Difference between DOMContentLoaded and load events


What is the difference between DOMContentLoaded and load events?

From the Mozilla Developer Center:

The DOMContentLoaded event is fired when the document has been completely loaded and parsed, without waiting for stylesheets, images, and subframes to finish loading (the load event can be used to detect a fully-loaded page).


The DOMContentLoaded event will fire as soon as the DOM hierarchy has been fully constructed, the load event will do it when all the images and sub-frames have finished loading.

DOMContentLoaded will work on most modern browsers, but not on IE including IE9 and above. There are some workarounds to mimic this event on older versions of IE, like the used on the jQuery library, they attach the IE specific onreadystatechange event.


See the difference yourself:

DEMO

From Microsoft IE

The DOMContentLoaded event fires when parsing of the current page is complete; the load event fires when all files have finished loading from all resources, including ads and images. DOMContentLoaded is a great event to use to hookup UI functionality to complex web pages.

From Mozilla Developer Network

The DOMContentLoaded event is fired when the document has been completely loaded and parsed, without waiting for stylesheets, images, and subframes to finish loading (the load event can be used to detect a fully-loaded page).


DOMContentLoaded==window.onDomReady()

Load==window.onLoad()

A page can't be manipulated safely until the document is "ready." jQuery detects this state of readiness for you. Code included inside $( document ).ready() will only run once the page Document Object Model (DOM) is ready for JavaScript code to execute. Code included inside $( window ).load(function() { ... }) will run once the entire page (images or iframes), not just the DOM, is ready.

See: http://learn.jquery.com/using-jquery-core/document-ready/


  • domContentLoaded: marks the point when both the DOM is ready and there are no stylesheets that are blocking JavaScript execution - meaning we can now (potentially) construct the render tree. Many JavaScript frameworks wait for this event before they start executing their own logic. For this reason the browser captures the EventStart and EventEnd timestamps to allow us to track how long this execution took.

  • loadEvent: as a final step in every page load the browser fires an “onload” event which can trigger additional application logic.

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Here's some code that works for us. We found MSIE to be hit and miss with DomContentLoaded, there appears to be some delay when no additional resources are cached (up to 300ms based on our console logging), and it triggers too fast when they are cached. So we resorted to a fallback for MISE. You also want to trigger the doStuff() function whether DomContentLoaded triggers before or after your external JS files.

// detect MSIE 9,10,11, but not Edge
ua=navigator.userAgent.toLowerCase();isIE=/msie/.test(ua);

function doStuff(){
    //
}
if(isIE){
    // play it safe, very few users, exec ur JS when all resources are loaded
    window.onload=function(){doStuff();}
} else {
    // add event listener to trigger your function when DOMContentLoaded
    if(document.readyState==='loading'){
        document.addEventListener('DOMContentLoaded',doStuff);
    } else {
        // DOMContentLoaded already loaded, so better trigger your function
        doStuff();
    }
}