What is this doing: "var timer = duration, minutes, seconds"?

Question

Trying to make a timer and came across this:

function timer(duration){
    var timer = duration, minutes, seconds;
    setInterval(function() {
        minutes = parseInt(timer / 60, 10);
        seconds = parseInt(timer % 60, 10);...

I've never seen a variable defined like this.

minutes and seconds are defined in setInterval callback, how is timer getting these?

Moreover, how can timer be more than one thing? There's something I'm missing here.

Original Fiddle

Thanks,

Danny