How to use setInterval and clearInterval?

setInterval sets up a recurring timer. It returns a handle that you can pass into clearInterval to stop it from firing:

var handle = setInterval(drawAll, 20);

// When you want to cancel it:
clearInterval(handle);
handle = 0; // I just do this so I know I've cleared the interval

On browsers, the handle is guaranteed to be a number that isn't equal to 0; therefore, 0 makes a handy flag value for "no timer set". (Other platforms may return other values; NodeJS's timer functions return an object, for instance.)

To schedule a function to only fire once, use setTimeout instead. It won't keep firing. (It also returns a handle you can use to cancel it via clearTimeout before it fires that one time if appropriate.)

setTimeout(drawAll, 20);

clearInterval is one option:

var interval = setInterval(doStuff, 2000); // 2000 ms = start after 2sec 
function doStuff() {
  alert('this is a 2 second warning');
  clearInterval(interval);
}

Use setTimeout(drawAll, 20) instead. That only executes the function once.