Flow

The Rust port is just the start. Flow in 2026 isn't like you remember: flow.org

Most of the Rust port was done by a single engineer over ~8 months, while the rest of the team kept shipping features: no code freeze. Read the full story: medium.com/flow-type/flow…

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

@justboriss Yes, you can fix it by changing the code. If only there was a tool that would point out the problem in the first place - so you knew you had to make the fix 🤔

@flowtype Fixed the typescript code. It is an error as well now: #code/C4TwDgpgBAMg9gcxgSwHbQLxQM7AE5oJQA+UAonnnHgNwBQdAZgK6oDGwycqUAhgCb8A6rzypCAHgAqUCAA9gEVP2zlK1EjnyEAfAAoANogBcUAIKVeIaToCUUAN50oUIwgB0YZtgAWe9ADualR4egBE-MjYANZ8BgC2cLhQLAYGYba29AC+DGzcyfEQ2Ni8CMWmFnhWErgEqAg6UFgA2gC69ALCouINekUlZcVZDAOl5dgtAAxt7sBwAKpgkHgAwrzYEHpZQA" target="_blank" rel="nofollow noopener">typescriptlang.org/play/?#code/C4…

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

@vpetryniak Doesn't seem like a good assumption 😬

@flowtype Typescript implicit assumes the data is immutable. The way to use it to prevent such things is with const/Readonly types, but ecosystem is not adopted for it unfortunately. So if you try to use Readonly for everything there will be type errors with libraries code

@jaydenfyi Exactly, better to have your type-checker actually enforce you're doing the right thing and prevent your app from crashing! The actual Flow error message is longer and describes: - Either make `Array<string>` and `Array<LogLine>` the same - Or make array a `ReadonlyArray`

@flowtype In fairness, functions that mutate args like that are usually a code smell.

Flow catches the crash before you ship it. More examples where Flow is safer: #toc-variance" target="_blank" rel="nofollow noopener">flow.org/en/docs/flow-v…

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: #code/C4TwDgpgBAMg9gcxgSwHbQLxQM7AE5oJQA+UAonnnHgNwBQdAZgK6oDGwycqUAhgCb8A6rzypCACgA2iAFxQAgpV4gAPPCRoIAPgCUUAN50oUGQgB0YZtgAWE9AHdylahIBE-ZNgDWfKQFs4XCgWKSk3XV16AF8GNm5g-whsbF4EZPklPBVVXAJUBG0oLABtAF16AWFRcQKJJJS05KiGBtT07BKABjLzYDgAVTBIPABhXmwICSigA" target="_blank" rel="nofollow noopener">typescriptlang.org/play/?#code/C4… Flow Playground, rejects with an error: #1N4Igxg9gdgZglgcxALlAIwIZoKYBsD6uEEAztvhgE6UYCe+JADpdhgCYowa5kA0I2KAFcAtiRQAXSkOz9sAD0bZKcEYIncAdJCgkJ+RlQxjO3PgMXLV6rRJINaUDfMnTZIFhjATN0p9ZQQDCEJCBEMCTgwEH4hMnwhKABrKAgAdyh8OEywCLAAC3wANyo4LFxscWQuHnc2bCL8aFxaTQlaJXwWZlNa-nrmbFyJbDYAWnalXvMSJOwJAtox4VxcafdZ+cXl0RxKddioMAqqCY7sA5BEqCHKkiol7JKVDCdL69uSe8oliEZI6DcMYFDDZd66DAwbBjBDzEaUEhjMgSeFVGrmRJxUZjZhhOBkS4KJQqNROLQYL7KfR-ZQRCD7apmbAAX34kHqgUm2AABAAZCAIXnZHkAXm5ehUUAQ3IAPtyAKLUekAbgAOlB1TBEt44NBuew2AB1KhQbIIAAURAQyG5AEFqHQADz8wXCgB8AEpucB1dzuVbNIw4vlzTc0gqlZRzaqQGx8Ul9bgRKQJNytasYx6PWqoMz1eqdHpuWovhhYSQbfaaLRHRKzW7uWKANoAXRzBuNlFNUvNJfu5ez+agfbLlSbAAYW20IABVRjEgDCFOw5uzMRARWUJF1UECRXHmgAzABGAAcmnHIGZQA" target="_blank" rel="nofollow noopener">flow.org/try/#1N4Igxg9g…

👀 Is this Flow, or TypeScript? Is Flow coming back? - Flow syntax aligns with TypeScript - Flow has pattern matching - Useful React hooks/component/renders syntax - Compiler has been recently ported to Rust I'll keep an eye on it for you, but this is promising!

JavaScript Pattern Matching with `match`, available now: medium.com/flow-type/java…

Flow for TypeScript Users in 2026: 20+ side-by-side comparisons, verified against TypeScript 6.0.3 with 'strict' enabled: flow.org/en/docs/flow-v…

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.

If your "Flow vs TypeScript" take is from 2021, it's out of date. Flow looks like TypeScript now - but it has pattern matching and React component syntax, while catching runtime crashes TypeScript waves through. Examples below. 👇

SWC has added support for Flow: swc.rs/docs/usage/flow

Flow now supports Mapped Types, inspired by TypeScript. Read more about them in our new blog post: medium.com/flow-type/anno…