gistfile1.js

var pressedKeys = {};

var tick = function(msDuration) {
  gamejs.event.get().forEach(function(e) {
    if (e.type == gamejs.event.KEY_DOWN) {
      pressedKeys[e.key] = true;
    } else if (e.type == gamejs.event.KEY_UP) {
      delete pressedKeys[e.key];
    }
  });

  for (var keyCode in pressedKeys) {
    if (pressedKeys.hasOwnProperty(keyCode)) {
      // Do something with the key
    }
  }
};

gamejs.time.fpsCallback(tick, this, 25);