JavaScript問題

想請問各位邦友，下面Javascript的作用，兩段都是鍵盤的設定嗎? 謝謝
第一段
var keysDown = {};

addEventListener("keydown", function (e) {
keysDown[e.keyCode] = true;
}, false);

addEventListener("keyup", function (e) {
delete keysDown[e.keyCode];
}, false);

第二段
var update = function (modifier) {
if (38 in keysDown) { // Player holding up
hero.y -= hero.speed * modifier;
}
if (40 in keysDown) { // Player holding down
hero.y += hero.speed * modifier;
}
if (37 in keysDown) { // Player holding left
hero.x -= hero.speed * modifier;
}
if (39 in keysDown) { // Player holding right
hero.x += hero.speed * modifier;
}