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# Event

As web pages get richer, they tend to get slower. One way to make your pages as responsive as possible is to carefully storyboard the page-load and page-paint processes so that the interactions most central to the page's purpose are enabled as early as possible. The window object's `load` event won't happen until the full DOM and all image data have loaded. Putting off script execution until after the page loads can be optimal for some scripts, but sometimes you won't want to wait that long to begin interacting with the page via script.

The Event Utility gives you three additional interesting moments that occur during a page's load process: `available`, `contentready`, and `domready`.

In the example box below, all three events are all in use. A `<div>` (with a green background) loads; it has 100 children; one of those children is an arbitrarily large image that will take awhile to load.

Note: The order of the events isn't guaranteed because `available` and `contentready` are fired from a polling mechanism, and not all browsers support a native event to signal `domready`, so that will fall back to a timer as well. Using these events, you can trust the state of the DOM per subscription, but don't expect strict ordering between events.

Internet Explorer note: It isn't always safe to modify content during the available/contentready until after the `domready` moment.

## Source Code for This Example:

### Markup:

The markup used to create the DOM is very simple, consisting of a `<div>` that holds a `<ul>` with 100 child `<li>`s and a single ~3MB image. The `<ul>` will take a little time to load, and the image (loading over the internet) will take a few seconds to load even on a fast connection. That should allow us to see in the browser console some time deltas between when the `<div>` whose ID is `contentContainer` becomes available, when its children (those 100 `<li>`s) are ready, when the DOM is ready (including all the navigation elements on the page), and lastly when the page loads (ie, when that ~3MB image is fully loaded).

    <div id="contentContainer">

        <!--a ul with an arbitrarily large number of children:-->
        <ul>
            <li>...</li>
            <!--...100 more of these-->
        </ul>

        <img src="http://developer.yahoo.com/yui/docs/assets/examples/exampleimages/large/uluru.jpg" width="500" alt="Uluru" id="image" />

    </div>

### CSS:

The CSS colors the contentContainer element and hides the big list to keep the example more compact.

    <style type="text/css">
        #contentContainer {padding:1em; background:#999966;}
        #contentContainer ul {height:0px; overflow:hidden;}
    </style>

### JavaScript:

In the script, we subscribe to the four events in which we're interested and, in each case, log a message to the console to express the timing of the events as they fire.

    YUI().use('*', function(Y) {
        var results = Y.one('#demo');

        //assign page load handler:
        Y.on("load", function () {
            results.append("<p>The window load event fired.  The page and all of its image data, including the large image of Uluru, has completed loading.</p>");
        }, Y.config.win);

        //assign domready handler:
        Y.on("domready", function () {
            results.append("<p>The DOMContentLoaded event fired.  The DOM is now safe to modify via script.</p>");
        });

        //assign 'contentready' handler:
        Y.on("contentready", function () {
            results.append("<p>The 'contentready' event fired for the element 'contentContainer'.  That element and all of its children are present in the DOM.</p>");
        }, "#contentContainer");

        //assign 'available' handler:
        Y.on("available", function () {
            results.append("<p>The 'available' event fired on the element 'contentContainer'.  That element is present in the DOM.</p>");
        }, "#contentContainer");

        results.append("<p>As the page loads, you'll see the 'available', 'contentready', 'domready', and window load events logged here as they fire in sequence.</p>");

    });