|9|@]|C|$G|H]]|$1|I|3|-4|5|6|7|V|8|@]|9|@]|C|$]]]|J|$K|$5|L|M|N|C|$O|P]]]]

From <a href="http://www.satyakomatineni.com/akc/display?url=displaynoteimpurl&amp;ownerUserId=satya&amp;reportId=2129" rel="nofollow">here</a> 

<pre><code>if(typeof jQuery == "undefined") { 
 document.write("undefined"); 
}else{ 
 document.write("Exists"); 
} 
</code></pre>

blocks|key|1145088|text|流程1:|type|unstyled|depth|inlineStyleRanges|offset|length|style|BOLD|entityRanges|data|1145089|if+(jQuery)+{++
++++//+jQuery+is+loaded++
}+else+{
++++//+jQuery+is+not+loaded
}|code-block|syntax|javascript|1145090|流程2:|1145091|if+(typeof+jQuery+==+'undefined')+{++
++++//+jQuery+is+not+loaded++
}+else+{
++++//+jQuery+is+loaded
}|1145092|entityMap^0|0|4|0|0|0|4|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@$9|R|A|S|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|T|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|U|8|@$9|V|A|W|B|C]]|D|@]|E|$]]|$1|M|3|N|5|H|7|X|8|@]|D|@]|E|$I|J]]|$1|O|3|-4|5|6|7|Y|8|@]|D|@]|E|$]]]|P|$]]

Process 1:

<pre><code>if (jQuery) { 
 // jQuery is loaded 
} else {
 // jQuery is not loaded
}
</code></pre>

Process 2:

<pre><code>if (typeof jQuery == 'undefined') { 
 // jQuery is not loaded 
} else {
 // jQuery is loaded
}
</code></pre>

blocks|key|5442259|text|您的代码中名为"jQuery“的变量从未声明过，因此它将抛出类似”xxx(变量名)未定义“的错误。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|5442260|您可以使用typeof运算符检查变量是否未定义|offset|length|style|CODE|5442261|if+(typeof(jQuery)+==+"undefined")|code-block|syntax|javascript|5442262|entityMap^0|0|5|6|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|P|8|@$D|Q|E|R|F|G]]|9|@]|A|$]]|$1|H|3|I|5|J|7|S|8|@]|9|@]|A|$K|L]]|$1|M|3|-4|5|6|7|T|8|@]|9|@]|A|$]]]|N|$]]

The variable called "jQuery" in your code has never been declared, so it will throw an error like "xxx(variable name) is not defined".

You can use the <code>typeof</code> operator to check is a variable is undefined or not

<pre><code>if (typeof(jQuery) == "undefined")
</code></pre>

blocks|key|1145132|text|有几种解决方案：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1145133|1145134|使用typeof。它是一个特殊的运算符，永远不会产生ReferenceError。对于undefined值或上下文中不存在的变量，它的计算结果为“未定义”。我不是它的粉丝，但它似乎很常见。|ordered-list-item|offset|length|style|CODE|1145135|使用window.jQuery。这将强制执行“属性查找”：属性查找永远不会失败，如果所述属性不存在，则返回undefined。我在一些框架中看到过它的使用。它的缺点是假设一个上下文(通常是window).|1145136|Make，确保变量是“声明的”：var+jQuery;+if+(jQuery)+{+/*+yay+*/+}。看起来不是很常见，但它是完全有效的。请注意，var只是一个注释，并且已被提升。在全局上下文中，这将创建property.|1145137|Catch+ReferenceError的"jQuery“。老实说，我从来没有见过它，我也不推荐它，但它会起作用的。|1145138|1145139|祝你编码愉快。|1145140|entityMap^0|0|0|2|6|Q|E|17|9|0|2|D|1H|9|2M|6|0|G|11|24|3|0|6|E|0|0|0^^$0|@$1|2|3|4|5|6|7|U|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|V|8|@]|9|@]|A|$]]|$1|C|3|D|5|E|7|W|8|@$F|X|G|Y|H|I]|$F|Z|G|10|H|I]|$F|11|G|12|H|I]]|9|@]|A|$]]|$1|J|3|K|5|E|7|13|8|@$F|14|G|15|H|I]|$F|16|G|17|H|I]|$F|18|G|19|H|I]]|9|@]|A|$]]|$1|L|3|M|5|E|7|1A|8|@$F|1B|G|1C|H|I]|$F|1D|G|1E|H|I]]|9|@]|A|$]]|$1|N|3|O|5|E|7|1F|8|@$F|1G|G|1H|H|I]]|9|@]|A|$]]|$1|P|3|-4|5|6|7|1I|8|@]|9|@]|A|$]]|$1|Q|3|R|5|6|7|1J|8|@]|9|@]|A|$]]|$1|S|3|-4|5|6|7|1K|8|@]|9|@]|A|$]]]|T|$]]

There are several solutions:

<ol>
<li>Use <code>typeof</code>. It is a special operator and will never result in a <code>ReferenceError</code>. It evaluates to "undefined" for, well, the <code>undefined</code> value or for a variable which does not exist in context. I'm not a fan of it, but it seems very common.</li>
<li>Use <code>window.jQuery</code>. This forces a "property lookup": property lookups never fail, and return <code>undefined</code> if said property does not exist. I've seen it used in some frameworks. Has the downside of assuming a context (usually <code>window</code>).</li>
<li>Make sure the variable is "declared": <code>var jQuery; if (jQuery) { /* yay */ }</code>. Doesn't seem very common, but it is entirely valid. Note that <code>var</code> is just an annotation and is hoisted. In the global context this will create the "jQuery" property.</li>
<li>Catch the <code>ReferenceError</code>. Honestly, I have never seen this nor do I recommend it, but it would work.</li>
</ol>

Happy coding.

blocks|key|5442370|text|您可以使用:+if(+jQuery+!==‘未定义’的类型)|type|unstyled|depth|inlineStyleRanges|entityRanges|data|5442371|或|5442372|按照mozilla的建议进行操作|5442373|if(window中的‘jQuery’)|5442374|https://developer.mozilla.org/en-US/docs/JavaScript/Reference/Global_Objects/undefined|offset|length|5442375|entityMap|0|LINK|mutability|MUTABLE|url^0|0|0|0|0|0|2E|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|T|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|U|8|@]|9|@]|A|$]]|$1|F|3|G|5|6|7|V|8|@]|9|@]|A|$]]|$1|H|3|I|5|6|7|W|8|@]|9|@$J|X|K|Y|1|Z]]|A|$]]|$1|L|3|-4|5|6|7|10|8|@]|9|@]|A|$]]]|M|$N|$5|O|P|Q|A|$R|I]]]]

You can use :
if( typeof jQuery !== 'undefined')

or

Do what is recommended by mozilla

if('jQuery' in window)

<a href="https://developer.mozilla.org/en-US/docs/JavaScript/Reference/Global_Objects/undefined" rel="nofollow">https://developer.mozilla.org/en-US/docs/JavaScript/Reference/Global_Objects/undefined</a>

I always thought I could just check an undefined var by comparing it to undefined, but this is the error I get in the chrome console :

<img src="https://i.stack.imgur.com/7JIQm.png" alt="enter image description here">

How how do i check for the object jQuery being undefined ?

EDIT :

<img src="https://i.stack.imgur.com/7JIQm.png" alt="enter image description here">

if(jQuery) is giving me problems too

EDIT :

solution :

<code>if(window.jQuery)</code> works. 
<code>typeof(jQuery) == 'undefined'</code> works too.

Could anyone explain why ?

How to check if a variable or object is undefined?

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

我一直认为我可以通过将未定义的变量与未定义的变量进行比较来检查它，但这是我在chrome控制台中得到的错误：​​如何检查未定义的对象jQuery？编辑：​​如果(JQuery)也给我出了问题编辑：解决方案：if(window.jQuery)起作用了。typeof(jQuery) == 'undefined'也可以工作。...

问如何检查变量或对象是否未定义？
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

问如何检查变量或对象是否未定义？EN