Async-loaded scripts with DOMContentLoaded or load event handlers not being called?

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By loading the script asynchronously, you are telling the browser that it can load that script independently of the other parts of the page. That means that the page may finish loading and may fire DOMContentLoaded BEFORE your script is loaded and before it registers for the event. If that happens, you will miss the event (it’s already happened when you register for it).

In all modern browsers, you can test the document to see if it’s already loaded (MDN doc), you can check:

if (document.readyState !== "loading")

to see if the document is already loaded. If it is, just do your business. If it’s not, then install your event listener.

In fact, as a reference source and implementation idea, jQuery does this very same thing with it’s .ready() method and it looks widely supported. jQuery has this code when .ready() is called that first checks to see if the document is already loaded. If so, it calls the ready function immediately rather than binding the event listener:

// Catch cases where $(document).ready() is called after the
// browser event has already occurred.
if ( document.readyState === "complete" ) {
    // Handle it asynchronously to allow scripts the opportunity to delay ready
    return setTimeout( jQuery.ready, 1 );
}

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