TechQA.

How to defer inline Javascript?

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I have the following html code:

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Document</title>
    <script src="https://cdn.jsdelivr.net/blazy/1.8.2/blazy.min.js" defer></script>
    <script src="https://code.jquery.com/jquery-2.1.4.min.js" integrity="sha256-8WqyJLuWKRBVhxXIL1jBDD7SDxU936oZkCnxQbWwJVw=" crossorigin="anonymous" defer></script>
    <script src="https://cdnjs.cloudflare.com/ajax/libs/lightbox2/2.9.0/js/lightbox.min.js" defer></script>
    <script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/js/bootstrap.min.js" integrity="sha384-Tc5IQib027qvyjSMfHjOMaLkfuWVxZxUPnCJA7l2mCWNIpG9mGCD8wGNIcPD7Txa" crossorigin="anonymous" defer></script>
    <!-- 26 dec flexslider js -->
    <script src="https://cdnjs.cloudflare.com/ajax/libs/flexslider/2.6.3/jquery.flexslider.min.js" defer></script>
    <script defer>
    (function($) {
        $(document).ready(function() {
            //do something with b-lazy plugin, lightbox plugin and then with flexslider
        });
    })(jQuery);
    </script>
</head>
<body>
</body>
</html>

I get an error, saying jQuery is not defined. Now even if I remove defer from my inline JS code, it says jQuery is undefined. For some reason I have to keep the jQuery plugins in the head and keep my JS code inline. My question is:

  1. Why doesn't inline Javascript code get deferred when defer attribute is present on it?

  2. Is there a way to imitate the defer behavior on my inline Javascript code? I can put that at the end of body tag if required.

javascript jquery html deferred-execution
Original Q&A
10

There are 10 answers

13
trincot On BEST ANSWER

The scripts with the defer attribute load in the order they are specified, but not before the document itself has been loaded. As defer has no effect on script tags unless they also have the src attribute, the first script that gets executed is your inline script. So at that time jQuery is not loaded yet.

You can solve this in at least two ways:

  • Put your inline script in a .js file and reference it with a src attribute (in addition to the defer attribute which you already had there), or

  • Let your inline script wait for the document and the deferred scripts to be loaded. The DOMContentLoaded event will fire when that has happened:

    <script>
    window.addEventListener('DOMContentLoaded', function() {
        (function($) {
            //do something with b-lazy plugin, lightbox plugin and then with flexslider
        })(jQuery);
    });
</script>

NB: Notice that in the latter case $(document).ready(function() is not included any more, as that would wait for the same event (DOMContentLoaded). You could still include it like you had in your original code, but then jQuery would just execute the callback immediately, which makes no practical difference.

4
Jai On

From MDN docs:

defer
This Boolean attribute is set to indicate to a browser that the script is meant to be executed after the document has been parsed, but before firing DOMContentLoaded. The defer attribute should only be used on external scripts.

This is called an IIFE (Immediately Invoked Function Expression) which gets executed before DOM is available. So, in that case jQuery is undefined because it it not in the DOM.

3
Brook Jordan On

You can create a Base64 URL out of the script and put it into the src!

<script src="data:text/javascript;base64,YWxlcnQoJ0hlbGxvIHdvcmxkIScpOw=="
        defer>
</script>

I built a quick test to see it in action. You should see an alert with Hello world! last if defer is working:

<script defer>
  alert('Why no defer?!?');
</script>

<!-- alert('Hello world!'); -->
<script src="data:text/javascript;base64,YWxlcnQoJ0hlbGxvIHdvcmxkIScpOw=="
        defer></script>

<script>
  alert('Buh-bye world!');
</script>

Doing it manually is a little laborious so if you have the luxury of compiling your HTML in some way (Handlebars, Angular, etc.) then that helps a lot.

I'm currently using:

<script src="data:text/javascript;base64,{{base64 "alert('Hello world!');"}}"
        defer>
</script>
2
Zombo On

You can also use type="module":

<meta charset="utf-8">

<script type="module">
let t = document.getElementById('top');
console.log(t);
</script>

<h1 id="top">Top Questions</h1>

https://developer.mozilla.org/docs/Web/HTML/Element/script#attr-type

0
Turab On

I checked all the proposed solutions but all have their disadvantages. So I invented my own.

Put this inline script into your head tag or right after the start of body tag:

<script>var Defer = []; document.addEventListener('DOMContentLoaded', function() { while (Defer.length) Defer.shift().call(); }); </script>

This one liner will collect all the inline scripts you want to defer and run them respectively as soon as document is fully loaded. Now anytime you need to run an inline script deferred, just register it like:

<script>
  alert('This alert will show immediately.');

  Defer.push(function() {
   alert('This alert will show only after document is loaded.');
   // You can use anything which is not loaded yet, like jQuery
   $(".selector").doSomeJqueryStuff();
  });

  // You can use it as many times as you like and in any place of your DOM.
  Defer.push(function() {
    // Any inline code you want to defer
  });
</script>

This inline script will run only after document is loaded. That means you can run inline jQuery script having your jQuery stay at the end of your DOM.

0
zakariamouhid On

You can use this data url as src attribute

data:application/javascript,eval(document.currentScript.textContent)

which takes this current script tag and evaluate its content as if it was inside an external file. it also works with lazy attribute. it uses document.currentScript which not supported by IE browsers.

<script defer src="https://cdn.jsdelivr.net/npm/vue"></script>
<script defer src="data:application/javascript,eval(document.currentScript.textContent)">
    console.log('defered', typeof Vue); // function
</script>
<script>
    console.log('not defered', typeof Vue); // undefined
</script>

1
Michal Miky Jankovský On

defer loading with plain text Data URI - Chrome and FF

#noLib #vanillaJS

suggest not to use on Cross Browser PRODuction

until MS IE dies and MS Edge will adopt the Chromium open source ;)

the only way to defer script is external file or Data_URI (without using event DOMContentLoaded)

defer

spec script#attr-defer (MDN web docs): "This attribute must not be used if the src attribute is absent (i.e. for inline scripts), in this case it would have no effect.)"

Data_URI

spec Data_URI

with right type "text/javascript" there is no need to base64 at all... ;)

using plain text so you can use simple:

<script defer src="data:text/javascript,

//do something with b-lazy plugin, lightbox plugin and then with flexslider

lightbox.option({
  resizeDuration: 200,
  wrapAround: true
})

">

yes, it's little bit weird hack, but <script type="module"> are deferred by default, there is no other option to mix following in exact order:

  • module external files - deferred by default
  • module inline scripts - deferred by default
  • external files - optionally deferred
  • inline scripts - only with this hack - as I know (without libraries/frameworks)
1
shinsenter On

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 (note: I am the author of this script) 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

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.

0
AnyWhichWay On

There is a somewhat less obscure way to accomplish deferral that does not require callbacks, promises, or data urls ... although it does a little DOM manipulation in the background. The tiny library (109 bytes compressed/gziped) https://www.npmjs.com/package/deferscript let's you do this. The example below is based on the original post.

<script src="https://cdnjs.cloudflare.com/ajax/libs/flexslider/2.6.3/jquery.flexslider.min.js" defer>
</script>
<script src="./deferscript.js" defer>
    (function($) {
        $(document).ready(function() {
            //do something with b-lazy plugin, lightbox plugin and then with flexslider
        });
    })(jQuery);
</script>

All you have to do is insert a src attribute with the value ./deferscript.js.

1
Manuel Córdoba On

If the problem is that jQuery variable $ is not defined, maybe you can create a fake $ function that returns a ready function waiting for the DOMContentLoaded?

All my inline scripts has $(document).ready(..... and the problem is that $ is not defined as the header scripts are deferred.

So, just add a fake $ in an inline script in head:

<script type="text/javascript">
var $ = function(element) {
    return {
        ready: function(callback) {
            // in case the document is already rendered
            if (document.readyState!="loading") callback();
            // modern browsers
            else if (document.addEventListener) 
                document.addEventListener("DOMContentLoaded", callback);
            // IE <= 8
            else document.attachEvent("onreadystatechange", function(){    
                if (document.readyState=="complete") callback();
            });
        }
    };
};
</script>

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