Does the <script> tag position in HTML affects performance of the webpage?
Javascript assets, by default, tend to block any other parallel downloads from occurring. So, you can imagine if you have plenty of <script> tags in the head, calling on multiple external scripts will block the HTML from loading, thus greeting the user with a blank white screen, because no other content on your page will load until the JS files have completely loaded.
In order to combat this issue, many developers have opted to placing JS at the bottom of the HTML page (before the </body> tag). This seems logical because, most of the time JS is not required until the user begins interacting with the site. Placing JS files at the bottom also enables progressive rendering.
Alternatively, you can choose to load Javascript files asynchronously. There are plenty of existing methods which this can be accomplished by:
var xhrObj = getXHRObject();xhrObj.onreadystatechange = function() { if ( xhrObj.readyState != 4 ) return; eval(xhrObj.responseText); };xhrObj.open('GET', 'A.js', true);xhrObj.send('');Script DOM Element
var se = document.createElement('script');se.src = 'http://anydomain.com/A.js';document.getElementsByTagName('head')[0].appendChild(se);Meebo Iframed JS
var iframe = document.createElement('iframe');document.body.appendChild(iframe);var doc = iframe.contentWindow.document;doc.open().write('<body onload="insertJS()">');doc.close();To name a few...
Note: Only a maximum of five scripts can be loaded in parallel in current browsers.
ForIE there is the defer attribute you can use like so:
<script defer src="jsasset.js" type="text/javascript"></script>When set, this boolean attribute provides a hint to the user agent that the script is not going to generate any document content (e.g., no "document.write" in javascript) and thus, the user agent can continue parsing and rendering.
So I know this is an old discussion, but I've been reading about this topic and reading other answers on StackOverflow...
It actually doesn't matter where you put the jQuery tag anymore:
Finally, a word about persistent folklore. You may have encountered the frequently repeated advice to “always place JavaScript at the bottom of the page just before the closing tag”. This was once true because web browsers loaded scripts sequentially and blocked loading and rendering until each script was complete. This is no longer true; modern browsers do “preload scanning” and begin loading all scripts in parallel, whether listed in the head element or at the bottom of the page. External JavaScript often is loaded asynchronously and is written so it won’t execute until the page is loaded and the DOM is ready. Loading a script in the head element is no longer a bad practice.
http://railsapps.github.io/rails-javascript-include-external.html
Script elements at the top of the document are available sooner (so you can bind event handlers and have JS run as soon as an element becomes available), but they do block parsing of the rest of the page.
Script elements at the bottom aren't (so you can't) and there isn't any significant page left, so it doesn't matter if that is blocked.
Which is best depends on the relative importance priority (which has to be determined on a case-by-case basis) of having the JS running Vs. having the HTML rendering.
Note that a script element at the bottom must appear inside the body element. So it should be:
<script>...</script></body></html>and not
</body><script>...</script></html>