What is Lerna Publish?

Lerna publish is a powerful command within the Lerna ecosystem designed to automate and streamline the release process for multiple JavaScript/TypeScript packages managed within a single repository, also known as a monorepo. As a fast, modern build system for managing and publishing packages from the same repository, Lerna's publish command is central to its utility, enabling developers to release new versions of their packages to a registry like npm with efficiency and consistency.

Understanding Lerna's Role in Monorepos

Before diving into lerna publish, it's helpful to understand Lerna itself. Lerna serves as a build system that helps organize and manage projects where multiple distinct packages share a single Git repository. This approach, known as a monorepo, offers benefits such as simplified dependency management, easier cross-package testing, and unified tooling. Lerna then provides commands to orchestrate tasks across these packages, with publish being one of the most critical.

How Lerna Publish Works

The lerna publish command automates a series of crucial steps required to release updated packages, eliminating manual errors and standardizing the release workflow. Here's a breakdown of its typical operation:

1. Detecting Changes: Lerna first identifies which packages have had changes since the last release. It compares the current state of your repository with the latest Git tag to pinpoint modified packages.
2. Versioning: Based on the detected changes and your project's chosen versioning strategy, Lerna proposes or automatically bumps the version numbers for the affected packages.
3. Git Tagging and Committing: It then creates a Git commit for the version bumps and adds a Git tag (e.g., v1.2.3 or [email protected]) to mark the release point in your repository's history.
4. Publishing to Registry: Finally, Lerna executes npm publish for each updated package, releasing them to your configured package registry (most commonly, npm).

Key Features and Benefits

Utilizing lerna publish offers significant advantages for monorepo management:

• Automated Version Management: Reduces the overhead of manually tracking and updating package versions.
• Consistent Release Workflow: Ensures that all packages adhere to a standardized release process, minimizing human error.
• Dependency Synchronization: Helps manage inter-package dependencies within the monorepo, ensuring compatible versions are released together.
• Streamlined CI/CD: Integrates seamlessly into continuous integration and deployment pipelines, automating releases upon successful builds.
• Improved Developer Experience: Allows developers to focus on code rather than the complexities of publishing multiple packages individually.

Lerna Publish Strategies

Lerna offers different strategies for how packages are versioned and published:

1. Fixed/Synchronized Mode (Default)

In fixed mode (the default for new Lerna projects), all packages within the monorepo share the same version number. When any package changes and lerna publish is run, Lerna prompts to bump the entire project's version. All packages that have changed will then be published under this new, unified version.

• Pros: Simplifies versioning for closely related packages; easy to see the project's overall version.
• Cons: Can lead to unnecessary version bumps for packages that haven't changed significantly.
• Example lerna.json configuration:

  {
    "version": "1.0.0",
    "packages": ["packages/*"],
    "npmClient": "npm"
  }

2. Independent Mode

In independent mode, each package within the monorepo maintains its own distinct version number. When lerna publish is executed, Lerna identifies changed packages and prompts for a version bump for each individual package that has been modified.

• Pros: More granular control over individual package versions; avoids unnecessary version bumps.
• Cons: Can make it harder to track the overall state of the monorepo; more complex versioning scheme.
• Example lerna.json configuration:

  {
    "version": "independent",
    "packages": ["packages/*"],
    "npmClient": "npm"
  }

Practical Examples and Common Options

Here are some common ways to use lerna publish and notable options:

Option / Command	Description
lerna publish	The basic command. Prompts for version bumps and publishes changed packages.
lerna publish --conventional-commits	Automates version bumping based on Conventional Commits in your Git history, generating changelogs. Highly recommended for automated releases.
lerna publish --canary	Publishes a "canary" version (e.g., 1.0.0-alpha.0.commitsha) for all changed packages. Useful for continuous integration and testing pre-releases.
lerna publish --force-publish <packages>	Forces the publishing of specified packages, even if Lerna doesn't detect changes. Useful for re-releasing or fixing corrupted packages.
lerna publish --dry-run	Simulates the publish process without actually making any changes or publishing. Great for testing your release configuration.

Example Workflow with Conventional Commits:

Let's say you have a monorepo with package-a and package-b. After making changes to package-a and committing with feat: added new feature to package-a, you can run:

lerna publish --conventional-commits

Lerna will then:

1. Detect changes in package-a.
2. Suggest a minor version bump for package-a based on the feat: commit.
3. Ask for confirmation.
4. Update package-a's package.json version.
5. Create a Git commit and tag.
6. Publish package-a to npm.

Conclusion

lerna publish is an indispensable command for anyone managing a JavaScript/TypeScript monorepo. It transforms the often tedious and error-prone process of releasing multiple packages into an automated, consistent, and efficient workflow, leveraging Lerna's capabilities as a robust build system.