Flow is now written in Rust 🦀 The migration from OCaml to Rust brought up to 2x faster type-checking on large codebases.

TypeScript lets you pass Array<string> where Array<string | Error> is expected, so the function pushes an Error to your string array. Flow rejects it. TS Playground, 0 errors: typescriptlang.org/play/?#co… Flow Playground, rejects with an error: flow.org/try/#1N4Igxg9gdgZgl…

Crashes at runtime - but TypeScript accepts it: zero errors under strict. Can you spot the bug?

Another: TypeScript only validates the signature of a type-guard, not the body. Flow validates the body and rejects predicates that lie about what they check.

Where the two still diverge, Flow keeps the stricter, safer default. Extract a method off an instance and it loses its 'this' binding. TypeScript lets the extraction through, so tick() crashes without a valid 'this'. Flow rejects it right where 'this' is lost.

First-class component syntax for React. 'renders' types compose through wrappers - MainHeader renders Header slots anywhere a Header is expected, and unrelated elements are rejected.

Flow also has features TypeScript lacks. Pattern matching as an expression. It's exhaustively checked, with structural patterns that destructure as they match. Forget a case, like 'remove', and match flags it and tells you what to add.

TypeScript or Flow? You can't tell. Flow has keyof, readonly props, unknown, indexed access types T[K], and extends generic bounds. Plus conditional types, mapped types, and type guards.

Over the past few weeks we've been picking up our slack on pull requests. Thank you for all the contributions!

Flow just reached 10,000 stars on GitHub! github.com/facebook/flow