One minute countdown timer with combined start/reset button.

countdown.js

/* Requires:
	1 time container (var Timer)
	1 start/reset button (var startReset)
*/

// ------------------------------------- Start & Reset Functions
function startTimer() {
	sec = 59;
	countdown = setInterval(currentTime, 1000);
	toggle = false;
	startReset.innerHTML = 'Reset';
}

function resetTimer(){
	clearInterval(countdown);
	toggle = true;
	startReset.innerHTML = 'Start';
	timer.innerHTML = "1:00";
}

// ------------------------------------- Countdown Output
function currentTime() {
	switch(true){
		case (sec < 10 && sec > 0):
			timer.innerHTML = 0 + ':0' + sec;
			break;
		case (sec === 0):
			timer.innerHTML = 0 + ':0' + sec;
			clearInterval(countdown);
			break;
		default:
			timer.innerHTML = 0 + ':' + sec;
			break;
	}
	sec--;
}

// ------------------------------------- Variables
var countdown, sec;
var toggle = true;
var timer = document.getElementById('timer');
var startReset = document.getElementById('startReset');

// ------------------------------------- Start/Reset Events
startReset.onclick = function(){
	switch(toggle){
		case true:
			startTimer();
			break;
		case false:
			resetTimer();
			break;
	}
};