Here’s an update for modern browsers in 2019
let playerSpriteX = 0;
document.addEventListener('keyup', (e) => {
if (e.code === "ArrowUp") playerSpriteX += 10
else if (e.code === "ArrowDown") playerSpriteX -= 10
document.getElementById('test').innerHTML = 'playerSpriteX = ' + playerSpriteX;
});Click on this window to focus it, and hit keys up and down
<br><br><br>
<div id="test">playerSpriteX = 0</div>Original answer from 2013
window.onkeyup = function(e) {
var key = e.keyCode ? e.keyCode : e.which;
if (key == 38) {
playerSpriteX += 10;
}else if (key == 40) {
playerSpriteX -= 10;
}
}