GreaseMonkey script to prevent websites from messing with my keyboard shortcuts

keepMyKeys.js

// ==UserScript==
// @name         forbid rebinding my keys
// @namespace    http://tampermonkey.net/
// @version      0.1
// @description  Get off my lawn!
// @author       Roland Kuhn
// @include      *
// @grant        none
// ==/UserScript==

(function() {
    'use strict';

    const SHIFT = 1
    const CTRL = 2
    const ALT = 4
    const META = 8

    const p = window.Event.prototype.preventDefault
    window.Event.prototype.preventDefault = function() {
        const mod =
              (this.shiftKey ? SHIFT : 0) |
              (this.ctrlKey ? CTRL : 0) |
              (this.altKey ? ALT : 0) |
              (this.metaKey ? META : 0)
        if (this.code == 'KeyM' && mod == (SHIFT | META)) { return }
        if ((this.code == 'ArrowLeft' || this.code == 'ArrowRight') && mod == (ALT | META)) { return }
        if (this.code == 'KeyF' && mod == (SHIFT | META)) { return }
        if (this.code == 'KeyR' && mod == META) { return }
        if (this.code == 'KeyW' && mod == META) { return }
        if (this.code >= 'Digit0' && this.code <= 'Digit9' && mod == META) { return }
        if (this.code == 'KeyJ' && mod == (ALT | META)) { return }
        p.apply(this)
    }
})();