tsdown.config.ts

import { readFile, writeFile } from 'node:fs/promises';
import { defineConfig } from 'tsdown';

const CLIENT_IMPORT = '"use client"; import { useSay as GET_SAY } from "./client.mjs";';
const SERVER_IMPORT = 'import { getSay as GET_SAY } from "./server.mjs";';

export default defineConfig({
  entry: ['src/runtime/index.ts', 'src/runtime/client.ts', 'src/runtime/server.ts'],
  target: 'es2020',
  async onSuccess() {
    const index = await readFile('dist/index.mjs', 'utf8');
    await writeFile('dist/index.server.mjs', SERVER_IMPORT + index);
    // The `"use client"` build stays at `dist/index.mjs`. Naming it
    // `index.client.mjs` made framework import guards that deny `**/*.client.*`
    // in a server environment (TanStack Start) reject it, even though a
    // `"use client"` module is meant to be server-rendered.
    await writeFile('dist/index.mjs', CLIENT_IMPORT + index);
  },
});