function Key () {
  var pressed = [],
      eventHandlers = {
    press : [],
        down : [],
        up : {}
      };

  function getKeyEvent (e) {...};
  document.addEventListener()...

  ['press', 'down', 'up'].forEach(function(event) {
    this[event] = function (callback) {
      eventHandlers[event].push(callback);
    }
  }, this);

  this.pressed = function (keycode, callback){...}
}