Adding "0" to game countdown timer?

Using this script for a game countdown. Works fine but I would like to add “0” before the single digit #'s to make her look purdy.

Any help?

//the amount of time in seconds to count down from
timer = 10;
//create a new function
function countDown() {
 //subtract one from timer
 //if timer equals zero
 if (timer == 0) {
  //clears the countInt interval
//sets countInt to call the countDown function every 1000 milliseconds (1 second)
countInt = setInterval(countDown, 1000);