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Simply mimic the 'defer' attribute for inline scripts across all browsers (jQuery helper)
<!-- Somewhere along your page, you have some javascript which you want to defer. Just set its type to text/javascript/defer -->
<script type="text/javascript/defer">
$('body').css('background', 'green');
<!-- maybe even an external file -->
<script type="text/javascript/defer" src=""></script>
<!-- then near the bottom, you load libraries & dependencies like you should -->
<script src=""></script>
<!-- Assuming one of them is jQuery, just add this thingy and voila! Your scripts will run on `ready`. Note: Execution order isn't guaranteed for external files -->
<!-- Unminified, if you're interested:
$(this).clone().attr('type', '').insertAfter(this);
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wizzwizz4 commented Apr 12, 2018

It was never a great solution, since jQuery has to be undeferred in order for it to work (unless you have this script external and then defer it). If reimplemented in plain JavaScript it still wouldn't be great, since text/javascript/defer isn't a valid value for type (though most web browsers will just "deal with it"). If changed to use a data- attribute, the only problem I can see is that it doesn't seem to stop the JavaScript from running initially, so if you had something like:

<script type="text/javascript/defer">
  var x = document.getElementById("example").textContent;
  document.getElementById("example").textContent = "";

then it would wipe the contents of #example before performing an HTML eval on the contents, since it could only perform that HTML eval the second time when it was run with jQuery present. Also there'd be a lot of errors posted to the console and not a lot you could do about it.

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shinsenter commented Mar 23, 2019

Defer/async script tags are not good enough

There is a common knowledge that you should use <script src=".." async defer> (or set script.async = true before assigning src, when you do it from JS) and/or put your scripts at the very bottom of the page, so that as much as possible of the page gets loaded and rendered to the user, as fast as possible.

defer.js is written in plain JavaScript, making lazy-loading other contents more fast and performant. You can defer any javascript files as well as inline script blocks efficiently.

Defer loading of JavaScript

Defer loading of JavaScript

All you need to do now is just wrap all your code in defer() function.

<script type="text/javascript">
    // This will defer showing an alert message after 2 seconds
    defer(function() {
        alert("This message is shown after 2 seconds after the 'load' event.");
    }, 2000);

     // Append a HTML content to the <body> tag using jQuery
     deferscript('', 'jquery', 500, function () {
         $('body').html('<p>Your awesome content</p>');

Or defer loading and execution of external scripts with deferscript() function.

<script type="text/javascript">
    // Alternative way to lazy load Google Tag Manager script
    deferscript('//', 'google-tag', 500, function() {
        // Run extra code right after the script has been loaded
        (window.dataLayer = window.dataLayer || []).push('config', 'UA-34520609-2');

If your page is just an HTML page enhanced with some JavaScript, then you're good with just <script async>. It takes time for browser to parse and execute those scripts, and each UI change may reflow your layout, make your load speed more slow, no one likes staring at a blank white page; users are impatient and will leave quickly.

In various cases, using async or defer does not deliver faster page speed than defer.js does.

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