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I’ve always been into building performant and accessible sites, but lately I’ve been taking it extremely seriously. So much so that I’ve been building a tool to help me optimize and monitor the sites that I build to make sure that I’m making an attempt to offer the best experience to those who visit them. If you’re into performant, accessible and SEO friendly sites, you might like it too! You can check it out at Optimize Toolset.


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Webpack stats plugin




Webpack Stats Plugin

npm version Actions Status MIT license Maintenance Status

This plugin will ingest the webpack stats object, process / transform the object and write out to a file for further consumption.

The most common use case is building a hashed bundle and wanting to programmatically refer to the correct bundle path in your Node.js server.


The plugin is available via npm:

$ npm install --save-dev webpack-stats-plugin
$ yarn add --dev webpack-stats-plugin


We have example webpack configurations for all versions of webpack. See., e.g. test/scenarios/webpack5/webpack.config.js.


If you are using webpack-cli, you can enable with:

$ webpack-cli --plugin webpack-stats-plugin/lib/stats-writer-plugin


A basic webpack.config.js-based integration:

const { StatsWriterPlugin } = require("webpack-stats-plugin")

module.exports = {
  plugins: [
    // Everything else **first**.

    // Write out stats file to build directory.
    new StatsWriterPlugin({
      filename: "stats.json" // Default

Custom stats Configuration

This option is passed to the webpack compiler's getStats().toJson() method.

const { StatsWriterPlugin } = require("webpack-stats-plugin")

module.exports = {
  plugins: [
    new StatsWriterPlugin({
      stats: {
        all: false,
        assets: true

Custom Transform Function

The transform function has a signature of:

 * Transform skeleton.
 * @param {Object} data           Stats object
 * @param {Object} opts           Options
 * @param {Object} opts.compiler  Current compiler instance
 * @returns {String}              String to emit to file
function (data, opts) {}

which you can use like:

const { StatsWriterPlugin } = require("webpack-stats-plugin");

module.exports = {
  plugins: [
    new StatsWriterPlugin({
      transform(data, opts) {
        return JSON.stringify({
          main: data.assetsByChunkName.main[0],
          css: data.assetsByChunkName.main[1]
        }, null, 2);

Promise transform

You can use an asynchronous promise to transform as well:

const { StatsWriterPlugin } = require("webpack-stats-plugin");

module.exports = {
  plugins: [
    new StatsWriterPlugin({
      filename: "stats-transform-promise.json",
      transform(data) {
        return Promise.resolve().then(() => JSON.stringify({
          main: data.assetsByChunkName.main
        }, null, INDENT));



  • opts (Object) options
  • opts.filename (String|Function) output file name (Default: "stats.json")
  • opts.fields (Array) fields of stats obj to keep (Default: ["assetsByChunkName"])
  • opts.stats (Object|String) stats config object or string preset (Default: {})
  • opts.transform (Function|Promise) transform stats obj (Default: JSON.stringify())
  • opts.emit (Boolean) add stats file to webpack output? (Default: true)

Stats writer module.

Stats can be a string or array (we'll have an array due to source maps):

"assetsByChunkName": {
  "main": [

fields, stats

The stats object is big. It includes the entire source included in a bundle. Thus, we default opts.fields to ["assetsByChunkName"] to only include those. However, if you want the whole thing (maybe doing an opts.transform function), then you can set fields: null in options to get all of the stats object.

You may also pass a custom stats config object (or string preset) via opts.stats in order to select exactly what you want added to the data passed to the transform. When opts.stats is passed, opts.fields will default to null.




The opts.filename option can be a file name or path relative to output.path in webpack configuration. It should not be absolute. It may also be a function, in which case it will be passed the current compiler instance and expected to return a filename to use.


By default, the retrieved stats object is JSON.stringify'ed but by supplying an alternate transform you can target any output format. See test/scenarios/webpack5/webpack.config.js for various examples including Markdown output.

  • Warning: The output of transform should be a String, not an object. On Node v4.x if you return a real object in transform, then webpack will break with a TypeError (See #8). Just adding a simple JSON.stringify() around your object is usually what you need to solve any problems.

Internal notes

In modern webpack, the plugin uses the processAssets compilation hook if available when adding the stats object file to the overall compilation to write out along with all the other webpack-built assets. This is the last possible place to hook in before the compilation is frozen in future webpack releases.

In earlier webpack, the plugin uses the much later emit compiler hook. There are technically some assets/stats data that could be added after processAssets and before emit, but for most practical uses of this plugin users shouldn't see any differences in the usable data produced by different versions of webpack.


Contributions welcome!

We test against all versions of webpack. For a full explanation of our functional tests, see test/

To get started, first install:

$ yarn

Our tests first do various webpack builds and then run mocha asserts on the real outputted stats files. Inefficient, but for our small sample size efficient enough.

# Lint and tests
$ yarn run lint
$ yarn run test

# All together
$ yarn run check

Maintenance Status

Active: Formidable is actively working on this project, and we expect to continue for work for the foreseeable future. Bug reports, feature requests and pull requests are welcome.