blocks|key|3120859|text|在完全构建DOM之前，“就绪”处理程序中的代码将不会运行。一旦遇到处理程序外部的代码，它就会立即运行。因此，您的alert()在处理程序中的代码运行之前运行，因此结果非常合理:全局变量尚未初始化，因此它的值为undefined。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|3120860|通过将alert()+(或者更好地说，是console.log())调用放在“就绪”处理程序中，您可以清楚地看到执行的顺序：|3120861|$(document).ready(function()+{
++console.log("In+the+'ready'+handler");
});
console.log("Outside+the+'ready'+handler");|code-block|syntax|javascript|3120862|当该程序运行时，您将首先看到记录的"Outside“消息。|3120863|entityMap^0|1K|7|2W|9|0|3|7|K|D|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@$9|R|A|S|B|C]|$9|T|A|U|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|V|8|@$9|W|A|X|B|C]|$9|Y|A|Z|B|C]]|D|@]|E|$]]|$1|H|3|I|5|J|7|10|8|@]|D|@]|E|$K|L]]|$1|M|3|N|5|6|7|11|8|@]|D|@]|E|$]]|$1|O|3|-4|5|6|7|12|8|@]|D|@]|E|$]]]|P|$]]

The code inside the "ready" handler will not run until the DOM has been fully built. The code outside the handler will run as soon as it is encountered. Thus, your <code>alert()</code> runs before the code in the handler runs, so the outcome makes perfect sense: the global variable has not yet been initialized, so its value is <code>undefined</code>.

You can see the sequence of execution clearly by putting <code>alert()</code> (or, better, <code>console.log()</code>) calls inside the "ready" handler:

<pre><code>$(document).ready(function() {
 console.log("In the 'ready' handler");
});
console.log("Outside the 'ready' handler");
</code></pre>

When that runs, you'll see the "Outside" message logged first.

blocks|key|3120926|text|因为alert()是在文档完全准备好之前执行的。您甚至可以尝试在$(document).ready()之前声明变量，但它仍将返回未定义的..|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|3120927|entityMap^0|2|7|W|J|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|$]]

Because the <code>alert()</code> is executed before your document is perfectly ready.. You may try even by declaring the variable before <code>$(document).ready()</code> still it will return undefined..

blocks|key|4588358|text|$(document).ready()在页面完全加载后触发|type|unstyled|depth|inlineStyleRanges|entityRanges|data|4588359|当脚本标记完全加载时，将执行警报。|4588360|所以|4588361|4588362|脚本标记已加载=>+Execute+alert|ordered-list-item|4588363|继续加载页面|4588364|页面已完全加载=>+|4588365|+$(document).ready|4588366|You+var+is+set=>|4588367|4588368|在设置var之前，将执行警报|4588369|entityMap^0|0|0|0|0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|W|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|X|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|Y|8|@]|9|@]|A|$]]|$1|F|3|-4|5|6|7|Z|8|@]|9|@]|A|$]]|$1|G|3|H|5|I|7|10|8|@]|9|@]|A|$]]|$1|J|3|K|5|I|7|11|8|@]|9|@]|A|$]]|$1|L|3|M|5|I|7|12|8|@]|9|@]|A|$]]|$1|N|3|O|5|6|7|13|8|@]|9|@]|A|$]]|$1|P|3|Q|5|I|7|14|8|@]|9|@]|A|$]]|$1|R|3|-4|5|6|7|15|8|@]|9|@]|A|$]]|$1|S|3|T|5|6|7|16|8|@]|9|@]|A|$]]|$1|U|3|-4|5|6|7|17|8|@]|9|@]|A|$]]]|V|$]]

The $(document).ready() gets fired after the page is fully loaded

When the script tag is fully loaded the alert gets executed.

So

<ol>
<li>Script tag is loaded => Execute alert</li>
<li>Continue loading page</li>
<li>Page completly loaded => fire $(document).ready</li>
<li>You var is getting set</li>
</ol>

The alert gets executed before your var is set

blocks|key|4588371|text|其他答案是正确的，但还需要注意的是，$(document).ready(...)也会将您的变量隐藏在全局作用域之外。您可以声明变量，然后在函数中更新它|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|4588372|var+myVar;

$(document).ready(function()+{
++++myVar+=+"test";
});

console.log(myVar)+++//+test|code-block|syntax|javascript|4588373|entityMap^0|I|M|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@$9|N|A|O|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|P|8|@]|D|@]|E|$I|J]]|$1|K|3|-4|5|6|7|Q|8|@]|D|@]|E|$]]]|L|$]]

The other answers are correct but it is probably important to also note the <code>$(document).ready(...)</code> is also hiding your variable from the global scope. You could declare your variable then update it within the function

<pre><code>var myVar;

$(document).ready(function() {
 myVar = "test";
});

console.log(myVar) // test
</code></pre>

blocks|key|3120950|text|执行计划就像是|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3120951|//this+statement+shall+fix+the+driver,+not+run+it
$(document).ready(function()+{//-->this+it's+an+event+handler+waiting+to+be+fired+when+content+is+fully+loaded
+++myVar+=+"test";//-->myVar+won't+exists+until+this+event+is+triggered
});
//this+execute+the+alert+function+but+myVar+not+exist+yet
alert(typeof(myVar));|code-block|syntax|javascript|3120952|$(document).ready()类似于分配一个将在内容加载之后执行的事件，这意味着alert(myVar)将在设置为内容加载事件的lambda执行之前运行。我希望你能理解我。|offset|length|style|BOLD|3120953|entityMap^0|0|0|0|J|18|C|0^^$0|@$1|2|3|4|5|6|7|O|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|Q|8|@$I|R|J|S|K|L]|$I|T|J|U|K|L]]|9|@]|A|$]]|$1|M|3|-4|5|6|7|V|8|@]|9|@]|A|$]]]|N|$]]

The execution plan it's like

<pre><code>//this statement shall fix the driver, not run it
$(document).ready(function() {//--&gt;this it's an event handler waiting to be fired when content is fully loaded
 myVar = "test";//--&gt;myVar won't exists until this event is triggered
});
//this execute the alert function but myVar not exist yet
alert(typeof(myVar));
</code></pre>

$(document).ready() is like to assign an event who will execute after the content is loaded, which means alert(myVar) will run before the lambda execution which was set as the content-loaded event. I hope you'll understand me.

I have observed a strange behavior while learning jQuery and Javascript. When I call a variable that is defined inside the $(document).ready, from outside these tags it appears undefined, even when I define it as a global variable,
For example:

<pre><code>$(document).ready(function() {
 myVar = "test";
});
alert(typeof(myVar));
//Results "undefined"
</code></pre>

If I call the same variable inside the document.ready tags it works as expected

<pre><code>$(document).ready(function() {
 myVar = "test";
 alert(typeof(myVar));
 //Results "String"
});
</code></pre>

The result is same even after using window prefix.

<pre><code>$(document).ready(function() {
 window.myVar = "test";
});
alert(typeof(window.myVar));
//Results "undefined"
</code></pre>

I understand about the variable scopes but why even global variables aren't working this way. I am so confused.

Why variable appears undefined outside document ready tags

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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在学习jQuery和Javascript时，我观察到了一个奇怪的行为。当我调用一个在$(.ready)文档内部定义的变量时，从这些标记外部看它似乎是未定义的，即使我将它定义为一个全局变量，例如：$(document).ready(function() {   myVar = "test";});alert(typeof...

问为什么变量在文档就绪标记之外显示为未定义
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问为什么变量在文档就绪标记之外显示为未定义EN