Serious issues with sourcemaps and multiple webpack configs.

[ddumont]

I tried playing around with this plugin over the weekend and ran into a few issues. One was just getting a better understanding of service workers :), but it was really hard to do because sourcemaps are completely borked because of the way this plugin operates.

So I did a bit of tinkering around, and I found out a few things.

1. Don't make your service worker an entrypoint in your main webpack config.
   It's way better to eliminate a bunch of the noise from other plugins that aren't intended to mess
   around with service worker code. Also, from what I read, you really don't want the sw.js filename
   changing with a hash every build, and you're probably using those (hashes) in your prod config.

   // webpack.config.js
   module.exports = [
     {
        // your main code for the page and all your plugins
     },
     {
        // your service worker code, and its plugins.
     }
   ]

2. You shouldn't be rewriting any source before it goes through babel, or the minifier. The best place to
   put that is in a loader. (just like babel!)

   // in the service worker webpack config
   module: {
     rules: [
       {
         test: /\.js$/,
         exclude: /node_modules/,
         use: [
           { loader: "babel-loader" },
           { loader: "asset-loader" }, // <-- This is the loader I made to do the asset injection.
         ],
       },
     ],
   },      

3. Multiple configs totally makes it harder to get the assets you want to cache! So I made a new plugin:

   // in the main webpack config
   plugins: [
     {
       apply: compiler => {
         compiler.plugin("done", ({ compilation }) => {
           global.assets = Object.keys(compilation.assets);
         });
       },
     },
   ],

   Cool... so I've squirreled away, in a really hackey way, the assets for the main layer! But the service
   worker config finishes waaaaaaay before the main one... so here's another small hack, and my loader.

   // asset-laoder.js
   module.exports = function(source) {
     const callback = this.async();
     const interval = setInterval(() => {
       if (!global.assets) return; // just wait till the main config is done compiling.
       clearInterval(interval);
       const assets = global.assets.map(asset => `/${asset}`);
       callback(null, source.replace(/'\{assets\}'/ig, JSON.stringify(assets)));
     }, 100);
   };

   // worker.js
   const filesToCache = '{assets}'
   .filter(asset => !asset.endsWith('map') && !asset.endsWith('manifest')); // I don't want to cache those.

[ddumont]

Feel free to take this approach for your plugin. I find that I have way more control over how my serviceworker gets packed this way. The sourcemaps also work perfectly now... well... as well as they do for everything else ;)

[ddumont]

Ah ha! I almost forgot. You have to do a bit more work when using hot module replace for your main code. (I'm not sure I want to think about doing this with the service worker.... I guess it can work?)

// my express server
if (config.get('dev:devmode')) {
  const webpack = require('webpack');
  const wdm = require('webpack-dev-middleware');
  const whm = require('webpack-hot-middleware');
  const wpconf = require('./webpack.config');
  const compiler = webpack(wpconf[0]);
  app.use(wdm(compiler, { publicPath: '/' }));
  app.use(whm(compiler));
  app.use(wdm(webpack(wpconf[1]), { publicPath: '/' }));
}

Without this, wdm/whm gets very upset about changes in your service worker, if you passed it the whole array of webpack confs as the compiler.