Simple MangaDex scroller by pressing up and down

mangadex_scroller.user.js

// ==UserScript==
// @name         MangaDex Scroller
// @namespace    https://gist.github.com/dm00z/dadf71588d8263086da9f67786689c87
// @version      0.3
// @description  Simple MangaDex scroller by pressing up and down
// @author       dm00z
// @match        https://mangadex.org/chapter/*
// @grant        none
// @downloadURL  https://gist.github.com/dm00z/dadf71588d8263086da9f67786689c87/raw/2d3ef0c07a6f1f3b0ad787e534e964ba2cea21c8/mangadex_scroller.user.js
// ==/UserScript==

document.onkeydown = checkKey;
var scrollBy = 300;
function checkKey(e) {
    e = e || window.event;
    if (e.keyCode == '38') {
        // up arrow
        e.stopPropagation();
        window.scrollBy(0, -scrollBy);
    }
    else if (e.keyCode == '40') {
        // down arrow
        e.stopPropagation();
        window.scrollBy(0, scrollBy);
    }
    else if (e.keyCode == '37') {
        // left arrow
    }
    else if (e.keyCode == '39') {
        // right arrow
    }
}