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Flow Guide

This repository aims to be a newbie-guide for writing typed JavaScript with Facebook's Flow - a static type checker for JavaScript (http://flowtype.org).

We will first discover how to set up our tools to get us running. Afterwards, we will dive into some examples of how we can leverage types to make our JavaScript code more predictable and even maybe more robust.

How to set up your Flowtyped Project

We assume that you are using following toolchain:

  • Babel 6 for transpiling ES6 -> ES5 code
  • ESlint for style checking
  • Flow >= 0.28.0

We will discuss the most minimalistic setup to get our tools running with flow. In this project we also added additional configuration like airbnb eslint rules, so don't get confused by that.

Flow Installation

# Either install it locally as npm-package
npm install flow-bin --save-dev

# Or globally via brew
brew install flow

Babel 6 Configuration

Since Flow's syntax supports an extended set of JavaScript, it is required to strip away all Flow-Type code before execution.

This can be done via the babel-preset-flow babel-plugin.

# Install babel stuff with ES6 support (you probably already did that)
npm install babel-cli babel-core babel-preset-env babel-preset-es2015 --save-dev

# Now install flow related stuff
npm install babel-preset-flowtype --save-dev

After installation, add the plugin and presets to your .babelrc file (or wherever your babel is being configured):

# .babelrc
{
  "presets": ["env", "flow", "es2015"],
  "plugins": ["transform-class-properties"]
}

Now, babel will always strip away flow source and your JS runtime can interpret the code. This is especially important for feeding eslint, so let's look into its configuration.

ESLint Configuration

Now that our babel configuration allows us to parse Flow typed JavaScript, we can now utilize the babel-eslint parser to pass in sanatized JavaScript code. Although this would already work by now, there will be some warnings about unused variables whenever you write type declarations.

The eslint plugin eslint-plugin-flowtype will mute those warnings, so let us install all the eslint stuff we need:

# ESlint stuff (you probably already did that)
npm install eslint babel-eslint --save-dev

# Now the flow related stuff
npm install eslint-plugin-flowtype

Done! Now if we run eslint src/some.js, eslint should run through and there shouldn't be any errors because of unknown syntax.

Editor Integration

We can recommend following editor flow plugins:

For some plugins, make sure your flow installation is in your $PATH variable, otherwise some plugins might not pick up the flow executable, or even worse, pick up the wrong version of it.

Using flow

First of all, before you can even use flow, you need to create a .flowconfig in your local project's folder. This can be done by running flow init, which will create a very basic INI file.

By default, flow will run a local server, which will parse your project for js and json files, so you might see some problems with third-party node_modules dependencies, which might contain stuff like malformed JSON files or other unuseful stuff. For these edge-cases, you can put a regex matcher in your .flowconfig's [ignore] section, which will cause the program to skip these defined files.

The flow server will provide important endpoints for retreiving data about parsed files, so it is usually launched by your IDE / Text-Editor (if you installed the proper plugins of course). Without that architecture, auto-completion would be hard, so we should appreciate that awesome feature!

Let us write some type-safe code!

To ease you in, we will get you familiar with flow's syntax and try to give you a general idea of how its type inference works. All examples are listed in the repositories ./tutorial directory.

The best way to enjoy this tutorial is to clone this repository and open your favorite editor to open the tutorial directory, read through the files and play around with the code. See what happens if you change things, try to break things intentionally and get a grasp of how flow's error messages lead you to your mistakes.

The tutorial gradually introduces flow concepts file by file (directory by directory), so make sure to read the files in order. Comments will lead you through the important sections of the code. To code grows while we introduce new concepts, sometimes we detach from previous examples, sometimes we built upon them. Don't get confused by previous examples, just concentrate on the current context (imports etc.) and you should be fine ;-)

Tutorial Chapters:

  • 00 - Basics
  • 01 - React

Enjoy!

Flow Style Guide

This repository also contains the style guide we use for our flowtyped projects. Maybe you find it useful as a reference or as a starter for your own rules.

We started this documentation as we are quite early flow adopters and aren't aware of any resources for best practises, so we thought it would be a good idea to get the discussion going!

So if you have any input or questions, be free to open an issue!

Extras

Acknowledgments

This project raised from an idea of having some common ground for using flowtype. I had a lot of support and motivation during my work at Runtastic.

I will continue this flow-guide to make it even more useful... so, thanks Runtastic for the trust in my work!

Maybe you wanna check out their career page? https://www.runtastic.com/career