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An inspection tool for Webpack frontend JavaScript bundles.

inspectpack provides insight into your webpack-built JS bundles and detailed analysis of opportunites to reduce module sizes, unneeded duplicates, etc. It can be used as a webpack plugin during your compilations or as an offline CLI tool to report on your previous builds.

It is also the engine for the handy webpack-dashboard plugin.


The DuplicatesPlugin identifies unnecessarily duplicated code in your webpack bundles with an actionable report to help you trim down wasted bytes.

To get started, install the plugin:

$ npm install --save-dev inspectpack # OR
$ yarn add --dev inspectpack

Then, add the plugin to your webpack.config.js file:

const { DuplicatesPlugin } = require("inspectpack/plugin");

module.exports = {
  // ...
  plugins: [
    // ...
    new DuplicatesPlugin({
      // Emit compilation warning or error? (Default: `false`)
      emitErrors: false,
      // Handle all messages with handler function (`(report: string)`)
      // Overrides `emitErrors` output.
      emitHandler: undefined,
      // List of packages that can be ignored. (Default: `[]`)
      // - If a string, then a prefix match of `{$name}/` for each module.
      // - If a regex, then `.test(pattern)` which means you should add slashes
      //   where appropriate.
      // **Note**: Uses posix paths for all matching (e.g., on windows `/` not `\`).
      ignoredPackages: undefined,
      // Display full duplicates information? (Default: `false`)
      verbose: false

And from there you'll get actionable reports!

A quick tour

Let's see the plugin in action with a quick scenario that has various duplicates from a simple examples repository. (Side note: we've got lots of other interesting inspection scenarios in our test fixtures directory.)

The problem

In this scenario, we have an application that imports a simplified, fake version of lodash in (1) the root application, (2) transitively via a one dependency, and (3) again via a two dependency. Abstractly, the dependency tree (with semver ranges from pacakge.json) looks like:

- my-app:             # Resolved
  - lodash@^4.1.0     # 4.2.3
  - one@1.2.3:        # 1.2.3
    - lodash@^3.0.0   # 3.1.0
  - two@2.3.4:        # 2.3.4
    - lodash@^3.0.0   # 3.1.0

Using modern npm or yarn to install this project gives us the following on-disk node_modules folder (with version resolutions noted):

node_modules          # Resolved
  lodash              # 4.2.3
  one                 # 1.2.3
      lodash          # 3.1.0 (Cannot be collapsed)
  two                 # 2.3.4
      lodash          # 3.1.0 (Cannot be collapsed)

Looking to our resulting bundle we have the following duplicated code sources:

  • node_modules/lodash/index.js (4.2.3): This code is similar to the code for 3.1.0.
  • node_modules/one/node_modules/lodash/index.js, node_modules/two/node_modules/lodash/index.js (3.1.0): These two files are identical code sources. They are only included twice in the bundle because npm/yarn could not flatten the dependencies during installation.

So, we've got inefficient code that we discovered via a manual inspection. Wouldn't it be nice to have a report that specifically highlighted problems like these with useful information?

... enter the DuplicatesPlugin.

Diagnosing duplicates

Simple report

With our plugin enabled in the standard configuration:

new DuplicatesPlugin()

we get a summary report of the duplicates upon running the webpack command:

WARNING in Duplicate Sources / Packages - Duplicates found! ⚠️

* Duplicates: Found 2 similar files across 3 code sources (both identical + similar)
  accounting for 703 bundled bytes.
* Packages: Found 1 packages with 2 resolved, 3 installed, and 3 depended versions.

## bundle.js
lodash (Found 2 resolved, 3 installed, 3 depended. Latest 4.2.3.)
  3.1.0 ~/one/~/lodash
    scenario-new-webpack-new-npm-unflattened@* -> one@1.2.3 -> lodash@^3.0.0
  3.1.0 ~/two/~/lodash
    scenario-new-webpack-new-npm-unflattened@* -> two@2.3.4 -> lodash@^3.0.0
  4.2.3 ~/lodash
    scenario-new-webpack-new-npm-unflattened@* -> lodash@^4.1.0

Breaking down this report, we get a webpack "warning" emitted by default with an initial summary of the report.

  • The Duplicates summary looks at what is in the webpack bundle. It tells us there are 2 files that are not identical, but the same package file path (e.g 3.1.0 vs 4.2.3 for lodash/index.js) and that there are 3 code sources that end up in our final bundle (which includes two for 3.1.0). We also get a byte count for all the files at issue (703 bytes), which presumably could roughly be cut by 2/3 if we could collapse to just one file to do the same thing.
  • The Packages summary looks at what npm installed to node_modules. This is the other "view" into our problems.
    • Terminology: Let's dig in to what things mean here.
      • Resolved: We have one package (lodash) that has 2 resolved versions (3.1.0 and 4.2.3). A "resolution" means that upon inspecting the dependency tree and what's in a registry source, these specific versions "match". The results may differ at a different point in time
      • Installed: These are actual packages installed to the local disk. In our case, we have three installs for 2 resolutions because we place an identical version twice.
      • Depended: These are the number of upstream packages that create a dependency from a unique path in the graph to a package. Put more concretely, in our case, three unique package.json files have an entry for lodash.
        • Note: This is a bit of a complicated assessment, since aside from the root package.json the rest of the dependency graph depends on what is resolved at the next level to give a dependent package.json and so on recusively.
    • ~ Note: The ~ shorthand represents the node_modules folder, which is a common abbreviation for webpack tools. E.g., ~/two/~/lodash really means node_modules/two/node_modules/lodash.
    • Note - Duplicates Only: Unlike the CLI --action=versions report, the DuplicatesPlugin only reports package version skews when there are actual duplicated files (either similar or identical). This means there may be multiple versions of a package with different files as part of your bundle. If you'd like to see these, use the CLI reporting tool!

After the plugin runs, we get a duplicates/package report for asset (e.g. outputted "bundle" files) with duplicate packages that produce duplicate sources in our bundles in the form of:

{PACKAGE_NAME} (Found {NUM} resolved, {NUM} installed, {NUM} depended. Latest version {VERSION}.)

Looking to our specific report for lodash, we see that we have:

  • Two installed paths (~/one/~/lodash, ~/two/~/lodash) for one resolved version (3.1.0). These are part of the dependency tree because of two depended paths:
    • ROOT -> one@1.2.3 -> lodash@^3.0.0
    • ROOT -> two@2.3.4 -> lodash@^3.0.0
  • One installed path (~/lodash) for another resolved version (4.2.3). This is part of the dependency tree because of the one root depended path (ROOT -> lodash@^4.1.0).
  • Take these numbers together and you get our summary of 2 resolved, 3 installed, and 3 depended packages from our summary besides the package name.

Thus for actionable information, there is a naive "quick out" that if we could switch the root dependency also to ^3.0.0 or something that resolves to lodash@3.1.0 all three packages would collapse to one using modern npm or yarn!

Verbose report

But, let's say we want a little more information on the dependency tree besides the packages that end up on disk. For this, we can enable verbose output, which will include information on the bundled files that webpack is bringing in.

new DuplicatesPlugin({
  verbose: true

Our resulting report is:

WARNING in Duplicate Sources / Packages - Duplicates found! ⚠️

* Duplicates: Found 2 similar files across 3 code sources (both identical + similar)
  accounting for 703 bundled bytes.
* Packages: Found 1 packages with 2 resolved, 3 installed, and 3 depended versions.

## bundle.js
lodash (Found 2 resolved, 3 installed, 3 depended. Latest 4.2.3.)
      * Dependency graph
        scenario-new-webpack-new-npm-unflattened@* -> one@1.2.3 -> lodash@^3.0.0
      * Duplicated files in bundle.js
        lodash/index.js (I, 249)

      * Dependency graph
        scenario-new-webpack-new-npm-unflattened@* -> two@2.3.4 -> lodash@^3.0.0
      * Duplicated files in bundle.js
        lodash/index.js (I, 249)

      * Dependency graph
        scenario-new-webpack-new-npm-unflattened@* -> lodash@^4.1.0
      * Duplicated files in bundle.js
        lodash/index.js (S, 205)

We've got the same summary and organization as our previous report, but now we additionally have the bundled code sources with some additional information. Let's look at our first one for 3.1.0 ~/one/~/lodash:

lodash/index.js (I, 249)

this takes the form of:


which means the file index.js from the lodash package is identical to at least one other file in the bundle (the I designation) is 249 bytes in size.

Looking at the last one for 4.2.3 ~/lodash:

lodash/index.js (S, 205)

we have the same file name as the others, but it is not identical to any other file in the bundle -- instead it is only similar (the S designation) and is 205 bytes in size.

So now, with this verbose report we can see:

  • The specific files in play that are duplicated sources in the bundle.
  • Whether they have any identical matches elsewhere in the bundle.
  • The byte size (and hence the impact) of each source.

Fixing bundle duplicates

Alright! The plugin has analyzed your webpack compilation and dumped out a lot of information about all the duplicate sources and packages. ... so what do we do about it?

The real-world answer is it's complicated.

Some things are relatively easy to fix. Others are not.

Focus first on identical code sources

For starters, if you're serious about fixing pre-existing duplicates in your bundle, run with the verbose: true option. What that gives you is a list of the identical sources used in the bundle. These pieces of code are completely equivalent, so there is a better chance that they will be able to be collapsed without any difference in functionality.

Of course, the complexity is that identical pieces of code may require or import other files that are not identical in bytes or even equivalent in functionality. So a replacement analysis might start with identical code sources, but must also include any other depended-on sources. But hey, it's a decent place to start looking.

Change dependencies in your root package.json

For a few issues, you may be able to change a dependency you control, usually in your root package.json (or any other dependency you control). In our example above, if the root package.json downgraded its dependency to a semver range that resolved to lodash@3.1.0 likely all the duplicates for that mini-scenario would be solved.

Set resolve.alias in your webpack configuration

If you cannot resolve the dependencies in package.jsons you control, you can have webpack do manual resolutions to a single package for you using the resolve.alias option in your webpack.config.js file.

A slight warning here in that you are probably creating a bundle wherein some code sources may end up using a dependency version that is out of their specified semantic version range.

Set the resolutions field with yarn

In parallel to webpack collapsing package references in the bundle, if you use the yarn package manager to install your dependencies, you can analogous collapse to single packages in your installed node_modules directory before webpack even enters the picture.

Specifying a resolutions field in your package.json allows fine-grain control over how packages with the same package dependency resolve to one or more actual version numbers.

Similar to resolve.alias, because you can get outside the guarantees of semantic versioning with this tool, be sure to check that your overall application supports the finalized code in the bundle.

Command line tool

First, install (usually globally);

$ npm install -g inspectpack

From there, you can run the inspectpack command line tool from anywhere!

Usage: inspectpack -s <file> -a <action> [options]

  --action, -a            Actions to take
                [string] [required] [choices: "duplicates", "sizes", "versions"]
  --stats, -s             Path to webpack-created stats JSON object
                                                             [string] [required]
  --format, -f            Display output format
                     [string] [choices: "json", "text", "tsv"] [default: "text"]
  --ignored-packages, -i  List of package names (space separated) to ignore
                                                           [array] [default: []]
  --bail, -b              Exit non-zero if duplicates/versions results found
                                                      [boolean] [default: false]
  --help, -h              Show help                                    [boolean]
  --version, -v           Show version number                          [boolean]

  inspectpack -s stats.json -a duplicates  Show duplicates files
  inspectpack -s stats.json -a versions    Show version skews in a project
  inspectpack -s stats.json -a sizes       Show raw file sizes

Generating a stats object file

inspectpack ingests the webpack stats object from a compilation to analyze project bundles and generate reports. To create a stats file suitable for inspectpack's --stats|-s flag you can add the following to your webpack.config.js:

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

module.exports = {
  // ...
  plugins: [
    new StatsWriterPlugin({
      fields: ["assets", "modules"],
      stats: {
        source: true // Needed for webpack5+

This uses the webpack-stats-plugin to output at least the assets and modules fields of the stats object to a file named stats.json in the directory specified in output.path. There are lots of various options for the webpack-stats-plugin that may suit your particular webpack config better than this example.

ℹ️ Webpack 5+ Note: If you are using webpack5+ you will need to enable the { source: true } options for the stats field for the plugin to include sources in stats output. In webpack versions previous to 5, this was enabled by default. The field is needed for internal determination as to whether or not a module is a real source file or a "synthetic" webpack added entry.

Note: Multiple entry points

If you configure entry with multiple entry points like:

module.exports = {
  entry: {
    foo: "./src/foo.js",
    bar: "./src/bar.js",

Then the created stats.json object from the previous webpack-stats-plugin configuration will cause inspectpack to analyze all of the bundled files across all of the entry points. The webpack-stats-plugin can be configured to split up separate stats files if desired in any manner (including splitting per entry point), but this is a more advanced usage not included in this document.


inspectpack can output reports in json, text, or tsv (tab-separated values for spreadsheets). Just pass these options to the --format|-f flag and get your information the way you want it!


Detect if there are modules in your bundle that should be deduplicated but aren't, meaning that you have the same code multiple times, inflating the size of your bundle.

Old versions of webpack used to deduplicate identical code segments in modules, but it no longer does so, relying instead on npm tree flattening. Unfortunately, npm may still resolve to multiple independent versions of an overall package that nonetheless contain identical or compatible duplicate modules in the ultimate bundle. The inspectpack duplicates actions shows you the instances in which this happens.

Let's see a duplicates report in action:

$ inspectpack -s /PATH/TO/stats.json -a duplicates -f text
inspectpack --action=duplicates

## Summary
* Extra Files (unique):         2
* Extra Sources (non-unique):   3
* Extra Bytes (non-unique):     172

## `bundle.js`
* foo/index.js
  * Meta: Files 2, Sources 3, Bytes 172
  0. (Files 1, Sources 1, Bytes 64)
    (64) /PATH/TO/MY_PROJECT/node_modules/different-foo/node_modules/foo/index.js
  1. (Files 1, Sources 2, Bytes 108)
    (54) /PATH/TO/MY_PROJECT/node_modules/foo/index.js
    (54) /PATH/TO/MY_PROJECT/node_modules/uses-foo/node_modules/foo/index.js

Let's decipher the report:

  • Each heading (e.g., ## bundle.js) is per outputted asset.
  • The first level is a unique file name (here, foo/index.js). inspectpack considers all modules that resolve to a package path as potential "duplicates".
  • Within our entry for a unique file path, we next have two "Files" comprising indexes 0 and 1. Each file at this level corresponds to a unique code block. This means, e.g., that node_modules/different-foo/node_modules/foo/index.js (0) and node_modules/foo/index.js have different sources (1).
  • Within each "File" are 1+ "Sources". These comprise multiple modules with identical sources. This means that node_modules/foo/index.js is completely identical to node_modules/uses-foo/node_modules/foo/index.js in our example for index 1.

A positive report for duplicates means that your identical sources are completely wasted bytes -- you're including literally the same code multiple times. And multiple matching file paths means you are potentially wasting bytes because the packages may be able to be collapsed.


The versions action is a bit more high-level and abstract than duplicates. Versions reports on multiple versions of packages installed in your node_modules tree that have version skews and have 2+ files included in your bundle under inspection. In this manner, inspectpack ignores all the multitudes of package versions skews of things that don't matter to your ultimate application or library.

  • Note - Duplicates: The versions report includes any packages that result in 2+ files from different installs of a package in your bundle. However, that doesn't mean that they're necessarily duplicate files, like you would find in the --action=duplicates report. For example, if your bundle includes lodash@3.0.0/get.js and lodash@4.0.0/has.js, you will get a versions report for the lodash versions, but would not see these files listed in a duplicates report.

Requirements: In order to get an accurate report, you must run inspectpack from the project root where the base installed node_modules directory is located. You also need to have installed all your node_modules there.

Let's create a versions report on a project with both scoped and unscoped packages:

$ inspectpack -s /PATH/TO/stats.json -a versions -f text
inspectpack --action=versions

## Summary
* Packages with skews:      2
* Total resolved versions:  4
* Total installed packages: 4
* Total depended packages:  5
* Total bundled files:      7

## `bundle.js`
* @scope/foo
  * 1.1.1
    * ~/@scope/foo
      * Num deps: 2, files: 2
      * scoped@1.2.3 -> @scope/foo@^1.0.9
      * scoped@1.2.3 -> flattened-foo@^1.1.0 -> @scope/foo@^1.1.1
  * 2.2.2
    * ~/uses-foo/~/@scope/foo
      * Num deps: 1, files: 1
      * scoped@1.2.3 -> uses-foo@^1.0.9 -> @scope/foo@^2.2.0
* foo
  * 3.3.3
    * ~/unscoped-foo/~/foo
      * Num deps: 1, files: 2
      * scoped@1.2.3 -> unscoped-foo@^1.0.9 -> foo@^3.3.0
  * 4.3.3
    * ~/unscoped-foo/~/deeper-unscoped/~/foo
      * Num deps: 1, files: 2
      * scoped@1.2.3 -> unscoped-foo@^1.0.9 -> deeper-unscoped@^1.0.0 -> foo@^4.0.0

Digging in to this report, we see:

  • Each heading (e.g., ## bundle.js) is per outputted asset.
  • A top-level hierarchy of package names (@scoped/foo and foo).
  • Within each package name, are different installed versions found in the tree (e.g., 1.1.1 for ~/@scope/foo and 2.2.2 for ~/uses-foo/~/@scope/foo). These different versions are actually installed on disk within node_modules and not flattened.
  • Within a version number (e.g. for 1.1.1:~/@scope/foo we have scoped@1.2.3 -> @scope/foo@^1.0.9 and scoped@1.2.3 -> flattened-foo@^1.1.0 -> @scope/foo@^1.1.1) we have listed the "logical" dependency hierarchy path of the full tree noted by semver ranges from package.json:dependencies (^1.0.9 and ^1.1.1), that are flattened by npm to just one actual installed path (node_modules/@scope/foo).

The versions report thus gives us a functional view of how the dependencies in a project correspond to what's actually installed on disk in node_modules, allowing you to infer what packages / dependencies are causing potential wasteful duplicate modules to show up in your bundle.


Sizes produces a simple report of the byte size of each module in a bundle.

Let's create a sizes report using one of the projects we used before:

$ inspectpack -s /PATH/TO/stats.json -a sizes -f text
inspectpack --action=sizes

## Summary
* Bytes: 9892

## `bundle.js`
* Bytes: 9892
* /PATH/TO/MY_PROJECT/node_modules/@scope/foo/bike.js
  * Size: 63
* /PATH/TO/MY_PROJECT/node_modules/@scope/foo/index.js
  * Size: 54
* /PATH/TO/MY_PROJECT/node_modules/bar/index.js
  * Size: 54
* /PATH/TO/MY_PROJECT/node_modules/bar/tender.js
  * Size: 69
* /PATH/TO/MY_PROJECT/node_modules/flattened-foo/index.js
  * Size: 103
* /PATH/TO/MY_PROJECT/node_modules/unscoped-foo/index.js
  * Size: 297
* /PATH/TO/MY_PROJECT/node_modules/unscoped-foo/node_modules/deeper-unscoped/index.js
  * Size: 182
* /PATH/TO/MY_PROJECT/node_modules/unscoped-foo/node_modules/deeper-unscoped/node_modules/foo/car.js
  * Size: 61
* /PATH/TO/MY_PROJECT/node_modules/unscoped-foo/node_modules/deeper-unscoped/node_modules/foo/index.js
  * Size: 64
* /PATH/TO/MY_PROJECT/node_modules/unscoped-foo/node_modules/foo/car.js
  * Size: 61
* /PATH/TO/MY_PROJECT/node_modules/unscoped-foo/node_modules/foo/index.js
  * Size: 64
* /PATH/TO/MY_PROJECT/node_modules/uses-foo/index.js
  * Size: 98
* /PATH/TO/MY_PROJECT/node_modules/uses-foo/node_modules/@scope/foo/index.js
  * Size: 54
* /PATH/TO/MY_PROJECT/src/index.js
  * Size: 655
Note: Source size calculations and the webpack lifecycle

The sizes reported are most likely of the uncompressed source of each module. Because inspectpack relies on the stats object output, the information reported in the sizes action reflects at what point the stats object was generated. For example, using the recommended webpack-stats-plugin, the source information would be after all loader processing, but potentially before any webpack plugins. Thus, the resultant, actual size of a given module in your ultimate bundle could be bigger (e.g., in a development bundle with webpack-inserted comments and imports) or smaller (e.g., your bundle is minified and gzipped).

Notes, tips, tricks

Special characters in file paths

Webpack loaders use a special syntax for loaders with ? and ! characters that will end up in the stats object identifier field (e.g., /PATH/TO/node_modules/css-loader/index.js??ref--7-1!/PATH/TO/node_modules/postcss-loader/lib/index.js??ref--7-2!/PATH/TO/src/bar/my-style.css") for a given module item.

We currently use a very naive solution to determine the "true" asset name by just stripping off everything before the last ?/! character. There are technically some potential use cases (e.g. those characters in real file paths) that might not be correctly handled. We have a tracking ticket for folks to comment on if you're hitting any issues.

Other useful tools

Other tools that inspect Webpack bundles:

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.