Flow: static typechecker for JavaScript
GitHub - facebook/flow: Adds static typing to JavaScript to improve developer productivity and code quality.
Flow

  
    
  
  
   
  
  
    
  
  
   
  
  
    
  
  
     
  

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

Requirements
Using Flow
Using Flow's parser from JavaScript
Building Flow from source
Join the Flow community
License

Requirements
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 5.2.0 is required).


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.




Validate the opam version is 2.x.x:

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.


Initialize opam:
# on Mac and Linux:
opam init

# on Windows:
scripts/windows/init_opam.sh


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note: If you find that you get an error looking for ocaml-base-compiler version, your local dependency repo may be out of date and you need to run opam update + opam upgrade


Build the flow binary:

This produces the bin/flow binary.


Build flow.js (optional):
opam install -y js_of_ocaml.5.7.2
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

Website: https://flow.org
Discord: https://discord.gg/8ezwRUK
irc: #flowtype on Freenode
Twitter: follow @flowtype and #flowtype to keep up with the latest Flow news.
Stack Overflow: Ask a question with the flowtype tag

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