Migrate build from tsup to tsdown (Rolldown)

[jbetancur]

Summary

Evaluate migrating the library build from tsup to tsdown (the Rolldown-based successor).

Motivation

tsup maintenance has slowed — latest publish was ~8 months ago (v8.5.1, 2025-11-12) — while tsdown is actively developed and is the direction the esbuild/Rolldown tooling ecosystem is consolidating toward. Rolldown also generally produces faster builds and better-optimized output. This is about staying on maintained tooling, not a bug in the current build.

Current build surface (what a migration must preserve)

tsup.config.ts has three build passes and leans on a few tsup-specific behaviors that need to be re-verified under tsdown:

• Dual format: esm + cjs, with dts: true, sourcemaps, minify, treeshake, external: ['react','react-dom'].
• injectStyle: true on the main bundle (CSS injected at runtime so most consumers need no CSS import). tsdown's CSS handling differs — this is the highest-risk item.
• CSS-only pass: a third build that emits dist/DataTable.css for SSR consumers (Next.js App Router), then deletes the duplicate JS via an onSuccess rm. Needs an equivalent.
• "use client" banner: added post-build via scripts/use-client-banner.mjs because tsup strips a configured banner during treeshake. Confirm whether tsdown preserves a top-of-file directive natively (may let us drop the script).
• Locales sub-bundle: ./locales entry, and the ./css export.

Acceptance criteria

• dist/ output is byte-compatible in structure: index.js/index.mjs/index.d.ts, locales/, DataTable.css, sourcemaps.
• All three exports map entries (., ./locales, ./css) resolve identically.
• "use client" is present at the top of both index.js and index.mjs.
• Runtime CSS injection still works for the default import path; the explicit ./css import still works for SSR.
• npm run build + the existing test/lint/typecheck all pass; smoke-test in a Next.js App Router app (Server Component import) and a plain Vite app.

Notes

Keep it behind a branch and diff the dist/ output against the tsup build before switching. If any of the CSS-injection or use client behaviors regress and can't be reproduced cleanly, hold — the current build works, so this is opportunistic, not urgent.