index.js

// ==UserScript==
// @name         old.reddit hide sidebar
// @namespace    http://hugo.hornquist.se
// @version      1.1.0
// @description  Adds toggle to hide Reddit sidebar
// @author       hugo@lysator.liu.se
// @match        https://*.reddit.com/*
// @match        https://old.reddit.com/*
// @match        https://www.reddit.com/r/*/wiki/*
// @match        https://np.reddit.com/*
// @updateURL    https://git.lysator.liu.se/hugo/web-monkey-scripts/raw/master/reddit-hide-sidebar/index.js
// @downloadURL  https://git.lysator.liu.se/hugo/web-monkey-scripts/raw/master/reddit-hide-sidebar/index.js
// ==/UserScript==

(function() {
	'use strict';

	let btn = document.createElement('button');
	let div = document.createElement('div');
	div.className = 'sidebar-toggle';
	div.appendChild(btn);

	let sidebar = document.getElementsByClassName('side')[0];
	sidebar.appendChild(div);

	btn.onclick = () => sidebar.classList.toggle('hidden');

	let style = document.createElement('style');

	style.innerText = `
.sidebar-toggle {
	position: absolute;
	z-index: 100;
	left: -2em;
		width: 2em;
		height: 2em;
		display: grid;
	top: 0;
}
.side {
	transition: 0.5s;
		position: relative;
}
.side.hidden {
	 width: 0px;
}
.side.hidden button::before {
	content: '<';
}
.side:not(.hidden) button::before {
	content: '>';
}`;

	document.getElementsByTagName('head')[0].appendChild(style);
})();