Timer for JavaScript.

metronome.js

function Metronome(interval, executions, func, context) {
  if (interval < 1) {
    throw "Interval must be a integer";
  }
  if (executions == 0) {
    throw "Number of executions must be a positive integer";
  }
  
  // Initialize mutable private members.
  var executionsLeft = executions;
  var nextRun = null;
  var timer = null;
  var timeLeft = null;
  
  // Run the closure asynchronously.
  function run(once) {
    nextRun = new Date().getTime() + interval;
    setTimeout(function() {
      func.call(context);
    }, 0);
    executionsLeft--;
    if (once) return;
    if (executionsLeft != 0) schedule();
  }
  
  // Schedule the closure.
  function schedule() {
    nextRun = new Date().getTime() + (timeLeft || interval);
    timer = setTimeout(run, timeLeft || interval);
  }

  // Is the metronome currently running.
  this.isRunning = function() {
    return !!timer;
  };
  
  this.executionsLeft = function() {
    if (executionsLeft < 0) return Infinity;
    return executionsLeft;
  };
  
  // Time until next run, in miliseconds.
  // Only meaningful if the metronome is currently running,
  // otherwise it will just return null.
  this.timeUntilNextRun = function() {
    if (!this.isRunning()) return null;
    return nextRun - new Date().getTime();
  };
  
  this.start = function() {
    if (this.isRunning() || !executionsLeft) return false;
    schedule();
    return this.isRunning();
  };
  
  this.stop = function() {
    if (!this.isRunning()) return false;
    clearTimeout(timer);
    timer = null;
    return !this.isRunning();
  };
  
  this.pause = function() {
    if (!this.isRunning()) return false;
    timeLeft = this.timeUntilNextRun();
    return this.stop();
  };
  
  this.resume = function() {
    if (this.isRunning()) return false;
    this.start();
    timeLeft = null;
    return this.isRunning();
  };
  
  // Runs the scheduled function _right now_. If the metronome was running,
  // this resets the countdown.
  this.runNow = function() {
    run(!this.stop());
  };
   
}