Mastering Lerna: A Guide to Managing JavaScript Monorepos

Table of Contents

1. Introduction
2. Chapter 1: What is Lerna?
   • Why Monorepos?
3. Chapter 2: Installing and Setting Up Lerna
   • Prerequisites
   • Step-by-Step Installation Guide
   • Setting up Your First Lerna Project
4. Chapter 3: Dependency Management in Lerna
   • Independent Dependencies
   • Hoisting Shared Dependencies
   • Bootstrapping Packages
5. Chapter 4: Running Scripts Across Packages
   • Executing Scripts Globally
   • Targeting Specific Packages
6. Chapter 5: Versioning and Publishing with Lerna
   • Fixed Mode vs Independent Mode
   • Publishing Packages to npm
7. Chapter 6: Using Lerna with Yarn Workspaces
   • Enabling Yarn Workspaces in Lerna
   • Optimizing Workflow with Lerna Yarn Workspaces
8. Chapter 7: Advanced Lerna Usage
   • Filtering Commands
   • Creating Custom Commands
9. Chapter 8: Best Practices for Lerna Monorepos
   • Logical Package Organization
   • Automating Tests and Builds
   • CI/CD Integration
10. Conclusion
11. Appendix: Common Lerna Commands

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Introduction

Managing large-scale JavaScript or TypeScript projects involving multiple interdependent packages can be a significant challenge for developers and development teams. Often, developers rely on multiple repositories for each package, which results in overhead in terms of code maintenance, dependency management, and collaboration.

Lerna, a powerful tool developed for managing monorepos, streamlines this process. Monorepos enable teams to host multiple packages in a single repository, simplifying dependency management and making collaboration across teams smoother.

This ebook aims to provide a complete guide to using Lerna to manage your monorepos efficiently. Whether you’re dealing with a component library or a large-scale project with several interconnected packages, you’ll find valuable insights to help you maximize your productivity with Lerna.

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Chapter 1: What is Lerna?

Lerna is an open-source tool that facilitates the management of multiple packages in a monorepo. It offers powerful features such as automatic dependency management, versioning, and publishing, making it easier to maintain JavaScript and TypeScript projects at scale.

Why Monorepos?

Monorepos are an architectural choice for many large-scale projects, as they provide several benefits:

• Shared Codebase: With monorepos, code reuse is easier. This reduces duplication and ensures consistency across projects.
• Simplified Collaboration: Developers can work together more effectively when all the packages are in one place.
• Unified Build Processes: Standardizing build, testing, and deployment across multiple packages becomes easier.

Despite these benefits, managing a monorepo can bring unique challenges, particularly in managing dependencies and versioning. Lerna is designed to tackle these challenges head-on, providing an optimized workflow for monorepos.

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Chapter 2: Installing and Setting Up Lerna

Prerequisites

Before starting, ensure you have Node.js and npm (or Yarn) installed. Lerna is compatible with both npm and Yarn.

Step 1: Installing Lerna

You can install Lerna globally via npm:

npm install --global lerna

Alternatively, you can add Lerna as a development dependency in your project:

npm install --save-dev lerna

Step 2: Initializing a Lerna Monorepo

Once installed, initialize your monorepo by navigating to your project directory and running:

lerna init

This will create the essential configuration files, including lerna.json, and set up a packages folder where your individual packages will reside.

Step 3: Adding Packages

In a Lerna project, each package lives in its own subfolder under packages. Each package must have its own package.json file for dependency management.

Sample structure:

/my-project
  /packages
    /package-a
    /package-b
  lerna.json
  package.json

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Chapter 3: Dependency Management in Lerna

Managing dependencies across multiple packages is one of Lerna’s core strengths.

Independent Dependencies

Lerna allows you to add dependencies to a specific package. For example, if only package-a needs lodash, you can run:

lerna add lodash --scope=package-a

Hoisting Shared Dependencies

When multiple packages share dependencies, you can hoist those dependencies to the root of your monorepo. This reduces redundancy and speeds up installations. To enable hoisting, add this to lerna.json:

{
  "hoist": true
}

Bootstrapping

To install dependencies and link packages that depend on one another, run:

lerna bootstrap

This ensures that all necessary external dependencies are installed and that packages can reference each other properly.

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Chapter 4: Running Scripts Across Packages

Lerna makes it easy to execute scripts (e.g., build, test, lint) across all packages in your monorepo.

Executing Scripts Globally

To run a script like build across all packages, use:

lerna run build

Targeting Specific Packages

If you only want to run a script in certain packages, use the --scope flag:

lerna run test --scope=package-a

This flexibility allows for more targeted execution, saving time during development.

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Chapter 5: Versioning and Publishing with Lerna

Lerna provides robust versioning and publishing features, allowing you to easily version and release packages.

1. Fixed Mode

In fixed mode, all packages share the same version number. When any package is updated, the version number is incremented for all.

2. Independent Mode

In independent mode, each package has its own version number. When a package is changed, only that package’s version is updated.

To switch to independent mode, modify lerna.json:

{
  "version": "independent"
}

Publishing Packages

To publish your packages to npm, run:

lerna publish

Lerna will handle versioning and publishing based on your configuration.

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Chapter 6: Using Lerna with Yarn Workspaces

Combining Lerna with Yarn Workspaces can further optimize dependency management by hoisting even more shared dependencies.

To enable Yarn Workspaces, modify your lerna.json file:

{
  "npmClient": "yarn",
  "useWorkspaces": true
}

Then update your package.json:

{
  "workspaces": ["packages/*"]
}

This integration boosts performance and simplifies managing large-scale projects.

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Chapter 7: Advanced Lerna Usage

Filtering Commands

Lerna allows filtering to run commands for specific packages or to exclude certain packages.

Example for running on specific packages:

lerna run build --scope=package-a --scope=package-b

Example for excluding packages:

lerna run build --ignore=package-c

Custom Commands

You can define custom Lerna commands within package.json for specialized workflows. These commands can then be run across your packages.

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Chapter 8: Best Practices for Lerna Monorepos

1. Organize Packages Logically: Group related packages together for better code reuse.
2. Use Hoisting: Hoisting shared dependencies saves space and speeds up install times.
3. Automate Testing: Use lerna run to automate testing across your entire monorepo.
4. CI/CD Pipelines: Implement continuous integration and deployment workflows to automatically test and deploy changes.
5. Yarn Workspaces: Leverage Yarn Workspaces with Lerna for better dependency management.

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Conclusion

Lerna is an invaluable tool for managing monorepos, offering features that simplify complex workflows, from dependency management to versioning and publishing. By adopting Lerna, teams can reduce complexity, streamline processes, and improve collaboration, making it easier to maintain large-scale projects.

Whether you’re working on a simple component library or a multi-package ecosystem, Lerna provides the tools needed to manage your project effectively. Keep experimenting with Lerna’s advanced features to unlock its full potential.

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Appendix: Common Lerna Commands

• lerna init: Initializes a Lerna monorepo.
• lerna bootstrap: Installs dependencies and links packages.
• lerna add [package] --scope=[package-name]: Adds a dependency to a specific package.
• lerna run [script]: Runs a script across all packages.
• lerna publish: Publishes packages to npm.

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Happy Coding :)

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