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    1.1.0 • Public • Published

    fastify-webpack-hot 🔥

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    A Fastify plugin for serving files emitted by Webpack with Hot Module Replacement (HMR).

    Basic HMR Setup

    import webpack from 'webpack';
    import {
    } from 'fastify-webpack-hot';
    const compiler = webpack({
      entry: [
        path.resolve(__dirname, '../app/main.js'),
      mode: 'development',
      plugins: [
        new webpack.HotModuleReplacementPlugin(),
    void app.register(fastifyWebpackHot, {



    Accessing Webpack Stats

    Stats instance is accessible under request.webpack.stats:

    app.get('*', async (request, reply) => {
      const stats = request.webpack.stats.toJson({
        all: false,
        entrypoints: true,
      // ...

    The most common use case for accessing stats is for identifying and constructing the entrypoint assets, e.g.

    for (const asset of stats.entrypoints?.main.assets ?? []) {
      if ('.js')) {
        htmlBody +=
          '<script defer="defer" src="/' + + '"></script>\n';

    Accessing Output File System

    You can access Output File System by referencing compiler.outputFileSystem. However, this will have the type of OutputFileSystem, which is incompatible with memfs, which is used by this package. Therefore, a better way to access outputFileSystem is by referencing request.webpack.outputFileSystem:

    app.get('*', async (request, reply) => {
      const stats = JSON.parse(
        await request.webpack.outputFileSystem.promises.readFile(
          path.join(__dirname, '../dist/stats.json'),
      // ...

    This example shows how you would access stats.json generated by webpack-stats-plugin.

    Note: You likely won't need to use this because fastify-webpack-hot automatically detects which assets have been generated and serves them at output.publicPath.

    Compressing Response

    This plugin is compatible with compression-webpack-plugin, i.e. This plugin will serve compressed files if the following conditions are true:

    • Your outputs include compressed file versions (either .br or .gz)
    • Request includes a matching accept-encoding header

    Example compression-webpack-plugin configuration:

    new CompressionPlugin({
      algorithm: 'brotliCompress',
      deleteOriginalAssets: false,
      filename: '[path][base].br',
      compressionOptions: {
        level: zlib.constants.BROTLI_MIN_QUALITY,
      minRatio: 0.8,
      test: /\.(js|css|html|svg)$/,
      threshold: 10_240,

    Note: You may also try using fastify-compress, however, beware of the outstanding issue that may cause the server to crash (fastify-compress#215).

    Difference from webpack-dev-server

    All of the above are relatively straightforward to implement, however, I didn't have a use-case for them. If you have a use-case, please raise a PR.


    Node.js Logging

    This project uses roarr logger to output the program's state.

    Export ROARR_LOG=true environment variable to enable log printing to stdout.

    Use roarr-cli program to pretty-print the logs.


    npm i fastify-webpack-hot

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    • gajus