Toggle double video playback rate

video-toggle.js

// ==UserScript==
// @name         Toggle double playback rate
// @namespace    http://tampermonkey.net/
// @version      0.1
// @description  Sets video playback rate to (3 - current rate)
// @author       You
// @match        *://*/*
// @icon         data:image/gif;base64,R0lGODlhAQABAAAAACH5BAEKAAEALAAAAAABAAEAAAICTAEAOw==
// @grant        none
// @run-at       context-menu
// ==/UserScript==

(function() {
    'use strict';

    // Your code here...
    [...document.querySelectorAll('video')].forEach(v => {
        const originalRate = v.playbackRate;
        v.playbackRate = Math.max(1,3 - originalRate);
        console.log(`Playback rate ${originalRate} => ${v.playbackRate} for ${v.currentSrc}`);
    });
})();