Getting started

What uf is

The problem it is aimed at

A Flow and React project today is an assembly job. Babel to strip the types, a preset to understand component, a plugin for the React Compiler, Vite for the dev server, a second config to tell Vite about the first one, Vitest with its own transform pipeline that has to be configured the same way, Prettier for formatting, ESLint with a parser plugin so it can read the syntax, and a tsconfig.json you do not use but something insists on.

Every one of those is a place where two tools can disagree about what your code means. When they do, the error surfaces somewhere unrelated: a test that passes and a build that fails, or a formatter that reflows syntax the linter then rejects.

uf's position is that there should be exactly one answer to "what does this file mean", that it should be given once, in Rust, and that everything else should use it.

What that buys

One parse of the modern syntax. component, hook, renders, match and Flow enums are understood by Meta's own Flow parser, compiled into the binary. The dev server, the build, the test runner, the formatter and the linter all read the same tree. There is no second parser to keep in step.

No Babel. Flow reaches JavaScript through the official Flow parser, ports of Hermes' own lowering passes, the official React Compiler crate in syntax mode, and oxc for JSX and codegen. Nothing in that list is a plugin you install.

One config. uf.config.js is the only config file. It is Flow, so it is type-checked, and it configures the runtime, the router, the build, the test runner and the formatter together.

One command. uf dev, uf build, uf test, uf fmt, uf lint, uf check — nothing to add to package.json for any of them.

What it does not do

uf does not replace Vite, and does not want to. The dev server and the production build are Vite 8 — uf decides what Vite is handed and drives it over a JSON protocol. Vite's plugin ecosystem keeps working, and a project that outgrows uf's opinions can reach past them.

uf does not type-check your code. Flow does that, and uf check runs it.

uf is not finished. It is at 0.0.0-alpha, every release so far is a prerelease, and parts of it lose to what they mean to replace — the test runner is about three times slower than Bun's, and its own page says so rather than leaving you to find out.

Where to go next

To try it, install it and then build something. To understand the Flow pipeline before trusting it with anything, read Flow, the modern parts.

Next: Why Flow →