User Script for generating email aliases

replace_email_address_placeholders.user.js

// ==UserScript==
// @name         Generate random email address from placeholders
// @namespace    https://github.com/mfakane
// @version      0.1
// @description  Automatically replaces placeholders in email addresses during input. Intended to use with plus aliases (e.g. yourusernamehere+XXXX@gmail.com) or custom domains (e.g. HOSTNAME+XXXX@example.com)
// @author       mfakane
// @match        http://*/*
// @match        https://*/*
// @icon         data:image/gif;base64,R0lGODlhAQABAAAAACH5BAEKAAEALAAAAAABAAEAAAICTAEAOw==
// @grant        none
// @updateURL    https://gist.githubusercontent.com/mfakane/4f613a4de336861b057d660fa847b7f8/raw/replace-email-address-placeholders.js
// @downloadURL  https://gist.githubusercontent.com/mfakane/4f613a4de336861b057d660fa847b7f8/raw/replace-email-address-placeholders.js
// @supportURL   https://gist.github.com/mfakane/4f613a4de336861b057d660fa847b7f8
// ==/UserScript==

(function() {
    'use strict';

    const generateRandomString = (length) => {
        const chars = "abcdefghijklmnopqrstuvwxyz0123456789";
        return [...Array(length).keys()]
            .map(x => chars[Math.floor(Math.random() * chars.length)])
            .join("");
    };

    const replacements = [
        { pattern: /X+/g, replacer: x => generateRandomString(x.length) },
        { pattern: /HOSTNAME/g, replacer: x => location.hostname.replace(/^(www|login|auth|accounts?)\./, "") },
    ];
    const generateAlias = (email, hostname) => {
        const atIndex = email.indexOf("@");
        const localPart = replacements.reduce(
            (str, x) => str.replace(x.pattern, x.replacer),
            email.slice(0, atIndex)
        );

        return `${localPart}@${email.slice(atIndex + 1)}`;
    };

    document.addEventListener("input", (e) => {
        const target = e.target;
        if (target.nodeName?.toLowerCase() !== "input") return;
        const value = target.value;
        if (typeof value === "string" &&
            value.match(/^.+@.+\..+$/) &&
            replacements.some(x => x.pattern.test(value))) {
            target.value = generateAlias(value);
        }
    }, true);
})();