Cross-browser compatible way to bind events on page load

Question

Using jQuery, I can use the following function to execute code as soon as the DOM has loaded:

$(function() {
    // do stuff here
});

Or equivalently:

$(document).ready(function() { 
    // do stuff here
});

In trying to get a better understanding of raw javascript, I'm using this code to achieve a similar effect:

window.onload = function() {
    // do stuff here
}

Is this method cross-browser compatible? Are there any better ways to achieve this functionality?

Answer 1

Yes it is cross-browser compatible, but onLoad waits for everything on the page to load before it fires. Internally jQuery.ready uses the DOMContentLoaded event and a few hacks to support older browsers that don't support DOMContentLoaded. Most modern browsers support DOMContentLoaded including IE starting with version 9. You can test whether a browser supports DOMContentLoaded using this page.

If you are not using a DOM library such as jQuery which has built in support for DOMContentLoaded, you could use DOMContentLoaded and then fallback to onLoad if the browser doesn't support it.

(function () { // Encapsulating our variables with a IIFE
  var hasRun = false;  // a flag to make sure we only fire the event once
                       // in browsers that support both events
  var loadHandler = function (evt) {
    if (!hasRun) {
      hasRun = true;
      console.log('loaded via ' + evt.type);
    }
  };
  
  document.addEventListener('DOMContentLoaded', loadHandler, false);
  window.addEventListener('load', loadHandler, false);
}());

Unless you are expecting visitors with very old browsers like IE8, you are totally safe to just use DOMContentLoaded without a backup.

document.addEventListener('DOMContentLoaded', function (evt) {
  console.log('loaded via ' + evt.type);
}, false);

Answer 2

This is similar to what JQuery does:

window.$ = {};
$.ready = function(fn) {
  if (document.readyState == "complete")
      return fn();

  if (window.addEventListener)
      window.addEventListener("load", fn, false);
  else if (window.attachEvent)
      window.attachEvent("onload", fn);
  else
      window.onload = fn;
}

And to use it:

$.ready(function() {
  // code here...
});

Answer 3

The window onload method is cross-browser compatible, but there is a better alternative.

• The jQuery ready event fires when the DOM is ready.
• The window onload event fires when all data is downloaded.

So, let's say you have lots of images (or one BIG one) on your page. The html file will finish downloading and be ready for manipulation long before the images are done downloading. So jQuery's ready event shoots and you can start doing great JavaScript stuff while all those pretty pics download.

That's one of the reasons it's a good idea to use a js library.

When there aren't that many images then the difference is negligible. Though, you can only set ONE method at a time on the onload event. You can, however, set jQuery's ready event multiple times and each method will get called sequentially.

Answer 4

Cross-browser compatibility would have to depend on how you define the term "browser". Like for instance if it's a text based browser, then it might not be what you're looking for.

To answer your question, it will be cross-browser compatible if that particular browser warrants window.onload feature.

As a general guide, we usually use libraries that are tested so that we allow the libraries to take care of such "cross-browser" compatibility and we deal with the actual application logic.

Hope it helps!