JSFiddle - React, Tailwind, and code Playground

HTML

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<br><br><br>
<div id="test">playerSpriteX = 0</div>

JavaScript

var playerSpriteX = 0;

window.onkeyup = function(e) {
    var key = e.keyCode ? e.keyCode : e.which;
    
    if (key == 38) {
        playerSpriteX += 10;
    }else if (key == 40) {
        playerSpriteX -= 10;
    }
    
    document.getElementById('test').innerHTML = 'playerSpriteX = '+playerSpriteX;
}