onAvailable: Executes the supplied callback when the item with the supplied id is found.

gistfile1.js

(function (window, document) {

	function onLoad(callback) {
		if (document.readyState === "complete") {
			setTimeout(callback, 1);
		} else if (document.addEventListener) {
			window.addEventListener("load", callback, false);
		} else {
			window.attachEvent("onload", callback);
		}
	}

	var isReady = false;
	onLoad(function () {
		isReady = true;
	});


	/**
	 * Milliseconds between retries
	 */
	var POLL_INTERVAL = 20;

	/**
	 * Number of attempts to locate element
	 */
	var POLL_RETRIES = 2000;

	var interval = null,
		listeners = [],
		pollRetries;

	/**
	 * Polls the DOM for an element matching the supplied id and fires the
	 * associated callback method
	 */
	function checkAvailable() {
		var i, l, listener, el, missed = 0;

		for (i = 0, l = listeners.length; i < l; i++) {
			listener = listeners[i];
			if (listener) {
				el = document.getElementById(listener.id);
				/**
				 * If the DOM element is found, execute callbacks for all related
				 * onAvailable listeners. If the DOM is ready or the next sibling
				 * is detected, execute callbacks for all onContentReady listeners.
				 */
				if (el && (!listener.checkContent || (listener.checkContent && (el.nextSibling || el.parentNode.nextSibling || isReady)))) {
					try {
						listener.callback.call(el, listener.obj);
					} catch (ex) {
						console.error(ex);
					}
					listeners[i] = null;
				} else {
					missed++;
				}
			}
		}

		pollRetries--;
		if (missed === 0 || pollRetries <= 0) {
			interval = window.clearInterval(interval);
			if (listeners.length) {
				listeners.splice(0, listeners.length);
			}
		}

		listener = el = null;
	}


	/**
	 * NOTE: If element is last the child of its parent node, include at
	 * least one character of whitespace or onContentReady will not fire
	 * until DOMReady.
	 */

	/**
	 * Executes the supplied callback when the item with the supplied
	 * id is found.  This is meant to be used to execute behavior as
	 * soon as possible as the page loads.  If you use this after the
	 * initial page load it will poll for a fixed time for the element.
	 * The number of times it will poll and the frequency are
	 * configurable.  By default it will poll for 40 seconds at a rate
	 * of 50 times per second.
	 *
	 * <p>The callback is executed with a single parameter:
	 * the custom object parameter, if provided.</p>
	 *
	 * @param {String} id id of element to look for
	 * @param {Function} callback method to execute when element is found
	 * @param {Object} [obj] optional object to passed as parameter to callback
	 * @param {Boolean} [checkContent] check child node readiness (onContentReady)
	 */
	function onAvailable(id, callback, obj, checkContent) {
		pollRetries = POLL_RETRIES;
		listeners.push({
			id: id,
			callback: callback,
			obj: obj,
			checkContent: !!checkContent
		});

		if (!interval) {
			interval = window.setInterval(checkAvailable, POLL_INTERVAL);
		}
	}

	window.onAvailable = onAvailable;

}(window, document));