blocks|key|2642745|text|简单答案：|type|unstyled|depth|inlineStyleRanges|offset|length|style|BOLD|entityRanges|data|2642746|在一个简单的场景中(标签是原始HTML文本的一部分)，浏览器肯定会一个接一个地执行它们。|2642747|带不同警告的详细讨论|2642748|JavaScript不一定是单线程的(这取决于您的JavaScript引擎的实现，例如参见Web+Workers)。|2642749|但是，各个<script>标记是按顺序执行的。|CODE|2642750|参考文档请参考JavaScript:+The+Definitive+Guide。引用"12.3.+JavaScript程序的执行“章节：|2642751|出现在和标记之间的|2642752|+JavaScript语句按出现的顺序执行；当一个文件中出现多个脚本时，脚本将按出现的顺序执行。如果脚本调用document.write(+)，则传递给该方法的任何文本都会紧跟在结束标记之后插入到文档中，并在脚本运行结束时由HTML解析器进行解析。相同的规则也适用于包含在具有src属性的单独文件中的脚本。|blockquote|2642753|2642754|2642755|2642756|请注意，上述情况只适用于“直接”执行标记中的代码。但是，顺序可能会受到以下因素的影响：|2642757|2642758|setTimeout()调用(duh)|unordered-list-item|2642759|defer+attribute|2642760|Dynamic+attachement+of+<script>标签-请参阅此答案的最后一节。|2642761|2642762|2642763|作为警告，请注意，通过<script+src="xxxx"+/>从外部加载的JavaScript代码仍将按顺序执行，但是，浏览器很可能会以并行方式下载代码-这取决于浏览器的实现(但仍会按正确的顺序安排下载代码片段的执行)。|2642764|这个警告很重要，以防你想进行一些奇怪的攻击，而JavaScript源代码的URL实际上是一个CGI脚本，它做了一些事情，并且你试图依赖于脚本中逻辑的正确下载顺序。|2642765|同样，它也不会影响浏览器JS引擎对这些脚本片段的执行顺序。|2642766|2642767|然而，一个更重要的警告是，如果你真的用外部源动态附加<script>标签(例如，通过appendChild()调用)，根据this+SO+post，以及MSDN+blog+the+post+was+based+on，非IE浏览器不保证执行的顺序！这将取决于哪个标签的代码首先完成下载！|2642768|entityMap|0|LINK|mutability|MUTABLE|url|http://en.wikipedia.org/wiki/Web_Workers|1|http://www.w3schools.com/tags/att_script_defer.asp|2|http://blogs.msdn.com/b/kristoffer/archive/2006/12/22/loading-javascript-files-in-parallel.aspx|3^0|0|5|0|0|6|4|0|19|B|0|0|5|8|0|7|W|0|0|0|0|0|0|0|0|0|C|0|0|F|1|0|N|8|0|0|0|2|2|B|L|0|0|0|0|Q|8|16|D|36|9|1P|C|2|24|V|3|0^^$0|@$1|2|3|4|5|6|7|1W|8|@$9|1X|A|1Y|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|1Z|8|@]|D|@]|E|$]]|$1|H|3|I|5|6|7|20|8|@$9|21|A|22|B|C]]|D|@]|E|$]]|$1|J|3|K|5|6|7|23|8|@]|D|@$9|24|A|25|1|26]]|E|$]]|$1|L|3|M|5|6|7|27|8|@$9|28|A|29|B|N]]|D|@]|E|$]]|$1|O|3|P|5|6|7|2A|8|@$9|2B|A|2C|B|N]]|D|@]|E|$]]|$1|Q|3|R|5|6|7|2D|8|@]|D|@]|E|$]]|$1|S|3|T|5|U|7|2E|8|@]|D|@]|E|$]]|$1|V|3|-4|5|6|7|2F|8|@]|D|@]|E|$]]|$1|W|3|-4|5|6|7|2G|8|@]|D|@]|E|$]]|$1|X|3|-4|5|6|7|2H|8|@]|D|@]|E|$]]|$1|Y|3|Z|5|6|7|2I|8|@]|D|@]|E|$]]|$1|10|3|-4|5|6|7|2J|8|@]|D|@]|E|$]]|$1|11|3|12|5|13|7|2K|8|@$9|2L|A|2M|B|N]]|D|@]|E|$]]|$1|14|3|15|5|13|7|2N|8|@]|D|@$9|2O|A|2P|1|2Q]]|E|$]]|$1|16|3|17|5|13|7|2R|8|@$9|2S|A|2T|B|N]]|D|@]|E|$]]|$1|18|3|-4|5|6|7|2U|8|@]|D|@]|E|$]]|$1|19|3|-4|5|6|7|2V|8|@]|D|@]|E|$]]|$1|1A|3|1B|5|6|7|2W|8|@$9|2X|A|2Y|B|C]|$9|2Z|A|30|B|N]]|D|@]|E|$]]|$1|1C|3|1D|5|6|7|31|8|@]|D|@]|E|$]]|$1|1E|3|1F|5|6|7|32|8|@]|D|@]|E|$]]|$1|1G|3|-4|5|6|7|33|8|@]|D|@]|E|$]]|$1|1H|3|1I|5|6|7|34|8|@$9|35|A|36|B|N]|$9|37|A|38|B|N]|$9|39|A|3A|B|C]]|D|@$9|3B|A|3C|1|3D]|$9|3E|A|3F|1|3G]]|E|$]]|$1|1J|3|-4|5|6|7|3H|8|@]|D|@]|E|$]]]|1K|$1L|$5|1M|1N|1O|E|$1P|1Q]]|1R|$5|1M|1N|1O|E|$1P|1S]]|1T|$5|1M|1N|1O|E|$1P|1U]]|1V|$5|1M|1N|1O|E|$1P|1U]]]]

<p><strong>Simple answer:</strong> </p>

<p>In a simple scenario (tags are part of original HTML text), the browser definitely executes them one after another.</p>

<p><strong>Detailed discussion with different caveats</strong></p>

<p>JavaScript isn't necessarily single-threaded (it depends on the implementation of your JavaScript engine, e.g. see <a href="http://en.wikipedia.org/wiki/Web_Workers" rel="noreferrer">Web Workers</a>).</p>

<p>BUT, the individual <code>&lt;script&gt;</code> tags are executed sequentially.</p>

<p>For reference, please see <code>JavaScript: The Definitive Guide</code>. Quoting Chapter "12.3. Execution of JavaScript Programs":</p>

<blockquote>
  <p>JavaScript statements that appear between  and  tags are executed in order of appearance; when more than one script appears in a file, the scripts are executed in the order in which they appear. If a script calls document.write( ), any text passed to that method is inserted into the document immediately after the closing  tag and is parsed by the HTML parser when the script finishes running. The same rules apply to scripts included from separate files with the src attribute. </p>
</blockquote>

<hr>

<p>Please note that the above is only true of "straight up" execution of code in tags. The order can, however, be affected by:</p>

<ul>
<li><p><code>setTimeout()</code> calls (duh)</p></li>
<li><p><a href="http://www.w3schools.com/tags/att_script_defer.asp" rel="noreferrer">defer attribute</a></p></li>
<li><p>Dynamic attachement of the <code>&lt;script&gt;</code> tags - see the last section of this answer.</p></li>
</ul>

<hr>

<p>As a <strong>caveat</strong>, please note that JavaScript code loaded externally via <code>&lt;script src="xxxx" /&gt;</code> would still be executed sequentially, BUT, it is quite possible that the browser would <strong>DOWNLOAD</strong> the code in parallel - depends on browser implementation (but still schedule the execution of downloaded code snippets in correct order). </p>

<p>This caveat is important in case you want to have some weird hack whereas the URL for the JavaScript source is actually a CGI script which does something and you try to depend on the correct order of downloads for the logic in the script. </p>

<p>Again, it would have no bearing on your browser JS engine's execution order of those script pieces.</p>

<hr>

<p>However, a far more <strong>important caveat</strong> is that if you actually attach the <code>&lt;script&gt;</code> tags with external sources dynamically (e.g. via <code>appendChild()</code> call), according to <a href="http://blogs.msdn.com/b/kristoffer/archive/2006/12/22/loading-javascript-files-in-parallel.aspx" rel="noreferrer">this SO post</a>, as well as the <a href="http://blogs.msdn.com/b/kristoffer/archive/2006/12/22/loading-javascript-files-in-parallel.aspx" rel="noreferrer">MSDN blog the post was based on</a>, non-IE browsers <strong>do NOT guarantee the order of execution!</strong> It will depend on which tag's code finished downloading first!</p>


blocks|key|2642801|text|在处理过程中，浏览器不是将所有的javascript编译成一个“文件”吗？例如，如果您在多个文件上有多个$(document).ready()调用，那么当浏览器对页面进行预处理时，它基本上会压缩所有内容以供执行-并按“看到”的顺序依次运行它。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2642802|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

<p>Doesn't the browser compile all the javascript anyway into one "file" during processing. For example, if you had multiple $(document).ready() calls across multiple files, when the browser pre-processes the page, it essentially condenses everything down for execution - and runs it sequentially in the order it was 'seen'. </p>


blocks|key|576032|text|浏览器按顺序执行JavaScript+(+jQuery也是如此，因为jQuery就是JavaScript)。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|576033|至于在HTML中有多个脚本标记，这没有理由成为问题。正如Nabab所问的，我很有兴趣看看你的消息来源。|576034|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|F|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|G|8|@]|9|@]|A|$]]|$1|D|3|-4|5|6|7|H|8|@]|9|@]|A|$]]]|E|$]]

<p>The browser executes JavaScript sequentially (the same is true for jQuery since jQuery is just JavaScript).</p>

<p>As for having multiple script tags in HTML, there's no reason why this would be a problem. As Nabab asked, I would be interested in seeing your source for that. </p>


blocks|key|2642878|text|实例化jQuery对象的调用越少，开销就越小--但即使是为运行在第二代硬件上的旧浏览器设计的，也要小心微优化。分析您的应用程序并修复实际成为瓶颈的部分。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2642879|至于浏览器处理多个脚本标记的方式--它因浏览器而异，因版本而异，有时甚至因操作系统而异。所有浏览器都按文档顺序执行每个脚本标记：|2642880|<script+src="scripts/some_script.js"></script>+
<script+src="scripts/some_other_script.js"></script>+
<script>
//+Some+JavaScript
</script>+
<script>
//+Some+More+JavaScript
</script>+|code-block|syntax|javascript|2642881|但是，其他行为没有定义，并且存在差异。例如，Opera+(至少在Windows+XP+For+version+10.6上)在其自己的上下文中运行每个脚本标记--因此，如果在第四个脚本块中引用，则第三个脚本块中的局部变量将超出范围。|2642882|<script>
var+i+=+42;
</script>
<script>
alert(i);
//+Alerts+"undefined"+in+Opera,+but+42+in+Firefox.
</script>|2642883|entityMap^0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|P|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|Q|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|R|8|@]|9|@]|A|$]]|$1|K|3|L|5|F|7|S|8|@]|9|@]|A|$G|H]]|$1|M|3|-4|5|6|7|T|8|@]|9|@]|A|$]]]|N|$]]

<p>The fewer calls you make that instantiate a jQuery object, the less overhead you have -- but even if you are designing for old browsers running on 2nd generation hardware be wary of micro-optimizations.  Profile your application and fix the parts that <em>actually</em> are the bottlenecks.</p>

<p>As for the way browsers handle multiple script tags -- it varies from browser to browser, from version to version, and sometimes even from operating system to operating system.  All browsers execute each script tag in document order:</p>

<pre><code>&lt;script src="scripts/some_script.js"&gt;&lt;/script&gt; &lt;!-- Executed 1st --&gt;
&lt;script src="scripts/some_other_script.js"&gt;&lt;/script&gt; &lt;!-- Executed 2nd --&gt;
&lt;script&gt;
// Some JavaScript
&lt;/script&gt; &lt;!-- Executed 3rd --&gt;
&lt;script&gt;
// Some More JavaScript
&lt;/script&gt; &lt;!-- Executed 4th --&gt;
</code></pre>

<p>However, other behaviors are not defined and there is variation.  For example, Opera (at least on Windows XP for version 10.6) ran each script tag in its own context -- so local variables in the 3rd script block would be out of scope if referred to in the 4th script block.</p>

<pre><code>&lt;script&gt;
var i = 42;
&lt;/script&gt;
&lt;script&gt;
alert(i);
// Alerts "undefined" in Opera, but 42 in Firefox.
&lt;/script&gt;
</code></pre>


blocks|key|2642913|text|是的，我们可以在标签中写任意数量的标签。并且浏览器按顺序访问它们。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2642914|+<html>
+++<body>

++++

++++++<script></script>
++++++<script></script>
++++++<script></script>
++++++++++++++.
++++++++++++++.
++++++++++++++.
++++++<script></script>

++++</body>
</html>|code-block|syntax|javascript|2642915|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|$]]

<p>Yes, we can write any number of  tags inside  tag. And browser access them in sequential order.</p>

<pre><code> &lt;html&gt;
   &lt;body&gt;

    &lt;!-- here your code--&gt;

      &lt;script&gt;&lt;/script&gt;
      &lt;script&gt;&lt;/script&gt;
      &lt;script&gt;&lt;/script&gt;
              .
              .
              .
      &lt;script&gt;&lt;/script&gt;

    &lt;/body&gt;
&lt;/html&gt;
</code></pre>


<p>I have read that it is not recommended to instantiate jQuery multiple times in your HTML. This makes perfect sense to me, but:
Isn't Javascript single-threaded anyway? And leaving jQuery behind, how does the browser execute these multiple script tags? In parallel or one after another?</p>

<p>Thanks, Philip</p>


Implications of multiple <script> tags in HTML

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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我读到过，不推荐在HTML中多次实例化jQuery。这对我来说很有意义，但是: Javascript不是单线程的吗？抛开jQuery不谈，浏览器如何执行这多个脚本标记？并行的还是一个接一个的？谢谢，菲利普

问多个<script>标签在超文本标记语言中的含义
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