Service worker 本质上充当 Web 应用程序、浏览器与网络（可用时）之间的代理服务器。这个 API 旨在创建有效的离线体验，它会拦截网络请求并根据网络是否可用来采取适当的动作、更新来自服务器的的资源。它还提供入口以推送通知和访问后台同步 API。

Service worker 运行在 worker 上下文：因此它无法访问 DOM，相对于驱动应用的主 JavaScript 线程，它运行在其他线程中，所以不会造成阻塞。

出于安全考量，Service worker 只能由 HTTPS 或 localhost 使用。

注册

if ('serviceWorker' in navigator) {
    navigator.serviceWorker
        .register('/service-worker.js', {
            scope: '/',
        })
        .then(function (registration) {
            // 注册成功
            console.log('ServiceWorker registered successful', registration.scope);
        })
        .catch(function (err) {
            // 注册失败
            console.log('ServiceWorker registration failed: ', err);
        });
}

scope 用来指定 ServiceWorker 的作用域，它的默认值是 ServiceWorker 脚本所在目录。

我们来看下控制台输出信息:

ServiceWorker registered successful http://localhost:7016/

Service Worker 生命周期

下载
安装
激活

self.addEventListener('install', function (event) {
    console.log('install');
});

self.addEventListener('activate', function (event) {
    console.log('activate');
});

self.addEventListener('fetch', function (event) {
    console.log('fetch');
});

控制台打印：

刷新页面，控制台只剩下 fetch 事件，安装和激活只会在首次下载 Service Worker 时执行。

离线缓存

const addResourcesToCache = async (resources) => {
    const cache = await caches.open('v1');
    await cache.addAll(resources);
};

const putInCache = async (request, response) => {
    const cache = await caches.open('v1');
    await cache.put(request, response);
};

const cacheFirst = async ({ request, preloadResponsePromise, fallbackUrl }) => {
    // First try to get the resource from the cache
    const responseFromCache = await caches.match(request);
    if (responseFromCache) {
        return responseFromCache;
    }

    // Next try to use the preloaded response, if it's there
    const preloadResponse = await preloadResponsePromise;
    if (preloadResponse) {
        console.info('using preload response', preloadResponse);
        putInCache(request, preloadResponse.clone());
        return preloadResponse;
    }

    // Next try to get the resource from the network
    try {
        const responseFromNetwork = await fetch(request);
        // response may be used only once
        // we need to save clone to put one copy in cache
        // and serve second one
        putInCache(request, responseFromNetwork.clone());
        return responseFromNetwork;
    } catch (error) {
        const fallbackResponse = await caches.match(fallbackUrl);
        if (fallbackResponse) {
            return fallbackResponse;
        }
        // when even the fallback response is not available,
        // there is nothing we can do, but we must always
        // return a Response object
        return new Response('Network error happened', {
            status: 408,
            headers: { 'Content-Type': 'text/plain' },
        });
    }
};

const enableNavigationPreload = async () => {
    if (self.registration.navigationPreload) {
        // Enable navigation preloads!
        await self.registration.navigationPreload.enable();
    }
};

self.addEventListener('activate', (event) => {
    console.log('activate');
    event.waitUntil(enableNavigationPreload());
});

self.addEventListener('install', (event) => {
    console.log('install');
    event.waitUntil(
        addResourcesToCache([
            '/',
            '/index.html',
            '/index.css',
            'https://cdnjs.cloudflare.com/ajax/libs/babel-polyfill/7.12.1/polyfill.min.js',
            'https://cdnjs.cloudflare.com/ajax/libs/highlight.js/9.16.2/highlight.min.js',
        ])
    );
});

self.addEventListener('fetch', (event) => {
    console.log('fetch', event.request);
    event.respondWith(
        cacheFirst({
            request: event.request,
            preloadResponsePromise: event.preloadResponse,
            fallbackUrl: '/fallback',
        })
    );
});

使用 CacheStorage 来进行缓存。

首次访问后，刷新页面，查看 Network, 发现已经改为从 ServiceWorker 加载缓存的资源文件