JavaScript Variable setTimeout Loop

Recently I had the need for a function to be repeatedly called for infinity. Normally setInterval would do the trick, but I wanted the time in between firings to change dynamically.

Here is the solution I came up with.

//global var to change speed
var speedControl = 1;

//use the up/down keys to increase/decrease the speed
//I'm not handling zero here, beware
document.addEventListener('keyup', function(e){  
  if(e.which == 38){
  } else if(e.which == 40){

function timer(speedGlobal, cb){  
  //as the speedControl number rises, the intreval decreases, so speed increases.
  var speed = Math.floor(1000/speedGlobal); //cut out nasty decimals
  console.log('inside timer', speedGlobal, speed)
  //cb is setTimeout
    //call timer recursively, used the global var as the new speedGlobal 
    timer(speedControl, setTimeout); 
  }, speed); 

//start the timer
timer(speedControl, setTimeout);  

Currently, this only prints messages to the console. Later I'll show how I use it in a project.

I've been trying to step up my JavaScript game by using closures and functional programming. While I don't really consider this a closure, it does rely on of first class functions since a function is being passed as a parameter. This snippet probably isn't perfect, but it's pretty slick and solved a problem that I had.