What is Lerna?

Lerna is a popular tool that simplifies the management of multi-package repositories. Originally developed by Babel to handle its extensive ecosystem, Lerna has become the go-to solution for JavaScript developers looking to manage a monorepo. With Lerna, you can manage multiple packages within a single repository, allowing for unified versioning, dependency management, and build optimization across projects.

Key benefits of using Lerna include:

  • Shared Dependencies: Manage dependencies between packages in a centralized manner.
  • Streamlined Builds: Only rebuild the parts of the project that have changed.
  • Unified Versioning: Keep all packages in sync with either fixed or independent versioning.
  • Improved Collaboration: Teams can work across multiple projects within a single repository.

Setting Up a Lerna Monorepo

To start using Lerna, you first need Node.js installed. Once installed, you can add Lerna globally or as a dev dependency in your project:

npm install -g lerna

Alternatively, add it as a dev dependency:

npm install --save-dev lerna

Initializing a Lerna Project

With Lerna installed, you can initialize a new Lerna project by running the following command in the root of your project directory:

lerna init

This command creates a lerna.json file in the root directory, along with a packages folder where individual projects or packages will reside. The lerna.json file contains configuration settings for your monorepo, such as versioning strategy and dependencies.

Configuring Lerna

The lerna.json file is the central configuration file for Lerna projects. Some of the important configuration options include:

  • Version: Determines the versioning scheme. By default, Lerna uses fixed versioning (all packages are updated together).
  • Packages: Specifies the location of packages (typically set to ["packages/*"]).
  • npmClient: Specifies the package manager to use (e.g., npm or yarn).
  • Use Workspaces: If using Yarn, you can set "useWorkspaces": true to enable Yarn workspaces for improved dependency management.

An example lerna.json configuration might look like this:

{
  "packages": ["packages/*"],
  "version": "0.0.0",
  "npmClient": "yarn",
  "useWorkspaces": true
}

Versioning Strategies

Lerna supports two main versioning strategies:

  • Fixed/Locked: All packages are versioned together, making it easy to maintain consistent releases across packages.
  • Independent: Each package is versioned independently, which is useful when different packages have unique release cycles.

To use independent versioning, set the version property to "independent" in lerna.json.

Creating Packages in Lerna

In a Lerna monorepo, packages are the individual modules or projects contained within the repository. To create a new package, you can either manually add a folder under the packages directory or use Lerna's generate command to scaffold it.

Adding a Package

To add a new package:

mkdir packages/my-new-package

Then initialize it as a new Node.js package:

cd packages/my-new-package
npm init -y

Adding Dependencies Between Packages

Lerna allows you to define dependencies between packages. For example, if my-package depends on shared-utils, you can add it as a dependency:

lerna add shared-utils --scope=my-package

This command ensures that my-package depends on the local version of shared-utils, which is essential for managing shared code within a monorepo.

Running Commands in a Lerna Monorepo

Lerna makes it easy to run commands across all packages in the monorepo. You can use the lerna run command to execute npm scripts across multiple packages simultaneously. For instance, if you want to run the build script in all packages, use:

lerna run build

This command runs the build script in every package that has it defined. Similarly, you can use the lerna exec command to run arbitrary shell commands in each package:

lerna exec -- rm -rf dist

Managing Dependencies with Lerna

Dependencies in a monorepo can be complicated, especially when packages depend on one another. Lerna simplifies dependency management by linking packages and automatically handling shared dependencies.

Linking Local Dependencies

When packages within a Lerna monorepo depend on each other, Lerna links them. This means you don’t have to publish each package individually to test changes—Lerna will resolve local dependencies within the monorepo automatically.

Updating Dependencies

To add a new dependency to all packages, use:

lerna add lodash

This command installs lodash in each package that lists it as a dependency, ensuring consistency across your monorepo.

Optimizing Builds with Lerna

One of the advantages of using Lerna is its capability to optimize builds. By default, Lerna runs commands in all packages, which can be inefficient for large projects. To improve performance, Lerna provides the --scope and --ignore flags to target specific packages.

Targeting Specific Packages

For example, if you only want to run the test script for the my-package package, you can use:

lerna run test --scope=my-package

Using Lerna with CI/CD

Lerna integrates well with continuous integration and deployment (CI/CD) workflows. By configuring your CI/CD pipeline to run only the affected packages or scripts, you can speed up build times and reduce resource usage.

lerna run build --since master

This command will only run the build script for packages that have changed since the last commit to the master branch, streamlining the build process.

Testing in a Lerna Monorepo

Testing multiple packages within a monorepo can be challenging. Lerna makes this easier by allowing you to run tests across all packages with a single command. If each package includes a test script, you can run:

lerna run test

This command will run the test script in every package, which is useful for ensuring code quality across the entire monorepo.

Publishing with Lerna

Publishing packages from a Lerna monorepo can be done with the lerna publish command, which manages versioning and publishing to npm.

Fixed vs. Independent Versioning

When using lerna publish, you can choose between fixed and independent versioning. Fixed versioning keeps all packages at the same version, while independent versioning allows each package to have its own version number. Run the following command to publish:

lerna publish

Lerna will prompt you to select a new version and automatically update the package.json files in each package, as well as create a Git tag for the release.

Best Practices for Using Lerna

To make the most of Lerna, consider these best practices:

  • Use Scopes for Commands: Target specific packages with --scope to optimize build and test times.
  • Minimize Cross-Dependencies: Limit dependencies between packages to reduce complexity and potential for errors.
  • Automate Versioning: Use Lerna’s automatic versioning to keep packages in sync.
  • Leverage CI/CD: Integrate Lerna with your CI/CD pipeline for optimized, automated testing and publishing.

Common Challenges with Lerna

While Lerna simplifies monorepo management, there are common challenges developers encounter, such as dependency management and complex build pipelines. Here are a few solutions:

  • Managing Conflicts: Use dependency constraints and version locks to avoid conflicting dependencies.
  • Optimizing Builds: Run commands selectively with --scope to speed up builds in large monorepos.
  • Handling Cross-Dependencies: Carefully manage shared code and avoid circular dependencies where possible.

Conclusion

Lerna offers a robust solution for managing monorepos, providing tools for dependency management, optimized builds, and consistent versioning. By leveraging its powerful features and following best practices, developers can maintain efficient, scalable monorepos that enhance collaboration and streamline project workflows. Whether you’re managing a handful of packages or an extensive codebase, Lerna is an invaluable tool for any JavaScript or TypeScript developer working in a monorepo environment.