for example:
<body>
...all content is above the script...
<script src="https://foo/bar.js" defer></script>
</body>
Does it matter if we remove the defer from the script tag? By putting the script at the end of the body tag is already delaying the execution of the script so the above code should be the same as the following snippet right?
<body>
...all content is above the script...
<script src="https://foo/bar.js"></script>
</body>
According to Ben
The current best practice? Use deferred scripts in order in the head, unless you need to support older browsers (IE < 10, Opera Mini,
etc.) - 97.45% browser usage (ref)
Why? With defer, parsing finishes just like when we put the script at the end of the body tag, but overall the script execution finishes
well before, because the script has been downloaded in parallel with
the HTML parsing. This scenario will trigger the faster domInteractive
event that is used for page loading speed. With async, the order in
which your script will execute varies based on how fast the script is
fetched, so order can be compromised. Futhermore, async scripts are
executed inline and pause the parsing of the HTML.
Also from Mhmdrz_A
Browser will parse the document from top to bottom, hence putting the
scripts after all the main content will make the parser to reach that
later in time; which make the browser to download the script
later in time; The delay caused by putting the defer in the bottom is
absolutely non-sense, since the browser won't execute them (s) before (or during) HTML parser;
Thus it's better to let the browser load ( download ) them () as soon as possible, an have them available to execute as soon
as the all the main tasks are done.
Does it matter if we remove the defer from the script tag? By putting
the script at the end of the body tag is already delaying the
execution of the script so the above code should be the same as the
following snippet right?
Right you are! In this case, I'd say that there is no difference. As you correctly mention: the body tag is before the script, and that means it gets parsed first. Your html file has no idea there is any javascript until it has fully parsed the preceding code, and therefore defer does nothing.
Defer's only job is to run JavaScript after all the html has been parsed: see the MDN docs.
That said, with slightly different code, with say, another <script /> underneath is where the difference is found. That second (or more) <script /> tag will then begin to download in parallel (but executed in order). For example:
<script src="script1.js" />
<script src="script2.js" />
script1 is downloaded first, blocking script2 from downloading.
<script src="script1.js" defer />
<script src="script2.js" defer />
script1 is starts to download, then script2 also starts to download in parallel — but — script1 still gets executed in first, even if script2 finishes downloading first.
Further reading:
Detailed stackoverfow answer on script execution order in html
W3Schools description for <script /> (& it's attributes) in html
MDN docs on defer
Hope this helps!
Practically there will be no difference between these two.
Except that document.write (which shouldn't be used anyways) obviously is not allowed in defer.
Technically those are different. So the difference between a script having defer and one that does not, is that the one with defer is executed once the DOM is fully loaded, and the one without at the time where it is loaded.
This can make a difference if the client received the <script src="https://foo/bar.js" defer></script> tag, but the rest of the document is for some reason delayed due to whatever reason. The defer script can only execute if the complete response is sent.
But if that happens then you have other problems anyway.
How do I set the load and execution order of two external async Javascript files?
Given the following...
<script src="framework.js" async></script> // Larger file
<script src="scripts.js" async></script> // Small file
Though second in order scripts.js is downloading and executing before framework.js due to it's file size, but scripts.js is dependent on framework.js.
Is there a way natively to specify the load and execution order whilst still maintaining async properties?
You want to use defer if you want to preserve the execution order. What defer does is it async downloads the script, but defers execution till html parsing is done.
<script src="framework.js" defer></script>
<script src="scripts.js" defer></script>
However, you may want to start creating custom bundles once the number of scripts go higher.
You can see the difference here
If you need a solution for IE9 (since it does not support the defer attribute), I have created a small loader script:
https://github.com/mudroljub/js-async-loader
It loads all your scripts asynchronously and then executes them in order.
I know that in general, the scripts are loaded and executed in the order. But I have a problem that they are not executed in the right order, sometime the second script is executed and then the first script.
I am using jsp and tile. In the jsp template, I only load the common js files and then load specific js files for specific tile
It seems like browser loads and executes the scripts asynchronously
How to force it to load and execute scripts sequentially?
I've searched a lot of places for the answer but not work.
Template:
<html>
<head>
<title></title>
//load common js files
</head>
<body>
<tiles:insert attribute="header"/>
<tiles:insert attribute="content"/>
<tiles:insert attribute="footer"/>
</body>
</html>
Specific tile:
<div>
Test
</div>
//load js files
<script type="text/javascript" src="1.js"></script>
<script type="text/javascript" src="2.js"></script>
<script type="text/javascript" src="3.js"></script>
Edit:
These javascript files are very simple, they just print out number 1,2 and 3
But sometime It prints the wrong order.
The "content" div will be replaced by the tile and I don't know whether it is a type of loading resource dynamically: Dynamically loading JavaScript synchronously
There is a great article on html5rocks about script loading.
http://www.html5rocks.com/en/tutorials/speed/script-loading/
the simplest way would be to use defer but it's not supported by IED<10 and opera mini.
<script src="//other-domain.com/1.js" defer></script>
<script src="2.js" defer></script>
Spec says: Download together, execute in order just before
DOMContentLoaded. Ignore “defer” on scripts without “src”.
IE < 10 says: I might execute 2.js halfway through the execution of 1.js. Isn’t that fun??
The ultimate solution but less supported would a combination of dynamic inclusion and forcing async attribute to false.
[
'1.js',
'2.js'
].forEach(function(src) {
var script = document.createElement('script');
script.src = src;
script.async = false;
document.head.appendChild(script);
});
it requires a work around using onreadystatechange on IE<10
After doing some interesting reading on async, I couldn't find any information on what's supposed to happen if you first have a regular script tag, followed by one with async.
I tried a quick test and it seems to do what I would hope: if I put a non-async tag first, it seems to always load it before the later async attributed tag. Here's the code I'm testing with:
test.html:
<!doctype html>
<html>
<head><title>test</title></head>
<body>
<script src="https://code.jquery.com/jquery-2.1.1.min.js"></script>
<script src="test.js" async></script>
</body>
</html>
test.js:
alert(window.jQuery);
In every case it seems that jQuery is loaded. Maybe I'm missing something in my tests. Is this correct? Can I put dependencies in non-async tags and then the dependents using async? Is it particularly beneficial to do this?
Are there any gotchas? What would happen if you had some asyncs and then a non-async? Would the async'd js load first?
Here the questions I can answer. I'm not 100% sure its a long time ago.
That first script is sync loaded. That means that it will wiat until that file is fully loaded and after that it proceeds to the next line.
Depends on when you need this file to be loaded. If it has to be loaded before the page is loaded, then you shouldn't use async for that file.
The asyncs will load simultaneously with the one non-async. If you have two non-asyncs at the end. That two non-asyncs will load sync. (DUH)
The async will load when it is on that line.
By it I mean the reader
Non-async script tags block processing of the page. The browser may look ahead and load test.js before jquery has been executed, but the browser can not process the script as it does not know what jquery does to the page (document.writeln, changing the window object..).
So, yes it will work.
Doing this will make loading the page a bit faster, as one of the two script can be loaded asynchronously. It will probably not match the startup time of an optimizing, asynchronous loader though.
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.