flow  0.97.0
About: Flow is a static type checker, designed to find type errors in JavaScript programs.
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Flow Build Status Windows Build Status

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

For a background on the project, please read this overview.

Flow works with:

  • Mac OS X
  • Linux (64-bit)
  • Windows (64-bit, Windows 10 recommended)

There are binary distributions for each of these platforms and you can also build it from source on any of them as well.

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

The recommended way to install Flow is via the flow-bin npm package. Adding flow-bin to your project's package.json:

  • provides a smoother upgrade experience, since the correct version of Flow is automatically used based on the revision you check out
  • installs Flow as part of your existing npm install workflow
  • lets you use different versions of Flow on different projects
npm install --save-dev flow-bin

Although not recommended, you can also install Flow globally (for example, perhaps you don't use npm or package.json).

The best way to install globally is via flow-bin:

npm install -g flow-bin
flow # make sure `npm bin -g` is on your path

On Mac OS X, you can install Flow via the Homebrew package manager:

brew update
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=4.05.0 opam install flowtype eval opam config env flow –help

## Getting started
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
``` javascript
/* @flow */


// @flow
  • Run and see the magic happen
    flow check

More thorough documentation and many examples can be found at flow.org.

Flow is written in OCaml (OCaml 4.05.0 or higher is required). You can install OCaml on Mac OS X and Linux by following the instructions at ocaml.org.

For example, on Ubuntu 16.04 and similar systems:

sudo apt-get install opam
opam init --comp 4.05.0

On OS X, using the brew package manager:

brew install opam
opam init --comp 4.05.0

Then, restart your shell and install these additional libraries:

opam update
opam pin add flowtype . -n
opam install --deps-only flowtype

Once you have these dependencies, building Flow just requires running


This produces a bin folder containing the flow binary.

In order to make the flow.js file, you first need to install js_of_ocaml:

opam install -y js_of_ocaml

After that, making flow.js is easy:

make js

The new flow.js file will also live in the bin folder.

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

Flow can also compile its parser to JavaScript. Read how here.

This is a little more complicated. Here is a process that works, though it probably can be simplified.

The general idea is that we build in Cygwin, targeting mingw. This gives us a binary that works even outside of Cygwin.

  1. Install Cygwin 64bit from https://cygwin.com/install.html
  2. In powershell, run iex ((new-object net.webclient).DownloadString("https://raw.githubusercontent.com/ocaml/ocaml-ci-scripts/master/appveyor-install.ps1")) which will likely run a cygwin setup installer with a bunch of cygwin packages and stuff. This helps make sure that every package that opam needs is available.
  1. Open the cygwin64 terminal
  2. Download opam with curl -fsSL -o opam64.tar.xz https://github.com/fdopen/opam-repository-mingw/releases/download/
  3. tar -xf opam64.tar.xz
  4. cd opam64
  5. Install opam ./install.sh
  6. Initialize opam to point to a mingw fork: opam init -a default "https://github.com/fdopen/opam-repository-mingw.git" --comp "4.05.0+mingw64c" --switch "4.05.0+mingw64c"
  7. Make sure opam stuff is in your path: eval `opam config env`
  1. Clone flow: git clone https://github.com/facebook/flow.git
  2. cd flow
  3. Tell opam to use this directory as the flowtype project: opam pin add flowtype . -n
  4. Install system dependencies opam depext -u flowtype
  5. Install Flow's dependencies opam install flowtype --deps-only
  6. Finally, build Flow: make all

While Flow is written in OCaml, its parser is available as a compiled-to-JavaScript module published to npm, named flow-parser. Most end users of Flow will not need to use this parser directly (and should install flow-bin from npm above), but JavaScript packages which make use of parsing Flow-typed JavaScript can use this to generate Flow's syntax tree with annotated types attached.

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’`

Flow is MIT-licensed (LICENSE). The website and documentation are licensed under the Creative Commons Attribution 4.0 license (website/LICENSE-DOCUMENTATION).