In order to optimize the load of my document, I use to load jquery async like that
<script async type="text/javascript" src="js/jquery-1.12.3.min.js"></script>
Then I call a script using jquery :
<script type="text/javascript">
jQuery(document).ready(function() {
App.init();
OwlCarousel.initOwlCarousel();
FancyBox.initFancybox();
StyleSwitcher.initStyleSwitcher();
});
</script>
It returns me that jquery is not defined.
I don't know what should I use, I though that .readyfunction would wait untill all document is loaded before calling it.
the same for boostrap library, It tells me that jquery is not defined.
I've tried to ask the script to be loaded at the end, but it still does not work properly.
Anykind of help will be much appreciated.
Since jquery script is loaded asynchronously, jquery is not loaded on the moment your script is executing. So you need to wait for it to load by subscribing on load event like this:
<script async id="jquery" type="text/javascript" src="https://code.jquery.com/jquery-2.2.3.js"></script>
Then I call a script using jquery :
<script type="text/javascript">
document.getElementById('jquery').addEventListener('load', function () {
App.init();
OwlCarousel.initOwlCarousel();
FancyBox.initFancybox();
StyleSwitcher.initStyleSwitcher();
});
</script>
But I don't know why someone wants to do things like this.
To optimize page loading speed it is better to put all you javascript at the end of the body, so content will be loaded first, and scripts won't delay page rendering event if it's synchronously loaded.
Question Script Tag - async & defer has good answer to your problem.
In a nutshell you cannot load jQuery, or other library, asyncronously when some other script depends on it without some additional asyncronous handling for executing the scripts depending on the library.
That is my solution:
<script async type="text/javascript" src="path_to_jquery" id="jquery_script_tag">
</script>
<script>
if ( !('jQuery' in window) )
{
window.jQueryQueue = [];
//define temporary jQuery function
window.jQuery = function(){
//just save function parameters to queue
jQueryQueue.push( arguments );
}
document.getElementById('jquery_script_tag').addEventListener('load', function(){
//call jQuery with parameters saved in queue, after it loaded
for ( var i in jQueryQueue )
jQuery.apply( document, jQueryQueue[i] );
});
}
</script>
This code defines a temporary jQuery function if it is yet not defined. The function saves all jQuery function calls to queue while the real jQuery has not yet loaded (instead of calling undefined jQuery function). And when jQuery has loaded, it calls the jQuery functions in the queue with the same parameters as called before.
jQuery, and all components that depend on jQuery (including Bootstrap), depend on hooking the DOMContentLoaded event to set up events.
This means jQuery (and anything that uses $(function() {...})) must be downloaded before DOMContentLoaded fires, or it never hooks up its events.
In turn, that means <script async ... will be unreliable, breaking whenever jQuery takes longer to download than the page content does.
Unfortunately <script defer ... doesn't work either thanks to jQuery trying to fire as soon as the document.readyState === "interactive".
You can load content and then load jQuery by putting the <script> at the end of the <body> but this will result in a noticeable pause between the page appearing and any of the jQuery firing - users won't be able to click on anything, visual components won't appear, and so on.
You Used Async Loading When You Try Access Jquery It Not Loaded By Browser You Can Access Jquery After Page Loading Is Complete .
Load Jquery Normally To Fix Your Problem .
来源：https://stackoverflow.com/questions/37272020/jquery-loaded-async-and-ready-function-not-working