A timer utility class for javascript.

timer.js

function Timer(el, callbackFunction) {
	// init the timer
	// el - element for time to be updated
	// callbackFunction - string of function name to
	//    be called when the countdown is finished.
	this.init = function(el, callbackFunction) {
		this.seconds = 0;
		this.minutes = 0;
		this.counterCaller = null;
		this.timer = el;
		this.doneCallback = callbackFunction;
		this.running = true;
		this.updateTimer();
		this.running = false;
		this.isPaused = false;
	}

	// clears the timer - reset
	this.clear = function() {
		clearInterval(this.counterCaller);
		this.isPaused = false;
		this.seconds = 0;
		this.minutes = 0;
	}

	// start a countdown for timerMinutes minutes and timerSeconds seconds
	this.startTimer = function(timerMinutes, timerSeconds) {
		this.clear();
		this.running = true;
		this.seconds = timerSeconds;
		this.minutes = timerMinutes;
		this.updateTimer();
		var t = this;
		this.counterCaller = setInterval(function() {t.countDown();}, 1000);
	}

	// pauses the countdown.
	this.pauseTimer = function() {
		if (this.isPaused) {
			this.isPaused = false;
			this.running = true;
			var t = this;
			this.counterCaller = setInterval(function() {t.countDown();}, 1000);
		} else if (this.running) {
			this.isPaused = true;
			this.running = false;
			clearInterval(this.counterCaller);
		}
	}
	
	// countdown timer - alternatively could implement a countup timer
	this.countDown = function() {
		this.seconds--;
		if (this.seconds < 0 && this.minutes != 0) {
			this.seconds = 59;
			this.minutes--;
		}

		if (this.minutes == 0 && this.seconds == 0) {
			this.running = false;
		}
		this.updateTimer();
	}
	
	// calls the callback function when countdown is complete.
	this.notify = function() {
		eval(this.doneCallback+"();");
	}

	// updates the element content with the new time.
	this.updateTimer = function() {
		displaySeconds = this.seconds + '';
		displayMinutes = this.minutes + '';
		if (displaySeconds.length < 2) {
			displaySeconds = "0" + displaySeconds;
		}
		if (displayMinutes.length < 2) {
			displayMinutes = "0" + displayMinutes;
		}
		newTime = displayMinutes + ":" + displaySeconds;
		this.timer.innerHTML = newTime;
		if (!this.running) {
			this.clear();
			this.notify();
		}
	}

	// instantiates a new timer.
	this.init(el, callbackFunction);
};