poll-pattern.js

function poll(fn, callback, errback, timeout, interval) {
	var endTime = Number(new Date()) + (timeout || 2000);
	interval = interval || 100;

	(function p() {
			// If the condition is met, we're done! 
			if(fn()) {
				callback();
			}
			// If the condition isn't met but the timeout hasn't elapsed, go again
			else if (Number(new Date()) < endTime) {
				setTimeout(p, interval);
			}
			// Didn't match and too much time, reject!
			else {
				errback(new Error('timed out for ' + fn + ': ' + arguments));
			}
	})();
}

// Usage:  ensure element is visible
poll(
	function() {
		return document.getElementById('lightbox').offsetWidth > 0;
	},
	function() {
		// Done, success callback
	},
	function() {
		// Error, failure callback
	}
);