OJ

OJ is a Rust-native build tool for React apps.

It optimizes for memory and cold start, where running many builds (CI, agents, multi-tenant) under Vite gets expensive. OJ is meant to run real production React apps without changes to their source.

Server rendering

There is an SSR mode: oj dev --ssr src/entry-server.tsx in dev, and oj build --ssr for production. It streams the HTML out with renderToReadableStream instead of buffering, and the client hydrates through the normal dev pipeline, so Fast Refresh and HMR keep working over a server-rendered page.

In dev the server modules run in a small persistent Node process (a module runner using vm.SourceTextModule) that re-evaluates only what changed, instead of rebuilding a bundle per request.

Plugins

Vite/Rollup-style plugins run through a persistent Node plugin host. Drop an oj.plugins.mjs at the app root that default-exports a plugin array, or let oj read an app's vite.config.{ts,js,mjs} and pick up its plugins array directly.

A TypeScript config (including one that imports local .ts files) is loaded via Vite's own config loader when Vite is installed, or bundled with the app's esbuild otherwise. From a vite.config oj also adopts the app's base, server.port/host, define, and resolve.alias for any field its own config leaves unset, alias entries resolve alongside tsconfig paths, in both oj dev and oj build.

TanStack Start

Point oj at a TanStack Start app and it runs the framework directly, no source changes. It detects Start from the app itself:

oj dev web                                       # dev server for the ./web Start app
oj build web                                     # production build

Dev serves file-based routes with streaming SSR and client hydration, keeps Fast Refresh working, runs server functions in the module runner, and applies the app's vite.config plugins (React and the app's own). oj build emits a Node server.mjs, a Cloudflare worker.mjs (nodejs_compat), content-hashed client assets, and prerendered routes. oj's own docs site (www/) is a TanStack Start app built by oj and served from a Cloudflare Worker.

Quickstart

Install the CLI from crates.io, then run oj in any app:

cargo install oj --locked                       # install the `oj` CLI
oj dev                                           # dev server for the current app on :5199
oj build                                         # production build into ./dist

Or with Nix:

nix run github:lovablelabs/oj -- dev

Benchmarks

Generated fanout-10 React component trees, measured save-to-paint with Playwright against Vite 8.2.1 (Rolldown-based) on an M-series Mac, in both its default dev mode (vite) and its experimental bundled dev mode (vite-fbm). p50/p95 over 5 cold+warm restart cycles and 10 HMR edits.

1,000 components (p50/p95):

5,000 components (p50/p95):

10,000 components (p50/p95):

Bundle-mode oj wins cold start, warm start, and reload against Vite's default dev at every size (4-8x at 10k), and now also beats Vite's experimental bundled dev on all three at every size (cold, warm, and reload). HMR is a wash across all four (within ~10ms). oj's decisive, consistent win is memory: 43-115MB against Vite's 361MB-1.75GB, an 8-15x gap that widens with app size.

Production builds (oj build vs vite build) land at parity: same engine (Rolldown), byte-identical output sizes.

Reference reading

• oxc_transformer/examples/transformer.rs: the pipeline oj's compiler is based on
• oxc_transformer/src/jsx: JSX + ReactRefresh transform internals
• vitejs/vite-plugin-react: the Fast Refresh glue semantics oj replicates
• vite/packages/vite/src/node/server: HMR propagation, import.meta.hot protocol
• rolldown/rolldown: plugin hook filters, the prod linker oj embeds

Misc

cargo run -p oj -- dev                            # dev server for ./playground on :5199
cargo run -p oj -- dev --bundle                   # registry-runtime bundle mode
cargo run -p oj -- dev --ssr src/entry-server.tsx # streaming SSR + hydration
cargo run -p oj -- build playground               # production build into playground/dist
cargo test --workspace                            # rust unit tests
node --test e2e/unit/*.test.mjs                   # js unit tests (adapter helpers)
node e2e/run.mjs                                  # browser e2e suite (add --bundle for bundle mode)
node e2e/ssr-dev.mjs                              # SSR dev e2e, e2e/ssr-prod.mjs for the built server
node e2e/start.mjs                                # tanstack start integration (see e2e/fixtures/start-app)
node e2e/dep-optimize.mjs                         # dependency pre-bundle + cjs interop integration
node e2e/assets.mjs                               # asset url imports + new URL(import.meta.url)
node e2e/dynamic-import.mjs                       # dynamic import with variables (glob switch)
node e2e/wasm.mjs                                 # wasm ?init instantiation (dev + build)
node e2e/query-assets.mjs                         # ?url/?raw/?inline/?init in bundle mode
node e2e/rolldown-options.mjs                     # build.rollupOptions filenames + external
node e2e/preprocessors.mjs                        # less + stylus css (installs both, then dev+build)
node e2e/assets-inline.mjs                        # assetsInlineLimit: small assets become data uris
node e2e/config-proxy.mjs                         # adopt vite.config server.proxy + ignored-config warnings
node e2e/proxy-regex-context.mjs                  # server.proxy "^..." regex contexts (path + query), like Vite
node e2e/config-function.mjs                      # oj.config function form ({ command, mode }) => config
node e2e/build-target-raw-inline.mjs              # build.target downleveling + ?raw/?inline in build
node e2e/manual-chunks.mjs                        # rollupOptions output.manualChunks vendor splitting
node e2e/svgr.mjs                                 # svg as react component (?react), installs react
node e2e/worker-modes.mjs                         # ?worker in dev, bundle, and production build
node e2e/html-entry.mjs                           # relative index.html script entry (src="src/x")
node e2e/svelte.mjs                               # svelte 5 components in dev, bundle, and build
node e2e/build-mode.mjs                           # build --mode (import.meta.env.MODE + .env.<mode>)
node e2e/hmr-protocol.mjs                         # hmr client derives wss (behind https proxy)
node e2e/hmr-overlay-recovery.mjs                 # startup compile error: buffered overlay, reload on first update
node e2e/hmr-json-update.mjs                      # editing an imported .json hot-updates its boundary (?t= stamp)
node e2e/hmr-unresolved-import.mjs                # missing ./import fails with an overlay; creating the file recovers
node e2e/host-binding.mjs                         # dev/preview --host + server.host bind all interfaces
node e2e/plugin-ws.mjs                             # plugin server.ws custom events (send/on) round-trip
node e2e/plugin-ws-execute.mjs                     # post -> ws broadcast -> client reply -> collect (bridge execute)
node e2e/plugin-middleware.mjs                     # configureServer post body forwarding + transformIndexHtml
node e2e/hmr-gate.mjs                              # hmr gate holds updates until POST /__hmr_flush
node e2e/config-flag.mjs                           # oj dev --config <path> loads an override config
node e2e/config-wrapper.mjs                        # vite.config that calls an external defineConfig wrapper
node e2e/awkward-paths.mjs                         # percent-encoded filenames served, traversal contained
node bench/generate.mjs 1000                      # generate a benchmark app (then npm i inside it)
node bench/run.mjs 1000                           # p50/p95 benchmark vs vite
node bench/card.mjs                               # render bench/card.html to oj-benchmarks.png

Testing

cargo test --workspace                            # unit + integration suites
node e2e/run.mjs                                  # the end-to-end suite

The suite is organized by failure mode rather than by module ~ adversarial input, boundary shapes, contention, injected faults, and properties that hold for every input ~ and docs/development/testing.md describes the layers, the fuzz targets, and the behaviours that are deliberate boundaries rather than gaps.

License

MIT, see LICENSE.

OJ was created by Raphael Amorim and has since migrated to Lovable, which now maintains the project.