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.)
Here's what happens when a browser loads a website with a <script> tag on it:
Fetch the HTML page (e.g. index.html)
Begin parsing the HTML
The parser encounters a <script> tag referencing an external script file.
The browser requests the script file. Meanwhile, the parser blocks and stops parsing the other HTML on your page.
After some time the script is downloaded and subsequently executed.
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 and 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 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 and 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 and 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 src="path/to/script1.js" async></script>
<script 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 that script 2 is downloaded and executed before script 1.
According to http://caniuse.com/#feat=script-async, 97.78% of all browsers support this.
defer
<script src="path/to/script1.js" defer></script>
<script 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.
(To learn more and see some really helpful visual representations of the differences between async, defer and normal scripts check the first two links at the references section of this answer)
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 2% of browsers that do not support these attributes while speeding up the other 98%.
References
async vs defer attributes
Efficiently load JavaScript with defer and async
Remove Render-Blocking JavaScript
Async, Defer, Modules: A Visual Cheatsheet
Just before the closing body tag, as stated on Put Scripts at the 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.
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.
The standard advice, promoted by the Yahoo! Exceptional Performance team, is to put the <script> tags at the end of the document's <body> element so they don't block rendering of the page.
But there are some newer approaches that offer better performance, as described in this other answer of mine 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).
The modern approach is using ES6 'module' type scripts.
<script type="module" src="..."></script>
By default, modules are loaded asynchronously and deferred. i.e. you can place them anywhere and they will load in parallel and execute when the page finishes loading.
Further reading:
The differences between a script and a module
The execution of a module being deferred compared to a script(Modules are deferred by default)
Browser Support for ES6 Modules
If you are using jQuery then put the JavaScript code 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.
<script src="myjs.js"></script>
</body>
The script tag should always be used before the body close or at the bottom in HTML file.
The Page will load with HTML and CSS and later JavaScript will load.
Check this if required:
http://stevesouders.com/hpws/rule-js-bottom.php
The best place to put <script> tag is before closing </body> tag, so the downloading and executing it doesn't block the browser to parse the HTML in document,
Also loading the JavaScript files externally has its own advantages like it will be cached by browsers and can speed up page load times, it separates the HTML and JavaScript code and help to manage the code base better.
But modern browsers also support some other optimal ways, like async and defer to load external JavaScript files.
Async and Defer
Normally HTML page execution starts line by line. When an external JavaScript <script> element is encountered, HTML parsing is stopped until a JavaScript is download and ready for execution. This normal page execution can be changed using the defer and async attribute.
Defer
When a defer attribute is used, JavaScript is downloaded parallelly with HTML parsing, but it will be execute only after full HTML parsing is done.
<script src="/local-js-path/myScript.js" defer></script>
Async
When the async attribute is used, JavaScript is downloaded as soon as the script is encountered and after the download, it will be executed asynchronously (parallelly) along with HTML parsing.
<script src="/local-js-path/myScript.js" async></script>
When to use which attributes
If your script is independent of other scripts and is modular, use async.
If you are loading script1 and script2 with async, both will run
parallelly along with HTML parsing, as soon as they are downloaded
and available.
If your script depends on another script then use defer for both:
When script1 and script2 are loaded in that order with defer, then script1 is guaranteed to execute first,
Then script2 will execute after script1 is fully executed.
Must do this if script2 depends on script1.
If your script is small enough and is depended by another script
of type async then use your script with no attributes and place it above all the async scripts.
Reference: External JavaScript JS File – Advantages, Disadvantages, Syntax, Attributes
It 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).
The most conservative (and widely accepted) answer is "at the bottom just before the ending tag", 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.
It 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 at the top. If your styling is 100% CSS and you have all fallback options for the button actions then you can place it at the bottom.
Or the best thing (if that's not a concern) is you can make a modal loading box, place your JavaScript code 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.
Including scripts at the end is mainly used where the content/ styles of the web page 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 page.
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.
You can add JavaScript code in an HTML document by employing the dedicated HTML tag <script> that wraps around JavaScript code.
The <script> tag can be placed in the <head> section of your HTML, in the <body> section, or after the </body> close tag, depending on when you want the JavaScript to load.
Generally, JavaScript code can go inside of the document <head> section in order to keep them contained and out of the main content of your HTML document.
However, if your script needs to run at a certain point within a page’s layout — like when using document.write to generate content — you should put it at the point where it should be called, usually within the <body> section.
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 straightforward 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).
Script blocks DOM load until it's loaded and executed.
If you place scripts at the end of <body>, all of the DOM has a chance to load and render (the page will "display" faster). <script> will have access to all of those DOM elements.
On the other hand, placing it after the <body> start or above will execute the script (where there still aren't any DOM elements).
You are including jQuery which means you can place it wherever you wish and use .ready().
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 the head tag).
The best place to write your JavaScript code is at the end of the document after or right before the </body> tag to load the document first and then execute the JavaScript code.
<script> ... your code here ... </script>
</body>
And if you write in jQuery, the following can be in the head document and it will execute after the document loads:
<script>
$(document).ready(function(){
// Your code here...
});
</script>
If you still care a lot about support and performance in Internet Explorer before version 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 any more about in Internet Explorer before version 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 Internet Explorer before version 10, don't forget to wrap your code in a window.onload even, though!
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 the 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 a user's point of view, it would seems like that the page is loading faster.
Always, we have to put scripts before the closing body tag expect some specific scenario.
For Example :
`<html> <body> <script> document.getElementById("demo").innerHTML = "Hello JavaScript!"; </script> </body> </html>`
Prefer to put it before the </body> closing tag.
Why?
As per the official doc: https://developer.mozilla.org/en-US/docs/Learn/Getting_started_with_the_web/JavaScript_basics#a_hello_world!_example
Note: The reason the instructions (above) place the element
near the bottom of the HTML file is that the browser reads code in the
order it appears in the file.
If the JavaScript loads first and it is supposed to affect the HTML
that hasn't loaded yet, there could be problems. Placing JavaScript
near the bottom of an HTML page is one way to accommodate this
dependency. To learn more about alternative approaches, see Script
loading strategies.
I'm confused about where to place JavaScript functions:
When should they be put within the head
When inline within the body
And, when after the closing html tag?
thanks
The rules are fast and loose on this, There is no right or wrong way only better and less-better. (well after the </html> is wrong)
Generally speaking, javascript in the head of the document might block rendering of the page until the file is loaded in some browsers *cough*IE*cough*. This is due to a limit of simultaneous connections. So some people put them before the closing html tag. You can use a library to asynchronously load the javascript to avoid this blocking.
If you're using a library, or checking for the DOM to be loaded before executing code there's really no issue of where it's placed. However if you're not doing that it's probably better to put it at the end.
Javascript can always be safely placed in the head to make the functionality available to the entire page. Note that this can block loading of the rest of the document, so if you are loading very large or external Javascript, you may wish to load them inline near the end of the body.
Javascript placed inline will become available when executed. This allows you to conditionally load JS when page elements are loaded.
Javascript should always be placed in the <head> or <body>, never after </html>.
I agree, never seen (nor could I recommend) after html. Also, as noted, blocking is the concern. What I often go with is one script reference in the head to yepnope (an async js loader and testing tid bit (now included with modernizr)), and a tiny block of inline js at the end of the body tag that loads a bootstrap js file.
In that file, I use another yepnope request to load other needed assets asynchronously, and kick off initialization methods.
I've also taken to putting my Google Analytics code in a final yepnope block, so that it's the last thing to load, even after the js app boots.
I am about to embark on a new web project and I plan to put some JavaScripts in the <head> and also some before </body>, using the following scheme:
Scripts that are essential for the UX of the page: in the <head>. As I've picked up perusing the web - scripts in the <head> is loaded before the page loads, so it would make sense to put scripts that are essential to the user experience there.
Scripts that are non-essential to the design and UX (Google Analytics scripts etc.): before the </body>.
Is this a sensible approach?
Another approach would be to put all the scripts in the <head> and add defer attributes to the non-essential scripts. However, I read that older versions of Firefox don't pick up the defer attribute.
I think a lot of developers run JavaScript just before the </body> so that it is run after all the elements have been rendered.
However, if you organise your code correctly, the position on the page doesn't matter.
For example, when using jQuery, you can ensure the code isn't run until the page and its elements are fully rendered by doing the following:
$(document).ready(function(){
//Code here
});
Then the script reference can be put in the head tag.
Script tags should be referenced just before </body>. This prevents render blocking while the scripts load and is much better for site perception speed.
No obtrusive JavaScript should be used when using this technique.
JavaScript code should be placed at the end of the document so that it doesn't delay the parallel loading of page elements. This does then require that the JavaScript code is written in a specific way, but it does improve the speed of page loads.
Also, ideally you could host references like this under a different (sub)domain. References to jQuery should be pointed to Google's CDN too.
See Best Practices for Speeding Up Your Web Site for more information.
One of the reasons you'd want to put scripts before the </body> is if they manipulate the DOM without user interaction, so you'll need the DOM to be loaded in order to be manipulated. Another way to do that is to add an event listener and run the scripts when the page has loaded, but this will require additional code, which might get complicated if you have a lot of scripts, especially ones you haven't written yourself. Putting them at the end of the page also will speed up page load, though in the case of DOM manipulating scripts you might get some not-so-pretty results from that.
I'd say that's perfectly sensible. As you said, as long as you don't move essential scripts (e.g. jQuery, Modernizr, etc., etc.) out from the <head>, you shouldn't have problems.
Moving non-essential scripts to the bottom of the page should help with the perceived loading speed (that and minimizing / concatenating scripts).
It all depends on what you mean by "essential for UX". I agree with having Modernizr appear early for example, but not everything needs to load straight away. If you're trying to avoid a flash of unstyled text (FOUT), that's a good reason. Similarly, if you have scripts that affect how the page looks before the user does anything, you should load those early.
Don't forget though, speed is part of UX. There's no advantage in having some jQuery interaction ready to run when the user can't see the content it applies to yet. The difference between loading the scripts at the start of the end is a matter of seconds. If you let the page load first, the user will be using those seconds to take the page in, allowing you to load scripts unobtrusively.
Your page will load faster if you move scripts to the bottom of the page, and that makes a difference to your pagerank these days.
Also, some versions of Internet Explorer will throw errors if you try to run a script before the element it refers to has loaded.
Like Ed says, your scripts should be stored in a separate file, and in as few files as possible.
Put the JavaScript code in a separate file and place a link to it in the head part of the HTML.
I've read that it is better to place your <script> tags at the end of the document. The argument for doing this appears to be that the browser will stall rendering the page below a script tag until it has loaded and and executed the script. If your script tag is at the top of the page, rendering is stalled for a while, which is bad.
However, I am not sure if this is really true any more.
Looking around, I normally see the following locations...
In the <head> of the page or Just inside the <body> tag
Stackoverflow is an example of a site that puts the script tags in the head, and since they are normally rather obsessed with performance, I am starting to wonder if position in the page is important at all.
Last thing in the body element
The other common place to put javascript appears to be right at the very end of the <body> element. I am assuming this means that the page can render while the javascript downloads and gets on with doing its thing.
But which is better?
Does anyone have any thoughts or advice on this? I am looking to try and get our pages to perform and appear to the user as quickly as possible.
Does it matter? What are the advantages of being at the top of the page? Bottom of the page?
It really depends.
There is no catch all answer for this because it depends on what your javascripts are acting on.
Putting scripts at the end of the page is sometimes needed if your acting on a DOM element that needs to be loaded for the script to run. Say you want to focus on a control and your script is:
var mytext = document.getElementById("mytext2");
mytext.focus();
Well in this case you would want it to execute at the end of the page, after mytext2 control has already been loaded by the browser. This is less important for script blocks that only contain functions that are called by events.
If you have a big .js file that contains libraries of functions you may also want to put that at the end of the page so the browser will load the page faster before loading the large .js file.
Google analytics suggests putting their tracker at the end, to make sure the page has been delivered before you count the hits, but in some cases it suggests putting the script into the header too, it works both ways.
So, rule of thumb for me is, HEAD scripts for everything except things that execute in-line and act on DOM objects, or large scripts that you want to load after the page.
Rick Strahl just wrote a great blog on placement of Javascript in .net too:
The only validating way is to include it on the top (in the <head> section), but in the bottom will be faster to load - the rest of the page will load faster if you have script near the bottom, giving the user better response and making the experience better.
The problem is that most web browser stop rendering the HTML of the page until they've fetched and parsed all JavaScript code so far. So if you have a slow-loaded .js file included in the <head>, no HTML will be rendered and images will not even start to download before the .js have been downloaded and parsed, therefore frontend engineers propagate for putting the scripts as far down in the code as possible.
I usually just set a far-future Expires header for my .js files so they are cached in the browser for a long time and then include them in the <head> section. This gives good performance and doesn't look ugly :-)
But if you are serving external .js libraries (that are on other servers than your own), you will probably want them in the bottom because you can't change the Expires-header for other servers and you canät know that the other server always will be responsive.
yeah. Put Scripts at the Bottom
I think the size of the js file is much more important than the location of javascript. I always set highter number of the con-current connection to make sure they download in parallel.
I believe it's better to place script tags just before the closing body tag. Because:
Elements are blocked from rendering if they are below the script.
In IE6, IE7 resources in the page are blocked from downloading if they are below the script.
From this article. Also Yahoo's performance rule 6 is Move Scripts to the Bottom
Google Analytics always used to say to put the script tag at the bottom of the page. I believe the rationale was that if the Google servers ever went down, the page would fail to load if it were in the head (only for IE probably).