Throttling function, abstracted from Underscore

throttle.js

 var throttle = function (func, wait) {
  var context, args, timeout, result;
  var previous = 0;
  var later = function() {
    previous = new Date();
    timeout = null;
    result = func.apply(context, args);
  };
  return function() {
    var now = new Date();
    var remaining = wait - (now - previous);
    context = this;
    args = arguments;
    if (remaining <= 0) {
      clearTimeout(timeout);
      timeout = null;
      previous = now;
      result = func.apply(context, args);
    } else if (!timeout) {
      timeout = setTimeout(later, remaining);
    }
    return result;
  };
};

var socket = io.connect();
return {
  on: function (eventName, callback) {
    socket.on(eventName, throttle(function () {  // limit to once every 500ms
      var args = arguments;
      $rootScope.$apply(function () {
        callback.apply(socket, args);
      });
    }, 500));
  }
  // ...
};