[@rollup/plugin-typescript] Generate .d.cts files from entryFilenames

[drwpow]

• Rollup Plugin Name: @rollup/plugin-typescript
• Rollup Plugin Version: *

Feature Use Case

@rollup/plugin-typescript can be used to generate dual CJS + ESM builds, and it correctly generates *.cjs files using entryFilenames. However, for .cjs files, TypeScript requires a .d.cts matching extension. But this plugin will always output at *.d.ts, no matter the settings.

This means when you have a dual CJS + ESM build, you should be getting 2 copies of declarations, but one will always overwrite the other. Yes, the contents are identical. Yes, TypeScript should just figure it out. But TypeScript has chosen to require 1 definition to 1 file, always, which means .cjs files are untyped unless explicit .d.cts definitions exist.

Same for .mjs—if requested, those need .d.mts files as well, if using that extension.

Feature Proposal

import ts from '@rollup/plugin-typescript';

export default {
  input: 'src/index.ts',
  plugins: [ts({
    compilerOptions: {
      declaration: true
    }
  })],
  output: [
    {
      dir: './dist/',
      format: 'cjs',
      preserveModules: true,
      sourcemap: true,
      entryFileNames: '[name].cjs'
    }
  ],
};

Given this Rollup config, this should produce *.d.cts declarations. This may be a breaking change for some folks, but would satisfy TypeScript