Userscript that automatically runs on document-start - userscripts

Question

// ==UserScript==
// #name Button
// #run-at document-start
// #include *
// #grant none
// #noframes
// #version 0.0.1
// ==/UserScript==
(function () {
var button = document.createElement("Button")
button.innerHTML = "Title"
button.style = "top:0px;right:0px;position:absolute"
button.style.background='#4CAF50'
document.body.appendChild(button)
var don = document.querySelector('button')
button.onclick = function() {
alert('Hey') }
})()
Script to auto add button on everysite
Trying to inject above script through adguard extension as local title.user.js
But it wasn't working
if i load this script locally it doesnt work
but if i upload it somewhere and load it via url it works
Also on some websites it is not working is it possible that site is blocking javascript injection. If so how to bypass that and run this script on such websites
Ok seems like the website i am trying to inject is arabic and has right to left text and maybe some overlay html and hence button is not visible but when i changed right to left in above code it worked
But still adguard not injecting above code from local user.js only from remote url
Script also not working on stackoverflow.com itself

Answer 1

Looking at the code, document.body is likely undefined, because your meta block #run-at is set to document-start. Try document-end instead.
Either that, or you should change document.body to document.documentElement.
// ==UserScript==
// #name New script - stackoverflow.com
// #namespace Violentmonkey Scripts
// #include *
// #grant none
// #version 1.0
// #run-at document-start
// ==/UserScript==
(() => {
const button = document.createElement('Button');
button.innerHTML = 'Title';
button.style = 'top:0px;right:0px;position:fixed;z-index:10000;';
button.style.background = '#4CAF50';
document.documentElement.appendChild(button);
button.onclick = () => {
alert('Hey');
};
})();