HardSourceWebpackPlugin is a plugin for webpack to provide an intermediate caching step for modules. In order to see results, you'll need to run webpack twice with this plugin: the first build will take the normal amount of time. The second build will be significantly faster.

Install with npm install --save-dev hard-source-webpack-plugin or yarn add --dev hard-source-webpack-plugin. And include the plugin in your webpack's plugins configuration.

// webpack.config.js
var HardSourceWebpackPlugin = require('hard-source-webpack-plugin');

module.exports = {
  context: // ...
  entry: // ...
  output: // ...
  plugins: [
    new HardSourceWebpackPlugin()
  ]
}

You can optionally set where HardSource writes and reads its cache to and from, and the hash values that determine when it creates new caches.

new HardSourceWebpackPlugin({
  // Either an absolute path or relative to webpack's options.context.
  cacheDirectory: 'node_modules/.cache/hard-source/[confighash]',
  // Either a string of object hash function given a webpack config.
  configHash: function(webpackConfig) {
    // node-object-hash on npm can be used to build this.
    return require('node-object-hash')({sort: false}).hash(webpackConfig);
  },
  // Either false, a string, an object, or a project hashing function.
  environmentHash: {
    root: process.cwd(),
    directories: [],
    files: ['package-lock.json', 'yarn.lock'],
  },
  // An object.
  info: {
    // 'none' or 'test'.
    mode: 'none',
    // 'debug', 'log', 'info', 'warn', or 'error'.
    level: 'debug',
  },
  // Clean up large, old caches automatically.
  cachePrune: {
    // Caches younger than `maxAge` are not considered for deletion. They must
    // be at least this (default: 2 days) old in milliseconds.
    maxAge: 2 * 24 * 60 * 60 * 1000,
    // All caches together must be larger than `sizeThreshold` before any
    // caches will be deleted. Together they must be at least this
    // (default: 50 MB) big in bytes.
    sizeThreshold: 50 * 1024 * 1024
  },
}),

Some further configuration is possible through provided plugins.

  plugins: [
    new HardSourceWebpackPlugin(),

    // You can optionally exclude items that may not be working with HardSource
    // or items with custom loaders while you are actively developing the
    // loader.
    new HardSourceWebpackPlugin.ExcludeModulePlugin([
      {
        // HardSource works with mini-css-extract-plugin but due to how
        // mini-css emits assets, assets are not emitted on repeated builds with
        // mini-css and hard-source together. Ignoring the mini-css loader
        // modules, but not the other css loader modules, excludes the modules
        // that mini-css needs rebuilt to output assets every time.
        test: /mini-css-extract-plugin[\\/]dist[\\/]loader/,
      },
      {
        test: /my-loader/,
        include: path.join(__dirname, 'vendor'),
      },
    ]),

    // HardSource includes an experimental plugin for parallelizing webpack
    // across multiple processes. It requires that the extra processes have the
    // same configuration. `mode` must be set in the config. Making standard
    // use with webpack-dev-server or webpack-serve is difficult. Using it with
    // webpack-dev-server or webpack-serve means disabling any automatic
    // configuration and configuring hot module replacement support manually.
    new HardSourceWebpackPlugin.ParallelModulePlugin({
      // How to launch the extra processes. Default:
      fork: (fork, compiler, webpackBin) => fork(
        webpackBin(),
        ['--config', __filename], {
          silent: true,
        }
      ),
      // Number of workers to spawn. Default:
      numWorkers: () => require('os').cpus().length,
      // Number of modules built before launching parallel building. Default:
      minModules: 10,
    }),
  ]

The cacheDirectory is where the cache is written to. The default stores the cache in a directory under node_modules so if node_modules is cleared so is the cache.

The cacheDirectory has a field in it [confighash] that is replaced by the configHash option when webpack is started. The [confighash] field is here to help with changes to the configuration by the developer or by a script. For example if the same webpack configuration is used for the webpack cli tool and then the webpack-dev-server cli tool, they will generate different configuration hashes. webpack-dev-server adds plugins for its reloading features, and the default hash function produces a different value with those plugins added.

configHash turns a webpack configuration when a webpack instance is started and is used by cacheDirectory to build different caches for different webpack configurations.

Configurations may change how modules are rendered and so change how they appear in the disk cache hard-source writes. It is important to use a different cache per webpack configuration or webpack cli tool. webpack and webpack-dev-server for example needed separate caches, configHash and [confighash] in the cacheDirectory will create separate caches due to the plugins and configuration changes webpack-dev-server makes.

The default value for configHash is

configHash: function(webpackConfig) {
  return require('node-object-hash')({sort: false}).hash(webpackConfig);
}

This uses the npm node-object-hash module with sort set to false to hash the object. node-object-hash hashes as much as it can but may have issue with some plugins or plugins and loaders that load an additional configuration file like a babel rc file or postcss config. In those cases you can depend on node-object-hash and extend what it hashes to best cover those changes.

configHash can also be set to a string or it can be a function that generates a value based on other parts of the environment.

configHash: function() {
  return process.env.NODE_ENV   '-'   process.env.BABEL_ENV;
}

When loaders, plugins, other build time scripts, or other dynamic dependencies change, hard-source needs to replace the cache to make sure the output is correct. The environmentHash is used to determine this. If the hash is different than a previous build, a fresh cache will be used.

The default object

environmentHash: {
  root: process.cwd(),
  directories: [],
  files: ['package-lock.json', 'yarn.lock']
}

hashes the lock files for npm and yarn. They will both be used if they both exist, or just one if only one exists. If neither file is found, the default will hash package.json and the package.json under node_modules.

You can disable the environmentHash by setting it to false. By doing this you will manually need to delete the cache when there is any dependency environment change.

Control the amount of messages from hard-source.

Sets other defaults for info. Defaults to 'test' when NODE_ENV==='test'.

The level of log messages to report down to. Defaults to 'debug' when mode is 'none'. Defaults to 'warn' when mode is 'test'.

For example 'debug' reports all messages while 'warn' reports warn and error level messages.

hard-source caches are by default created when the webpack configuration changes. Each cache holds a copy of all the data to create a build so they can become quite large. Once a cache is considered "old enough" that it is unlikely to be reused hard-source will delete it to free up space automatically.

Caches older than maxAge in milliseconds are considered for automatic deletion.

For caches to be deleted, all of them together must total more than this threshold.

hard-source needs a different cache for each different webpack configuration. The default configHash may not detect all of your options to plugins or other configuration files like .babelrc or postcss.config.js. In those cases a custom configHash is needed hashing the webpack config and those other values that it cannot normally reach.

webpack-dev-server needs a different cache than webpack or other webpack cli tools. Make sure your cacheDirectory and configHash options are hashing the changes webpack-dev-server makes to your webpack config. The default hard-source values should do this.

If you are using multiple webpack instances in separate processes make sure each has its own cache by changing cacheDirectory or configHash.

This is can be due to module context dependencies. require.context or loaders that watch folders use this webpack feature so webpack rebuilds when files or folders are added or removed from these watched directories. Be careful about using require.context or context aware loaders on folders that contain a lot of items. Both require.context and context loaders depend on those folders recursively. hard-source hashes every file under a require.context or context loader folder to detect when the context has changed and dependent modules are rebuilt.

Make sure you don't have a require.context or context loader on the root of your project. Such a context module means webpack is watching the hard source cache and when the cache is written after a build, webpack will start a new build for that module. This normally does not happen with webpack-dev-server because it writes the built files into memory instead of the disk. hard-source cannot do that since that would defeat its purpose as a disk caching plugin.

If you encounter any issues or have an idea for hard-source-webpack-plugin could be better, please let us know.