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Chrome for me has always been a reference on web standards, unfortunately the defer isn't supported and IE supports it since version 5.5. Why ?
js.js
document.getElementById ("hi").innerHTML = "Hi :)"; HTML
<!DOCTYPE html> <html> <head> <meta charset = "utf-8"> <script defer="defer" src="js.js"></script> <title>Hi</title> </head> <body> <div id="hi"></div> </body> </html> 
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The defer attribute is a boolean attribute. If the defer attribute is set, it specifies that the script is downloaded in parallel to parsing the page, and executed after the page has finished parsing. Note: The defer attribute is only for external scripts (should only be used if the src attribute is present).
You should use defer for all other scripts. defer is great because it: Gets loaded as soon as possible — so it reduces load times. Doesn't execute until everything you need is ready — so all the DOM you need is there.
The defer is a Boolean value, used to indicate that script is executed after the document has been parsed. It works only with external scripts (i.e., works only when we are specifying the src attribute in <script> tag). It declares that the script will not create any content.
The data-pagespeed-no-defer attribute can change the order of script execution because those scripts using it are executed inline while those not using it are deferred. This can cause errors if these scripts depend on each other in any way.
defer and async mean?By default, a <script src=...></script> tag is evil! The browser must halt parsing the HTML until the script is downloaded and executed (since the script might call document.write(...) or define global variables that later scripts depend on). This means that any images and stylesheets that are after the script tag don't start downloading until after the script has finished downloading and executing. External scripts typically make the Web load much more slowly, which is why NoScript has become so popular.
Microsoft introduced defer to solve the problem. If you use <script defer src=...></script>, you promise not to call document.write(...). A defer external script will start downloading immediately but won't execute until after the page is rendered. After the page has rendered, all defer scripts are executed in the same order that they were declared. Not all browsers implement defer yet.
HTML5 introduced the async attribute which may execute any time--possibly before the page has finished parsing or even before other defer/async scripts that are still downloading. But it's harder to use multiple async scripts because their execution order is not guaranteed. Like defer, not all browsers implement async yet.
After all defer and async scripts have executed, the DOMContentLoaded and load events fire.
defer and async defer.defer, but unfortunately it doesn't say exactly when defer scripts execute (All in order? Before onload?). Thus, no other browsers implement defer because no one wants to reverse-engineer IE's behavior or break scripts that might depend on IE's peculiarities. (See the Mozilla feature request, for example).defer: defer scripts should all be executed in order after the rest of the page is parsed, and before onload. It also introduces async to specify scripts that can execute whenever they are downloaded without having to wait for each other. Unfortunately, HTML5 contradicts IE by not allowing inline defer scripts. This breaks the invariant that all defer scripts are executed in order (if some defer scripts have src and some have inline content).defer.async.defer and async are checked into Webkit. You should see it in Chrome and Safari very soon (it's already in the Chrome dev channel but it's a bit buggy).defer and async and for IE to implement async.There's no single rule to follow at this time. You have to choose the solution that best balances simplicity, page render latency, and script execution latency for the set of browsers that access your website.
<script async defer src=...></script>: This allows rendering to continue in parallel to script downloading for IE and the newest HTML5 browsers but causes pre-HTML5 browsers (including all versions of Opera) to block.defer (but not async) and they will be executed in the order that they were declared (except IE<=9 in certain conditions can execute them out of order). Again, this allows rendering to continue in parallel to script downloading in IE and HTML5-aware Gecko/Webkit, but older browsers and Opera will suffer. It's a good idea to use defer even if the scripts are at the bottom of the page so that they download in parallel with each other.defer for inline scripts because the HTML5 draft has taken away the execution order guarantee.async attribute. In other words, most older browsers treat it like a script at the bottom of the page, and newest browsers recognize the async. But Opera users get the worst of both worlds, because Opera begins execution immediately and doesn't understand async. This is the pattern recommended by Google Analytics for the urchin on many webpages.snippet:
<script> (function() { var script = document.createElement('script'); script.src = '...'; script.async = true; var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(script, s); })(); </script> Update: If you have scripts split into modules and want to improve performance, I recommend the "Coupling Asynchronous Scripts" chapter of Even Faster Web Sites by Steve Souder. It contains tips/tricks for not only controlling execution order but also to delay parsing of scripts to improve performance.
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defer is supported only by internet explorer. you don't need to rely on it. there are other ways to get the same effect, like placing your scripts at the end of the page just before the </body> tag, as others have pointed out.
The purpose of defer is to tell the externally linked script to wait until the page finishes loading until it runs. The same thing can be accomplished via good unobtrusive JavaScript methods, which usually include code that prevents scripts from executing until the DOM is finished loading.
The advantage of defer occurs in connection with Internet Explorer, since that browser is the only one that supports the defer attribute. So, if you need a quick script to run only in IE, and you don’t mind if the entire page loads before it starts to execute, then simply add defer="defer" in your tag and that will quickly take care of that problem. Fixing a transparent PNG issue in IE6 is one possible practical use for defer.
The defer attribute must be used when hiding a script from other browsers with a conditional comment that targets an IE-only script — otherwise the script will run normally in other browsers.)
Reference: http://www.impressivewebs.com/10-javascript-quick-tips-and-best-practices/
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