Modern web applications are becoming more complex, and they rely on an increasing number of dependencies. The larger the dependency tree is, the more likely it is to encounter version conflicts, bundle duplicate dependencies, ship unnecessary code, and negatively impact user performance.
On the front-end side, modern web applications include in the bundle a median of 98 direct and transitive dependencies1, 108% growth in the last three years.
Why the bundle contains duplicate dependencies
When a package is depended upon by multiple other dependencies and/or the application, npm resolves the dependency tree, and if a common version does not satisfy all the semver queries, it installs multiple instances of the same package. By installing multiple instances of the same dependency, npm meets the dependent package.json
and avoids breaking existing functionality.
If the duplicate package is a front-end dependency, it is bundled multiple times, increasing the size of the bundle, negatively impacting the application's web performance and, in some cases, causing Javascript runtime errors (ex: Duplicate React).
Resources:
Example
For example, in an application that depends on @ant-design/icons@4.7.0
(resolved to v4.7.0
) and antd@4.24.14
,
npm installs two different instances of @ant-design/icons
because antd@4.24.14
depends on @ant-design/icons@^4.7.0
(resolved to v4.8.1
):
{ "name": "example-app", // ... package information "dependencies": { "@ant-design": "4.7.0", "antd": "4.24.14" }}
npm list @ant-design/icons
example-fix-npm-dependencies@1.0.0 repo├── @ant-design/icons@4.7.0└─┬ antd@4.24.14 └── @ant-design/icons@4.8.1
As a result, the bundle includes two versions of the same dependency:

Identify duplicate dependencies
RelativeCI analyzes the bundle stats modules data, extracts the list of bundled dependencies, and identifies duplicate dependency instances. The information is presented as custom insights, metrics, and a list that you can filter and sort to discover the regressions quickly.
Duplicate packages insight
The duplicate packages insight shows the status and the result of the duplicate packages analysis, allowing you to easily identify when there are regressions(new duplicate instances) or improvements (removed duplicates). The insight information is available on the bundle analysis report summary, job list, and all the integration summaries.



Duplicate packages metric
The number of duplicate packages is available as a distinct metric on the report summary, the job list, and all the integration summaries.


Also, you can use the metric as a condition on the GitHub Commit Status Review or the Slack notification.

Duplicate packages data
To see the list of duplicate packages, go to the Packages section of the bundle analysis report and filter the results by the Duplicate filter. This will give you an overview of all the duplicate dependencies and their impact on the bundle size.

Fix npm duplicate dependencies
The strategies for resolving npm duplicate dependencies may vary depending on your setup, dependencies, and versions. However, the following methods can help you eliminate duplicate packages, reduce bundle size, and enhance your application's web performance.
npm dedupe
npm dedupe can help to quickly fix any existing issues in the package-lock.json
file by flattening the dependency tree and regenerating the package-lock.json
.
For example, the app dependency depends on @ant-design/icons@^4.6.0
and the correct semver resolution should be 4.8.1
, though the package-lock.json
resolves to 4.6.0
:
npm explain @ant-design/icons
@ant-design/icons@4.6.0node_modules/@ant-design/icons @ant-design/icons@"^4.6.0" from the root project
@ant-design/icons@4.8.1node_modules/antd/node_modules/@ant-design/icons @ant-design/icons@"^4.7.0" from antd@4.24.14 node_modules/antd antd@"^4.18.6" from the root projects
Running npm dedupe
will correct the package-lock.json
and remove the duplicate dependency:
npm dedupenpm list @ant-design/iconsexample-fix-npm-dependencies@1.0.0 repo├── @ant-design/icons@4.8.1└─┬ antd@4.24.14 └── @ant-design/icons@4.8.1 deduped

Upgrade/downgrade to a common version
Update and/or downgrade the dependency to a common version that the dependents can share.
In the example above, we can update the application version to v4.8.1
, the same as the version resolved for antd
. npm will resolve both dependencies to v4.8.1
npm install --save @ant-design/icons@^4.8.1example-fix-npm-dependencies@1.0.0 repo├── @ant-design/icons@4.8.1└─┬ antd@4.24.14 └── @ant-design/icons@4.8.1 deduped
Npm overrides
npm overrides configuration allows to explicitly set package versions and override the npm resolution for any dependency, direct or transitive.
In our example, we can override the resolved version of @ant-design/icons
to 4.7.0
.
{ "name": "app", /* ... */ "dependencies": { "@ant-design/icons": "^4.6.0", "antd": "^4.18.6", /* ... */ }, "overrides": { "antd": { "@ant-design/icons": "4.7.0" } }}
npm list @ant-design/icons
example-fix-npm-dependencies@1.0.0 /Users/vio/work/relative-ci/examples/fix-npm-duplicates├── @ant-design/icons@4.7.0└─┬ antd@4.24.14 └── @ant-design/icons@4.7.0 deduped
Webpack configuration
Webpack allows dependency imports to be overridden at bundle time using resolve.alias
configuration option.
To point all the app imports of @ant-design/icons
to the same version, we can add an entry to resolve.alias
:
module.exports = { // ... webpack config resolve: { alias: { '@ant-design/icons': path.join(__dirname, 'node_modules/@ant-design/icons'), } }};
[1]: Data based on open-source projects analyzed by RelativeCI
2020 | 2021 | 2022 | 2023 | |
---|---|---|---|---|
Median package count | 47 | 52 | 95 | 98 |
Mean package count | 46 | 76 | 130 | 138 |