Adds static typing to JavaScript to improve developer productivity and code quality.


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

For more background information on the project, please read this overview.



Flow works with:

  • macOS (x86_64)
  • Linux (x86_64 and arm64)
  • Windows (x86_64, 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.

Using Flow

Check out the installation instructions, and then the usage docs.

Using Flow's parser from JavaScript

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, but JavaScript packages which make use of parsing Flow-typed JavaScript can use this to generate Flow's syntax tree with annotated types attached.

Building Flow from source

Flow is written in OCaml (OCaml 4.14.0 is required).

  1. Install system dependencies:

    • Mac: brew install opam

    • Debian: sudo apt-get install opam

    • Other Linux: see opam docs

    • Windows: cygwin and a number of dependencies like make, gcc and g++ are required.

      One way to install everything is to install Chocolatey and then run .\scripts\windows\install_deps.ps1 and .\scripts\windows\install_opam.ps1. Otherwise, see the "Manual Installation" section of OCaml for Windows docs and install all of the packages listed in our install_deps.ps1.

      The remainder of these instructions should be run inside the Cygwin shell: C:\tools\cygwin\Cygwin. Then cd /cygdrive/c/Users/you/path/to/checkout.

  2. Validate the opam version is 2.x.x:

    opam --version

    The following instructions expect 2.x.x. Should your package manager have installed a 1.x.x version, please refer to the opam docs to install a newer version manually.

  3. Initialize opam:

    # on Mac and Linux:
    opam init
    # on Windows:
  4. Install Flow's OCaml dependencies:

    # from within this git checkout
    make deps

    note: If you find that you get an error looking for ocaml-base-compiler version, your local dependency repo may be out of date and you need to run opam update + opam upgrade

  5. Build the flow binary:

    eval $(opam env)

    This produces the bin/flow binary.

  6. Build flow.js (optional):

    opam install -y js_of_ocaml.4.0.0
    make js

    This produces bin/flow.js.

    The Flow parser can also be compiled to JavaScript. Read how here.

Running the tests

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'

Join the Flow community


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

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