Enabling Brotli on CDNs which don't support it yet

brotli-sw.js

// MIT License

// Copyright (c) 2016 Dexecure

// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:

// The above copyright notice and this permission notice shall be included in all
// copies or substantial portions of the Software.

// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
// SOFTWARE.


var db;

// return true if this is a text asset which can be Brotli compressed (like JS / CSS)
function isTextAsset(url) {
    return /(\.js|\.css)$/.test(url);
}

// return true if this is a domain you control
function isFirstPartyDomain(url) {
    return new URL(url).hostname === "example.com";
}

// Singleton class to return an Indexed Database instance
function getIDB() {
    return new Promise((resolve, reject) => {
        if (db) {
            // returning the existing IDB instance
            resolve(db);
        } else {
            var request = indexedDB.open("Dexecure", 1);
            request.onerror = function (event) {
                reject(request.error);
            };
            request.onsuccess = function (event) {
                db = request.result;
                resolve(request.result);
            };
        }

    });
}

// read from IDB and get Brotli status
function getBrotliStatus() {
    return getIDB()
        .then((db) => {
            return new Promise((resolve, reject) => {
                var transaction = db.transaction("config");
                var objectStore = transaction.objectStore("config");
                var request = objectStore.get(1);
                request.onerror = function (event) {
                    // safely asssume brotli not supported
                    reject(false);
                };
                request.onsuccess = function (event) {
                    resolve(event.target.result.isBrotliSupported);
                };
            });
        })
}

self.addEventListener('install', (event) => {
    event.waitUntil(
        fetch('https://blog.dexecure.com/content/images/perm/test.br')
        .then((res) => {
            if (res.ok) {
                return res.text();
            }
        })
        .then((resp) => {
            // check if the browser was able to decode the Brotli response correctly
            var isBrotliSupported = (resp === 'Dexecure');
            return isBrotliSupported;
        })
        .catch(() => {
            // assume brotli is not supported if request fails to be safe
            var isBrotliSupported = false;
            return isBrotliSupported;
        })
        .then((isBrotliSupported) => {
            // save Brotli Support status into IndexedDB
            return new Promise((resolve, reject) => {
                var request = indexedDB.open("Dexecure", 1);
                request.onerror = function (event) {
                    reject(request.error);
                };

                request.onsuccess = function (event) {
                    db = request.result;
                    var transaction = db.transaction("config", "readwrite");
                    var store = transaction.objectStore("config");
                    var config = {
                        isBrotliSupported: isBrotliSupported
                    }
                    var request2 = store.put(config, 1);
                    request2.onsuccess = function () {
                        resolve();
                    }
                };

                request.onupgradeneeded = function (e) {
                    var thisDB = e.target.result;
                    if (!thisDB.objectStoreNames.contains("config")) {
                        thisDB.createObjectStore("config");
                    }
                }
            });
        })
        .then(() => self.skipWaiting())
    );
});

self.addEventListener('fetch', function (event) {
    var url = event.request.url;
    // send a custom header to your server announcing Brotli support for future requests
    if (isTextAsset(url) && isFirstPartyDomain(url)) {
        event
        .respondWith(
            getBrotliStatus()
            .then((isBrotliSupported) => {
                var headersToSend = new Headers({});
                if (isBrotliSupported) {
                    headersToSend = new Headers({
                        'brotli-supported': 'true'
                    });
                }

                return fetch(url, {
                        mode: 'cors',
                        headers: headersToSend
                    })
                    .then(response => {
                        if (response.ok) {
                            return response;
                        } else {
                            throw new Error('Unable to fetch optimised image');
                        }
                    })
                    .catch(err => fetch(event.request))
            })
        );
    }
});