Disable YouTube Autoplay automatically

disableAutoplay.js

// ==UserScript==
// @version      1.0.0
// @name         Autoplay Off
// @description  Sets Autoplay to `Off` by default
// @include      http*://*.youtube.com/*
// @run-at       document-end
// @grant        none
// @noframes
// ==/UserScript==
(function () {
  'use strict';
  function disableAutoplay() {
    setTimeout(function() {
      var checkbox = document.getElementById('autoplay-checkbox');
      
      if (checkbox && checkbox.checked === true) {
        checkbox.click();
      }
    }, 2000);
  }

  window.addEventListener('readystatechange', disableAutoplay, true);
}());