oj README

The project's README from github.com/lovablelabs/oj. As one markdown file: README.md.

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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

Supported targets

oj embeds V8, so it builds on the targets rusty_v8 ships prebuilt static libs for: Linux glibc x64/arm64, Linux musl x64/arm64, macOS x64/arm64, and Windows MSVC x64/arm64.

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):

tool cold start warm start reload HMR server RSS
oj --bundle 311/315ms 240/243ms 41/42ms 56/57ms 43MB
oj (unbundled) 434/582ms 352/354ms 170/173ms 56/59ms 43MB
vite 701/734ms 653/659ms 172/176ms 55/73ms 434MB
vite-fbm 326/344ms 334/338ms 49/52ms 55/58ms 361MB

5,000 components (p50/p95):

tool cold start warm start reload HMR server RSS
oj --bundle 750/783ms 588/604ms 132/133ms 59/61ms 75MB
oj (unbundled) 1281/1402ms 1058/1080ms 744/839ms 60/63ms 65MB
vite 2649/2656ms 2424/2532ms 758/807ms 56/147ms 949MB
vite-fbm 927/970ms 965/973ms 174/177ms 59/80ms 976MB

10,000 components (p50/p95):

tool cold start warm start reload HMR server RSS
oj --bundle 1315/1333ms 1129/1442ms 232/237ms 67/72ms 115MB
oj (unbundled) 2492/2543ms 2068/2303ms 1523/1687ms 67/176ms 94MB
vite 5693/6085ms 5304/6289ms 1781/1820ms 59/64ms 1552MB
vite-fbm 1528/2225ms 1482/2176ms 302/414ms 59/65ms 1751MB

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

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.

oj (lovablelabs/oj)

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604 ·GitHub
Oj
What is this?OJ is a Rust-native build tool for React apps — a Vite-style dev server and bundler optimized for memory and cold start, built for setups that run many builds (CI, coding agents, multi-tenant).
Is this a replacement for Vite, or does it still need Vite to run?It's a standalone replacement, not a Vite plugin: point oj at an existing React app and it runs with no source changes, reading your vite.config where available. Production output comes from Rolldown, the same engine Vite uses.
How do I try it on my existing React app?Install the CLI from crates.io (Nix also works), then run oj in the app directory. No code changes are needed — it picks up your existing setup.
Will my Vite config and plugins carry over?Mostly yes — oj can read vite.config.{ts,js,mjs} and take its plugins array directly, or you can drop an oj.plugins.mjs at the app root. It also adopts base, server.port/host, define and resolve.alias for fields its own config leaves unset.
Does it work on macOS, Linux and Windows?All three, on x64 and arm64. Since oj embeds V8, it builds only where rustyv8 ships prebuilt static libs: Linux glibc and musl, macOS, and Windows MSVC.
Why build this when Vite already exists?Running many builds under Vite — CI jobs, coding agents, multi-tenant setups — gets expensive in memory and cold-start time. OJ optimizes exactly those costs while keeping Vite's plugin model and output.
How does the dev server stay fast between edits?In dev, server modules run in a small persistent Node process that re-evaluates only what changed, instead of rebuilding a bundle on every request.
Is HMR still as fast as Vite's?Yes — the benchmarks show HMR is essentially a wash across oj and both Vite dev modes, staying within roughly 10ms at every app size tested.
Is it ready for real projects today?The project calls itself experimental, but it is built to run real production React apps unmodified, and production builds are byte-identical to Vite's since both use the Rolldown engine.
What are similar projects?rolldown, router, oxc, esbuild, vite, rollup, rspack, nx
Which topics is it in?v8, bundler, ssr, cloudflare-workers, build-tool, vite, performance, rust, react, free, frontend, nodejs
Creator
lovablelabs
Size
6.8 MB
Stars
604
Updated
CommitOct 16Releasev0.2.36Oct 16
Age
2mo
Commits · 90 Days