Flow’s type checking is opt-in, which means you can gradually convert your existing JavaScript codebase to Flow while reaping incremental benefits. You don’t need to rewrite your existing codebase to start using Flow.

You can opt-in on a per file basis by simply adding a /* @flow */ comment to the top of your source file. Flow uses type inference to find errors without further guidance. You can add type assertions by annotating your program with types.


Flow is designed for JavaScript programmers. Writing code with Flow should feel like writing JavaScript, from the common idioms in the language to the fast development cycle.

Most JavaScript code is “boring.” Boring is good! Boring code is easy to read and understand for humans and computers alike. Flow is particularly well suited to code like this, but that’s not all Flow can do.

JavaScript also provides powerful tools for metaprogramming that traditional statically typed languages lack. Flow is designed to understand even very dynamic code which is often found in JavaScript programs.

We’re constantly improving Flow to understand more and more JavaScript, but if you need it, the any type lets you opt-out of type checking in a granular way, so you can keep writing JavaScript the way JavaScript was meant to be written.


When you start Flow, it performs an initial analysis of all the files in your codebase and stores the results in a persistent server. When you save a file, Flow incrementally rechecks the changes in the background.

Both the initial analysis and recheck are heavily optimized for performance, which preserves the fast feedback of developing plain JavaScript.

In short, you don’t need to wait for Flow to check your code.


Flow uses control flow analysis to deeply understand your code to find errors that other type systems can’t. Flow is designed to find errors and we take soundness seriously.

For example, Flow tracks null values which may propagate unintentionally through code and eventually cause a runtime error. Flow’s path sensitive analysis can uncover bugs like this, even through layers of indirection in the program’s control flow.


The type system has a rich and rapidly expanding feature set that allows it to understand common JavaScript idioms, even when annotations are absent through type inference.

In addition to inference, Flow also provides an expressive type language, covering the JavaScript core language and standard libraries found in browsers and platforms like Node.js.