gistfile1.js

var tick = function(){
  /* ... compute stuff, do the work that needs to be done ... */
  var data = ...; 
  sendMessage(data);

  // schedule the next tick according to our course correction

  /* observe what time it is and how long it's been 
     since a heartbeat and compute how much less than 
     500ms to delay for */
  var currentTime = (new Date()).getTime();
  var correctedDelay = ... /* compute scheduling correction */...;

  setTimeout(tick, correctedDelay);
};

// initially delay by our "ideal time". Subsequent timings will be corrected and less-than-ideal
setTimeout(tick, 500);