keyboard.user.js

// ==UserScript==
// @name reddit inbox keyboard commands
// @match https://*.reddit.com/message/*
// @description RES style keyboard commands, without the bulk. Meant for /r/counting, and only works in inbox.
// @author TehVulpez
// ==/UserScript==

var siteTable = document.getElementById("siteTable");

var currentNo = 0;
var currentEl;

function handleKeys(e) {
    if (document.activeElement.nodeName == "TEXTAREA") { // stop keyboard commands from activating while in textarea
	return;
    }
    else if (e.key == "j" && currentNo != 48) { // down
	currentEl.style.backgroundColor = "#fff";
	currentNo += 2;
	setMessage();
	currentEl.scrollIntoView({block:"center"});
    }
    else if (e.key == "k" && currentNo != 0) { // up
	currentEl.style.backgroundColor = "#fff";
	currentNo -= 2;
	setMessage();
	currentEl.scrollIntoView({block:"center"});
    }
    else if (e.key == "a") { // upvote
	currentEl.getElementsByClassName("arrow")[0].click();
    }
    else if (e.key == "z") { // downvote
	currentEl.getElementsByClassName("arrow")[1].click();
    }
    else if (e.key == "r" && !e.ctrlKey) { // reply
	currentEl.getElementsByClassName("buttons")[0].lastChild.lastChild.click();
	e.preventDefault();
    }
}

function setMessage() {
    currentEl = siteTable.children[currentNo];

    currentEl.style.backgroundColor = "#eef";
}

setMessage();

addEventListener("keydown",handleKeys);