Lazy Loading is a hot topic for optimization of the web application.

I was trying to optimize the React App and as we already have splitChunks in our webpack configuration, it was for granted to pay more attention to code splitting.

I thought of analyzing our bundle with Webpack Bundle Analyzer and see how splitChunks has a done the splitting.

For some reason, I could not identify the Chunks by name as they were pretty random as 1234.asdfd23534kjh346mn63m46.chunk.js

So to resolve this, I updated the chunkName in output of webpack config to [name].[contenthash].chunk.js

But still no luck! Bundle analyzer was still showing the chunk names similar to 1234.asdfd23534kjh346mn63m46.chunk.js

🚀 Web Search to the rescue, I found Magic Comments in Webpack

And to name my chunks I added magic comments similar to following on all dynamic imports in the codebase

export default Loadable({
  loader: () => import(
    /* webpackChunkName: Dasahboard */
    './containers/Dashboard'
  ),
  loadaing: () => <Spinner />
})

Still no luck 😕

Getting on to more Web Search 💪

Then I came across a comment in one of the web pack’s repo:

Turn the comment on in your babel configuration for the project

After struggling for a few minutes and a few trial and errors, I realised that I don’t need to configure comments in babel configuration. Its because I am using the presets in Babel; comments are on by default.

Still no luck 😫. Magic Comments are not reaching Webpack.

If Magic Comments (or Any Comment) are not reaching webpack, then they are lost in transpiling process. Which means I need to dig deeper in Babel Configuration.

Then I started going through all of the plugins in the Babel configuration.

{
  ...
    "plugins": [
    "dynamic-import-webpack",
    "@babel/plugin-proposal-class-properties",
    "@babel/plugin-syntax-object-rest-spread",
    [
      "@babel/plugin-transform-runtime",
      {
        "corejs": 3
      }
    ]
  ],
  ...
}

From this list of plugins, the only plugin that might be the culprit is dynamic-import-webpack

A small plugin to make dynamic imports i.e. import() work. Which you can see here: GitHub - airbnb/babel-plugin-dynamic-import-webpack: Babel plugin to transpile import() to require.ensure, for Webpack

What’s special here? The First line of the Readme of the repo:

Babel plugin to transpile import() to require.ensure, for Webpack.

And this is what is causing all the trouble. As imports are transformed to require.ensure there are no more magic comments.

So as a solution, removed this plugin dynamic-import-webpack from Babel and Magic Comments take effect in Webpack.

Now the Chunks have names similar to List. asdfd23534kjh346mn63m46.chunk.js

Conclusion

Configuring webpack can be tricky when there are so many things going on. If you want the Chunks to be named properly; I would suggest going through the following checklist:

• chunkName in output is configured properly
• Magic comment /* webpackChunkName: MyChunk */ is used to name the chunk
• Babel is configured to NOT remove the comments
• And remove plugin dynamic-import-webpack

Bonus tip

Use webpackPrefetch: true magic comment with webpackChunkName . And consider adding service workers with good caching strategy.

This will cache the Files on Browser and avoid problems related to Chunks not found (Chunk loading failed) with multiple deploys.

As you are using [contenthash] in the output file names, only the changed modules will be cached again by service workers, not all the files.

Let me know through comments 💬 or on Twitter at @patel_pankaj_ and/or @time2hack

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Credits

Photo by Ferenc Almasi on Unsplash

Originally published at https://time2hack.com on Jan 21, 2021.