redirector.js

/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */


async function handleRequest(request) {  

    // The % of traffic we want to redirect (e.g. 0.05 == 5%)
    const SAMPLE_RATE = 0.05;

    // The current domain (e.g. 'https://www.allizom.org', 'https://www.mozilla.org').
    const domain = 'https://www.allizom.org';

    // The URL we're targeting.
    const targetURL = `${domain}/en-US/firefox/new/`;

    // The experimental URL we want to redirect to.
    const experimentURL = `${domain}/en-US/exp/firefox/new/`;

    // The website visitor's current URL.
    const currentURL = request.url;
    
    /**
     * Matches the current URL to the URL we're targeting.
     * If a match is found, then return a final URL for the
     * experimental page, preserving any original URL parameters.
     */
    function getFinalURL(currentURL, targetURL, experimentURL) {
        const url = currentURL.split('?');

        // is the current URL what we're explicitly targeting?
        if (url[0] === targetURL) {

            // preserve query params from current URL if they exist.
            const params = url.length > 1 ? url[1] : null;

            if (params) {
                return `${experimentURL}?${params}`;
            }

            return experimentURL;

        } else {
            return false;
        }
    }

    function isWithinSampleRate(SAMPLE_RATE) {
        return Math.random() < SAMPLE_RATE;
    }

    function isValidURL(finalURL, experimentURL) {
        return finalURL && finalURL.startsWith(experimentURL);
    }

    const finalURL = getFinalURL(currentURL, targetURL, experimentURL);

    // If final constructed URL looks correct, and is within sample rate, then do the redirect.
    if (isValidURL(finalURL, experimentURL) && isWithinSampleRate(SAMPLE_RATE)) {
        return Response.redirect(finalURL, 302)
    } 
    // Else return the original request.
    else {
        return await fetch(request)
    }

}

addEventListener('fetch', event => {
    event.respondWith(handleRequest(event.request))
})