User Script for Stand Still. Stay Silent

I discovered a wonderful comic named Stand Still. Stay Silent

I wanted to add the possiblity to use the arrow keys to browse the web comic, I then created a script for it :

// ==UserScript==
// @name         Stand Still. Stay silent Arrow Keys
// @namespace    http://aaflalo.me
// @version      0.1
// @description  Using the arrows key to navigate throught the comic
// @author       Antoine Aflalo
// @match        http://www.sssscomic.com/comic.php?page=*
// @include      http://www.sssscomic.com/comic.php?page=*
// @updateURL    https://gist.github.com/Belphemur/2920677efbfde686d0e1/raw/Stand%20Still.%20Stay%20Silent.user.js
// @downloadURL  https://gist.github.com/Belphemur/2920677efbfde686d0e1/raw/Stand%20Still.%20Stay%20Silent.user.js
// @grant        none
// ==/UserScript==
var basicURL = "http://www.sssscomic.com/comic.php?page=";
function getComicPage () {
	var re = /page=(\d+)/; 
	id = re.exec(document.location);
	return parseInt(id[1]);
}

function changePage(id) {
    if(id < 0)
        return;
    document.location.href = basicURL + id;
}

function checkKey(e) {

    e = e || window.event;

    if (e.keyCode == '37') {
        changePage(getComicPage() - 1);
    }
    else if (e.keyCode == '39') {
        changePage(getComicPage() +1);
    }
}

document.onkeydown = checkKey;