old_contents: // Kind of like a singleton
  const Keyboard  = {
      "KEY_UP": 38,
      "KEY_DOWN": 40,
      "KEY_LEFT": 37,
      "KEY_RIGHT": 39,
      "KEY_W": 87,
      "KEY_A": 65,
      "KEY_D": 68,
      "KEY_SPACE": 32,
      "keys": [],
      "keydown_handler": function(evt) {
          Keyboard.keys[evt.keyCode] = true; 
          if (evt.keyCode == Keyboard.KEY_UP ||
              evt.keyCode == Keyboard.KEY_DOWN ||
              evt.keyCode == Keyboard.KEY_LEFT ||
              evt.keyCode == Keyboard.KEY_RIGHT
          )
              evt.preventDefault();
      },
      "keyup_handler": function(evt) {
          //TODO: Remove from array to minimise memory consumption.
          Keyboard.keys[evt.keyCode] = false;
          if (evt.keyCode == Keyboard.KEY_UP ||
              evt.keyCode == Keyboard.KEY_DOWN ||
              evt.keyCode == Keyboard.KEY_LEFT ||
              evt.keyCode == Keyboard.KEY_RIGHT
          )
              evt.preventDefault();
      },
      "pressed": function(keyCode) {
          return Keyboard.keys[keyCode] === true;
      }
  }
  
new_contents: // Kind of like a singleton
  const Keyboard  = {
      "KEY_ESC": 27,
      "KEY_UP": 38,
      "KEY_DOWN": 40,
      "KEY_LEFT": 37,
      "KEY_RIGHT": 39,
      "KEY_W": 87,
      "KEY_A": 65,
      "KEY_D": 68,
      "KEY_SPACE": 32,
      "keys": [],
      "nKeysDown": 0,
      "keydown_handler": function(evt) {
          Keyboard.nKeysDown ++;
          Keyboard.keys[evt.keyCode] = true; 
          if (evt.keyCode == Keyboard.KEY_UP ||
              evt.keyCode == Keyboard.KEY_DOWN ||
              evt.keyCode == Keyboard.KEY_LEFT ||
              evt.keyCode == Keyboard.KEY_RIGHT
          )
              evt.preventDefault();
      },
      "keyup_handler": function(evt) {
          //TODO: Remove from array to minimise memory consumption.
          this.nKeysDown --;
          Keyboard.keys[evt.keyCode] = false;
          if (evt.keyCode == Keyboard.KEY_UP ||
              evt.keyCode == Keyboard.KEY_DOWN ||
              evt.keyCode == Keyboard.KEY_LEFT ||
              evt.keyCode == Keyboard.KEY_RIGHT
          )
              evt.preventDefault();
      },
      "pressed": function(keyCode) {
          return Keyboard.keys[keyCode] === true;
      },
      "anyKeyDown": function() {
          return Keyboard.nKeysDown > 0;
      }
  }
  
subject: Add any key test and ESC constant