Migrate Node Output To Esm

Make Node-target rsbuild builds runnable under `@rsbuild/core@2`

@rsbuild/core@2 changed the default output format for Node targets. In v1, output.target: 'node' produced minified CommonJS. In v2 it produces unminified ESM (output.module defaults to true, output.minify to false).

A v2 ESM bundle does not run as plain node dist/index.js unless one of the following is true:

a package.json with "type": "module" governs the output, or
the output files use the .mjs extension.

Without one of those, the build succeeds but the process crashes at runtime with Cannot use import statement outside a module.

This is workspace-specific — it depends on whether the project owns a package.json, what runs or references the bundle (a main field, a Dockerfile, a deploy script, a serve target), and whether other .js files in the project are CommonJS. So it ships as an AI-assisted instruction rather than a blind codemod.

What needs to change

For every project with an rsbuild.config.{ts,js,mjs,cjs} whose output.target is 'node':

Decide how the ESM output should be made runnable. Prefer, in order:
- The project owns its own package.json → set "type": "module" on it. This is rsbuild's recommended convention. Check first that no sibling .js files in the project rely on CommonJS (require, module.exports, __dirname); if any do, rename them to .cjs or convert them to ESM.
- The project has no package.json → add .mjs output to the rsbuild config instead, so the output is ESM regardless of any ancestor package.json:
  ts
```
export default defineConfig({
  output: {
    target: 'node',
    filename: { js: '[name].mjs' },
  },
});
```
Update anything that references the output filename. If you chose .mjs, fix references in package.json#main, Dockerfiles, serve targets, deploy scripts, and any import/require of the built file.

Do not pin back to CommonJS unless the project genuinely cannot move to ESM. If you must, restore the v1 behavior explicitly:

export default defineConfig({
  output: {
    target: 'node',
    module: false,
    minify: true,
  },
});

Sample Code Changes

Before — Node project, no project package.json

export default defineConfig({
  output: {
    target: 'node',
    distPath: { root: 'dist' },
  },
});

After — `.mjs` output so the ESM bundle runs

export default defineConfig({
  output: {
    target: 'node',
    filename: { js: '[name].mjs' },
    distPath: { root: 'dist' },
  },
});

Reference

@rsbuild/core v1 → v2 upgrade guide — Node output

Make Node-target rsbuild builds runnable under @rsbuild/core@2