keydown和鼠标点击问题

var Key =
{
    _pressed: {},
    LEFT: 37,
    UP: 38,
    RIGHT: 39,
    DOWN: 40,

    init: (function()
    {
        window.addEventListener('keydown', function(e) { Key._pressed[e.keyCode] = true; }, false);
        window.addEventListener('keyup', function(e) { delete Key._pressed[e.keyCode]; }, false);
    })(),

    isDown: function(keyCode) { return this._pressed[keyCode]; }
};