MessagesFinder.js

// ==UserScript==
// @name         Scratch Messages Finder
// @namespace    scratch.mit.edu/users/bob1171
// @version      1.1
// @description  Displays someone's message count
// @author       @bob1171
// @match        https://scratch.mit.edu/users/*
// @grant        none
// ==/UserScript==

var Url = window.location.href;
var user = Url.split('/')[4];

var number = getMessageCount(user);

if (number !=='NotFound","message":""')
{
alert(number);
}

else
{
alert('This user does not exist. You can make this account!');
}

function getMessageCount(user)
{
xmlHttp = new XMLHttpRequest();
xmlHttp.open(
"GET",
"https://api.scratch.mit.edu/users/" + user + "/messages/count", false);
xmlHttp.send(null);
var raw = xmlHttp.responseText;
var number = raw.substring(9, raw.length - 1);
return number;
}