Underscore.js Mixin that takes two functions as arguments, and returns a new function that when called will run the first function until it returns true. When the first function returns true, the second function is fired.

when.js

// When.js
// TJ Eastmond <tj.eastmond@gmail.com>, SpiteShow
// Simple Underscore.js Mixin that runs the first function until
// it returns true, then runs the second

(function() {

	// Pass in two functions. The first is checked until it returns true, then the second is run
	var when = function(truthy, func) {
		// Just making sure we were passed functions...
		if (!_.isFunction(truthy) || !_.isFunction(func)) { 
			return false;
		}

		// Return the new function with our truth check
		return function() {
			// Keep checking for a true result
			var timer = setInterval(function() {
				if (truthy() === true) {
					// Finally got a true, so stop the timer and call the function
					clearInterval(timer);
					func.call(arguments);
				}
			}, 1);
		};
	};

	// Mixin to Underscore.js
	_.mixin({ when : when });

}).call(this);