在 HTML 标记中，我应该在哪里放置脚本标记？

When embedding JavaScript in an HTML document, where is the proper place to put the <script> tags and included JavaScript? I seem to recall that you are not supposed to place these in the <head> section, but placing at the beginning of the <body> section is bad, too, since the JavaScript will have to be parsed before the page is rendered completely (or something like that). This seems to leave the end of the <body> section as a logical place for <script> tags.

So, where is the right place to put the <script> tags?

(This question references this question, in which it was suggested that JavaScript function calls should be moved from <a> tags to <script> tags. I'm specifically using jQuery, but more general answers are also appropriate.)

转载于:https://stackoverflow.com/questions/436411/where-should-i-put-script-tags-in-html-markup

Here's what happens when a browser loads a website with a <script> tag on it:

  1. Fetch the HTML page (e.g. index.html)
  2. Begin parsing the HTML
  3. The parser encounters a <script> tag referencing an external script file.
  4. The browser requests the script file. Meanwhile, the parser blocks and stops parsing the other HTML on your page.
  5. After some time the script is downloaded and subsequently executed.
  6. The parser continues parsing the rest of the HTML document.

Step #4 causes a bad user experience. Your website basically stops loading until you've downloaded all scripts. If there's one thing that users hate it's waiting for a website to load.

Why does this even happen?

Any script can insert its own HTML via document.write() or other DOM manipulations. This implies that the parser has to wait until the script has been downloaded & executed before it can safely parse the rest of the document. After all, the script could have inserted its own HTML in the document.

However, most JavaScript developers no longer manipulate the DOM while the document is loading. Instead, they wait until the document has been loaded before modifying it. For example:

<!-- index.html -->
<html>
    <head>
        <title>My Page</title>
        <script type="text/javascript" src="my-script.js"></script>
    </head>
    <body>
        <div id="user-greeting">Welcome back, user</div>
    </body>
</html>

Javascript:

// my-script.js
document.addEventListener("DOMContentLoaded", function() { 
    // this function runs when the DOM is ready, i.e. when the document has been parsed
    document.getElementById("user-greeting").textContent = "Welcome back, Bart";
});

Because your browser does not know my-script.js isn't going to modify the document until it has been downloaded & executed, the parser stops parsing.

Antiquated recommendation

The old approach to solving this problem was to put <script> tags at the bottom of your <body>, because this ensures the parser isn't blocked until the very end.

This approach has its own problem: the browser cannot start downloading the scripts until the entire document is parsed. For larger websites with large scripts & stylesheets, being able to download the script as soon as possible is very important for performance. If your website doesn't load within 2 seconds, people will go to another website.

In an optimal solution, the browser would start downloading your scripts as soon as possible, while at the same time parsing the rest of your document.

The modern approach

Today, browsers support the async and defer attributes on scripts. These attributes tell the browser it's safe to continue parsing while the scripts are being downloaded.

async

<script type="text/javascript" src="path/to/script1.js" async></script>
<script type="text/javascript" src="path/to/script2.js" async></script>

Scripts with the async attribute are executed asynchronously. This means the script is executed as soon as it's downloaded, without blocking the browser in the meantime.
This implies that it's possible to script 2 is downloaded & executed before script 1.

According to http://caniuse.com/#feat=script-async, 94.57% of all browsers support this.

defer

<script type="text/javascript" src="path/to/script1.js" defer></script>
<script type="text/javascript" src="path/to/script2.js" defer></script>

Scripts with the defer attribute are executed in order (i.e. first script 1, then script 2). This also does not block the browser.

Unlike async scripts, defer scripts are only executed after the entire document has been loaded.

According to http://caniuse.com/#feat=script-defer, 94.59% of all browsers support this. 94.92% support it at least partially.

An important note on browser compatibility: in some circumstances IE <= 9 may execute deferred scripts out of order. If you need to support those browsers, please read this first!

Conclusion

The current state-of-the-art is to put scripts in the <head> tag and use the async or defer attributes. This allows your scripts to be downloaded asap without blocking your browser.

The good thing is that your website should still load correctly on the 6% of browsers that do not support these attributes while speeding up the other 94%.

If you are using JQuery then put the javascript wherever you find it best and use $(document).ready() to ensure that things are loaded properly before executing any functions.

On a side note: I like all my script tags in the <head> section as that seems to be the cleanest place.

Just before the closing body tag, as stated on

http://developer.yahoo.com/performance/rules.html#js_bottom

Put Scripts at the Bottom

The problem caused by scripts is that they block parallel downloads. The HTTP/1.1 specification suggests that browsers download no more than two components in parallel per hostname. If you serve your images from multiple hostnames, you can get more than two downloads to occur in parallel. While a script is downloading, however, the browser won't start any other downloads, even on different hostnames.

The conventional (and widely accepted) answer is "at the bottom", because then the entire DOM will have been loaded before anything can start executing.

There are dissenters, for various reasons, starting with the available practice to intentionally begin execution with a page onload event.

XHTML Won't Validate if the script is anywhere other than within the head element. turns out it can be everywhere.

You can defer the execution with something like jQuery so it doesn't matter where it's placed (except for a small performance hit during parsing).

Depending on the script and its usage the best possible (in terms of page load and rendering time) may be to not use a conventional <script>-tag per se, but to dynamically trigger the loading of the script asynchronously.

There are some different techniques, but the most straight forward is to use document.createElement("script") when the window.onload event is triggered. Then the script is loaded first when the page itself has rendered, thus not impacting the time the user has to wait for the page to appear.

This naturally requires that the script itself is not needed for the rendering of the page.

For more information, see the post Coupling async scripts by Steve Souders (creator of YSlow but now at Google).

The standard advice, promoted by the Yahoo! Exceptional Performance team, is to put the <script> tags at the end of the document body so they don't block rendering of the page.

But there are some newer approaches that offer better performance, as described in this answer about the load time of the Google Analytics JavaScript file:

There are some great slides by Steve Souders (client-side performance expert) about:

  • Different techniques to load external JavaScript files in parallel
  • their effect on loading time and page rendering
  • what kind of "in progress" indicators the browser displays (e.g. 'loading' in the status bar, hourglass mouse cursor).

It makes more sense to me to include the script after the HTML. Because most of the time I need the Dom to load before I execute my script. I could put it in the head tag but I don't like all the Document loading listener overhead. I want my code to be short and sweet and easy to read.

I've heard old versions of safari was quarky when adding your script outside of the head tag but I say who cares. I don't know anybody using that old crap do you.

Good question by the way.

Depends, if you are loading a script that's necessary to style your page / using actions in your page (like click of a button) then you better place it on the top. If your styling is 100% CSS and you have all fallback options for the button actions then you can place it in the bottom.

Or best thing (if that's not a concern) is you can make a modal loading box, place your javascript at the bottom of your page and make it disappear when the last line of your script gets loaded. This way you can avoid users using actions in your page before the scripts are loaded. And also avoid the improper styling.

You can place where you want the scripts and one is not better than another practice.

the situation is as follows:

The page load linearly, "top-down", so if you put the script in the head ensures that it starts to load before everything, now, if you put it inside the body mixed with the code can cause page loads a unsightly manner.

identify good practice does not depend on where.

to support you, I will mention the following:

you can place:

and the page will load linearly

page is loaded asynchronously with other content

the content of the page will load before and after completion of loading the scripts are loaded

good practice here would be, when will implement each?

I hope I've been helpful, anything just answer me this issue.

Non-blocking script tags can be placed just about anywhere:

<script src="script.js" async></script>
<script src="script.js" defer></script>
<script src="script.js" async defer></script>
  • async script will be executed asynchronously as soon as it is available
  • defer script is executed when the document has finished parsing
  • async defer script falls back to the defer behavior if async is not supported

Such scripts will be executed asynchronously/after document ready, which means you cannot do this:

<script src="jquery.js" async></script>
<script>jQuery(something);</script>
<!--
  * might throw "jQuery is not defined" error
  * defer will not work either
-->

Or this:

<script src="document.write(something).js" async></script>
<!--
  * might issue "cannot write into document from an asynchronous script" warning
  * defer will not work either
-->

Or this:

<script src="jquery.js" async></script>
<script src="jQuery(something).js" async></script>
<!--
  * might throw "jQuery is not defined" error (no guarantee which script runs first)
  * defer will work in sane browsers
-->

Or this:

<script src="document.getElementById(header).js" async></script>
<div id="header"></div>
<!--
  * might not locate #header (script could fire before parser looks at the next line)
  * defer will work in sane browsers
-->

Having said that, asynchronous scripts offer these advantages:

  • Parallel download of resources:
    Browser can download stylesheets, images and other scripts in parallel without waiting for a script to download and execute.
  • Source order independence:
    You can place the scripts inside head or body without worrying about blocking (useful if you are using a CMS). Execution order still matters though.

It is possible to circumvent the execution order issues by using external scripts that support callbacks. Many third party JavaScript APIs now support non-blocking execution. Here is an example of loading the Google Maps API asynchronously.

Script blocks DOM load untill it's loaded and executed.

If you place scripts at the end of <body> all of DOM has chance to load and render (page will "display" faster). <script> will have access to all of those DOM elements.

In other hand placing it after <body> start or above will execute script (where there's still no DOM elements).

You are including jQuery which means you can place it wherever you wish and use .ready()

  • If you still care a lot about support and performance in IE<10, it's best to ALWAYS make your script tags the last tags of your HTML body. That way, you're certain that the rest of the DOM has been loaded and you won't block and rendering.

  • If you don't care too much anymore about IE<10, you might want to put your scripts in the head of your document and use defer to ensure they only run after your DOM has been loaded (<script type="text/javascript" src="path/to/script1.js" defer></script>). If you still want your code to work in IE<10, don't forget to wrap your code in a window.onload even, though!

<script src="myjs.js"></script>
</body>

script tag should be use always before body close or Bottom in HTML file.

then you can see content of the page first before loading js file.

check this if require : http://stevesouders.com/hpws/rule-js-bottom.php

I think it depends on the webpage execution. If the page that you want to display can not displayed properly without loading JavaScript first then you should include JavaScript file first. But If you can display/render a webpage without initially download JavaScript file, then you should put JavaScript code at the bottom of the page. Because it will emulate a speedy page load, and from an user's point of view it would seems like that page is loading faster.

You can place most of <script> references at the end of <body>,
But If there are active components on your page which are using external scripts,
then their dependency (js files) should come before that (ideally in head tag).

the best place to write your JavaScript code in the end of document code after tag to load the document then execute js code. and if u write JQuery code write

$(document).ready (function{

//your code here

});

At The End of HTML Document

So that it will not effect the loading of the HTML document in the browser at the time of execution.

Including scripts at the end is mainly used where the content/ styles of the website is to be shown first.

including the scripts in the head loads the scripts early and can be used before the loading of the whole web site.

if the scripts are entered at last the validation will happen only after the loading of the entire styles and design which is not appreciated for fast responsive websites.

first of all to imagine my chart and understand explanation well you should know this color what refer

enter image description here

you should put them in the end of body before closing tag (body) this help you to ignore any error

<body>
  <script></script>
</body>

but you can put them before head closing tag,

<head>
  <script></script>
</head>

but this leads to a very bad user experience since the browser isn't really doing anything useful while the script file is downloading. but what is happen When the browser comes across the above line in your markup, this is what happens.

  1. Pause parsing the document.
  2. Make a request to fetch the file.
  3. Execute the script after it has been downloaded.
  4. Resume parsing the document.

enter image description here

so when it's find any error this effect in our content, but i want to put my script in the top of my markup and i don't know i'v error or not to do this you have two attribute i think this will help you to download your markup well not effect with script

1.First Attribute

Async : When you add the async attribute to your script tag, the following will happen.

 <script src="myfile1.js" async></script>
 <script src="myfile2.js" async></script>
  1. Make parallel requests to fetch the files.
  2. Continue parsing the document as if it was never interrupted.
  3. Execute the individual scripts the moment the files are downloaded.

enter image description here

The great thing of this flow is that scripts can download in parallel while the document is being parsed. But there’s a caveat to this and that the third point – the script will be executed the moment it is downloaded. This can be a non-issue if a script is completely self contained. However, in many situations, scripts may depend on other scripts to have done some initialization before they can execute. e.g. jquery plugins require the jquery variable to already exist on the page.

ATTETION: Scripts that you programmatically insert into the DOM are async by default, unless you explicitly set their async attribute to false at insertion time.

  1. Second Attribute

Defer : Defer is very similar to async with one major difference. Here’s what happens when a browser encounters a script with the defer attribute.

<script src="myfile1.js" defer></script>
<script src="myfile2.js" defer></script>
  1. Make parallel requests to fetch the individual files.
  2. Continue parsing the document as if it was never interrupted.
  3. Finish parsing the document even if the script files have downloaded.
  4. Execute each script in the order they were encountered in the document.

As you can tell, defer is pretty much what you want to do in your files. However, due to limited browser support, its not a viable option at the time of writing. enter image description hereATTENTION: The async and defer attributes are ignored for scripts having no src attribute.

but When should I use what? Typically you want to use async where possible, then defer then no attribute. Here are some general rules to follow:

  • If the script is modular and does not rely on any scripts then use
    async.
  • If the script relies upon or is relied upon by another script then use defer.
  • If the script is small and is relied upon by an async script then use an inline script with no attributes placed above the async scripts.