Flow is a static typechecker for JavaScript. To find out more about Flow, check out flowtype.org.

For a background on the project, please read our launch blog post.

Flow works with:

• Mac OS X
• Linux (64-bit)

There are binary distributions for Mac OS X and many variants of Linux; you can also build it from source on almost any 64-bit Linux variant.

Flow is simple to install: all you need is the flow binary on your PATH and you're good to go.

You can install Flow via Homebrew package manager. To ensure you get the latest version, update Homebrew first:

brew update

Then simply type:

brew install flow

You can also build and install flow via the OCaml OPAM package manager. Since Flow has some non-ocaml dependencies, you need to use the depext package like so:

opam install depext
opam depext --install flowtype

If you don't have a new enough version of OCaml to compile Flow, you can also use OPAM to bootstrap a modern version. Install OPAM via the binary packages for your operating system and run:

opam init --comp=4.01.0
opam install flowtype
eval `opam config env`
flow --help

Getting started with flow is super easy.

• Initialize Flow by running the following command in the root of your project

flow init

• Add the following to the top of all the files you want to typecheck

/* @flow */

• Run and see the magic happen

flow check

More thorough documentation and many examples can be found at http://flowtype.org.

Flow is written in OCaml (OCaml 4.01.0 or higher is required) and (on Linux) requires libelf. You can install OCaml on Mac OS X and Linux by following the instructions at ocaml.org.

For example, on Ubuntu 14.04 and similar systems:

sudo apt-get install ocaml libelf-dev

On OSX, using the brew package manager:

brew install ocaml ocamlbuild libelf opam

Once you have these dependencies, building Flow just requires running

make

This produces a bin folder containing the flow binary.

Note: at this time, the OCaml dependency prevents us from adding Flow to npm. Try flow-bin if you need a npm binary wrapper.

To run the tests, first compile flow using make. Then run bash ./runtests.sh bin/flow

There is a make test target that compiles and runs tests.

To run a subset of the tests you can pass a second argument to the runtests.sh file.

For example: bash runtests.sh bin/flow class | grep -v 'SKIP'

• Website: http://flowtype.org/
• irc: #flowtype on Freenode
• Twitter: follow @flowtype and #flowtype to keep up with the latest Flow news.

Flow is BSD-licensed. We also provide an additional patent grant.