How to Use the Command 'lerna' (with examples)

Lerna is a tool that simplifies the management of JavaScript projects with multiple packages, known as monorepos. It is especially useful for projects where multiple interrelated packages are being developed in tandem, as it automates many of the tasks that come with dependency management, versioning, and publishing. More information can be found on the official Lerna website .

Initialize Project Files

Code:

lerna init

Motivation:

In any project, initialization is a critical first step to set up the necessary infrastructure and files needed to organize code systematically. Running lerna init creates the foundational files for a Lerna-managed monorepo, including lerna.json, package.json, and initiates a Git repository with a .git directory. This command is essential for scaffolding out the project structure, streamlining the management of multiple packages under one repository.

Explanation:

• lerna init: This standalone command takes care of bootstrapping a new Lerna project by creating the necessary configuration files. No additional arguments are needed because the command is designed to establish a standard structure automatically.

Example output:

lerna notice cli v4.0.0
lerna info Creating package.json and lerna.json files
lerna success Initialized Lerna files
Initialized a Lerna project.

Install External and Local Dependencies

Code:

lerna bootstrap

Motivation:

Managing dependencies is often a daunting task, especially when dealing with multiple packages that might rely on one another. lerna bootstrap is a powerful command that automates the installation of external dependencies in each package and establishes symbolic links between local packages that depend on each other. This means your local packages can be developed and tested as if they are installed like node modules, ensuring a synchronized development experience.

Explanation:

• lerna bootstrap: This command will scan through each package inside the monorepo, install any specified external dependencies, and link the local packages that are dependent on others within the same repository.

Example output:

lerna notice cli v4.0.0
lerna info Bootstrapping 4 packages
lerna info Symlinking packages and binaries
lerna info Installing external dependencies
lerna success Light bootstrap complete

Run Specific Scripts in Packages

Code:

lerna run build

Motivation:

In development, it’s often necessary to consistently run scripts across multiple packages, such as build, test, or lint scripts. lerna run script allows developers to execute a specific script, like a build process, across all packages that have it defined in their package.json. This command facilitates uniform script execution, saving time and reducing the room for error by ensuring that all components are built or tested in a unified manner.

Explanation:

• lerna run build: Here, run directs Lerna to execute scripts, while build is the specific script target. Replace build with any script name that your packages might have such as test or lint.

Example output:

lerna notice cli v4.0.0
lerna info Executing command in 3 packages: "npm run build"
package-1: > package-1@1.0.0 build
package-1: > webpack --config webpack.config.js
package-1:
package-1: Build successful
package-2: > package-2@1.0.0 build
package-2: > webpack --config webpack.config.js
package-2:
package-2: Build successful

Execute Arbitrary Shell Commands

Code:

lerna exec -- ls

Motivation:

Sometimes, there’s a need to perform system-level operations or execute a custom shell command across multiple packages. The lerna exec command enables users to perform these actions by running a specified shell command in the context of each package. For example, listing files or checking file sizes can be useful for debugging or verifying deployments.

Explanation:

• lerna exec: This Lerna sub-command runs an arbitrary command across all packages.
• -- is used to indicate the end of Lerna-specific options and the start of the shell command.
• ls: This is the shell command that will be executed, showing the contents of each package directory.

Example output:

lerna notice cli v4.0.0
lerna info Executing "ls" in 4 packages
packages/package-1: src dist README.md
packages/package-2: src dist README.md
packages/package-3: src dist README.md

Publish Updated Packages

Code:

lerna publish

Motivation:

Publishing new versions of packages can be tedious, especially when multiple packages have changed since the last release. The lerna publish command streamlines this process by identifying packages that have been modified, determining new version numbers, and publishing them to the npm registry. This compilation step ensures consistency in versioning and automates package updates, significantly simplifying release management.

Explanation:

• lerna publish: This standalone command searches for packages that have changes since the last version and prepares them for publishing. It announces each package’s new version number and handles any necessary release tasks to push them to the package registry.

Example output:

lerna notice cli v4.0.0
lerna info Checking for changed packages since v1.0.0
lerna info Wrote versions to lerna.json
lerna info Publishing packages to npm
lerna success Published package-1@1.1.0
lerna success Published package-2@1.1.0
lerna success Published package-3@2.0.0

Conclusion:

Lerna offers a robust suite of tools tailored for the efficient management of multi-package JavaScript projects. From initializing a monorepo, synchronizing dependencies, running scripts, executing shell commands, to publishing updated packages, Lerna simplifies numerous developer tasks. Leveraging Lerna’s capabilities can drastically reduce complexity and enhance productivity in projects with several interconnected components.