How to get rollup to include a dependency from another package in a lerna monorepo in its transpilation TypeScript

Getting Rollup to Play Nice: Including Dependencies from Other Packages in a Lerna Monorepo

Building a large, modular application with TypeScript and Lerna can be a powerful combination. However, managing transpilation with Rollup in a monorepo can present a unique challenge: how do you get Rollup to recognize and include dependencies from other packages within your monorepo?

This article will guide you through the process, providing a clear roadmap for incorporating dependencies from other packages into your Rollup configuration.

Understanding the Problem

When Rollup processes your code, it needs to know where to find the dependencies used in your project. In a typical project, these dependencies are usually found in node_modules. However, in a Lerna monorepo, your dependencies might reside in other packages within your repository.

This is where the challenge arises: Rollup might not automatically resolve these internal dependencies.

The Solution: Linking and Configuration

The key to resolving this lies in a two-step approach:

1. Linking Your Packages:

This involves creating symbolic links within your monorepo, pointing from the node_modules directory of your package to the respective folders of its dependencies within the monorepo.

• Using lerna bootstrap: This is the simplest method. The lerna bootstrap command will automatically link your packages, creating the necessary symbolic links within node_modules of each package. This ensures that Rollup can locate the dependencies during the build process.
• Manual Linking: If you prefer more control or face issues with lerna bootstrap, you can manually link your packages. Use npm link or yarn link to create the necessary links between the packages.

2. Configuring Rollup:

Once your packages are linked, you need to configure Rollup to recognize these links:

• Plugin Usage: Employ plugins like rollup-plugin-node-resolve to guide Rollup to look within the node_modules directory, including the linked folders. This plugin resolves dependencies from both your local node_modules and those within linked packages.
• Explicit Path Resolution: For granular control, you can explicitly provide the paths to your dependent packages within the input and output options of your Rollup configuration. This is especially helpful when dealing with non-standard dependency locations.

Here’s an example using rollup-plugin-node-resolve:

import resolve from 'rollup-plugin-node-resolve';

export default {
  input: 'src/index.ts',
  output: {
    file: 'dist/index.js',
    format: 'cjs',
  },
  plugins: [
    resolve({
      extensions: ['.ts', '.js']
    })
  ],
};

Best Practices for Smooth Rollup Integration

• Use Consistent Directory Structure: Maintaining a clear directory structure across your Lerna monorepo makes linking and resolving dependencies easier.
• Clear Package Dependencies: Use package.json files to define dependencies clearly for each package. This helps Rollup identify which packages to include.
• Leverage Lerna Features: Utilize Lerna’s features like lerna publish to manage package versioning and avoid potential conflicts during linking.

Conclusion

Understanding the challenges and implementing these solutions ensures that Rollup effectively transpiles your code, incorporating dependencies from other packages in your Lerna monorepo. By following these guidelines, you’ll streamline your build process and create a more robust and modular project structure.