So far the answers seem to rely on code being run instantly. If you set a timer for 1000ms, it will actually be around 1008 instead.
Here is how you should do it:
function timer(time,update,complete) {
var start = new Date().getTime();
var interval = setInterval(function() {
var now = time-(new Date().getTime()-start);
if( now <= 0) {
clearInterval(interval);
complete();
}
else update(Math.floor(now/1000));
},100); // the smaller this number, the more accurate the timer will be
}
To use, call:
timer(
5000, // milliseconds
function(timeleft) { // called every step to update the visible countdown
document.getElementById('timer').innerHTML = timeleft+" second(s)";
},
function() { // what to do after
alert("Timer complete!");
}
);