posts by Jesse Hallett

Flow Cookbook

Last updated 2017-01-07

Type-checking can be a useful asset in a Javascript project. A type checker can catch problems that are introduced when adding features or refactoring, which can reduce the amount of time spent debugging and testing. Type annotations provide a form of always-up-to-date documentation that makes it easier for developers to understand an unfamiliar code base. But it is important to use type-checking effectively to get its full benefit.

The Javascript community is fortunate to have a choice of two great type checkers. These recipes focus on Flow, and introduce patterns for using Flow effectively.

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These are primers on practical patterns for Flow. I recommend using these patterns in any project that uses Flow.

(Coming soon) Flow types are duck types - Flow is not a new, strait-jacketed OOP language. Flow is not Java or C#. Flow is a codification of previously-unwritten Javascript idioms.

(Coming soon) Uses for union types introduces a pattern for managing data that comes in different shapes. Union types are helpful for describing Redux actions, for unpacking incoming JSON data, and for passing messages over a channel. If you have been tempted to use subclasses, take a look at union types to see if they might be a better fit.

Unpacking JSON API data - Javascript’s flexibility is useful for handling incoming data in whatever form it may take. Flow is designed to be just as flexible when type-checking functions that process data. This is a case study that uses the Hacker News API as an example for type-safe data processing.

Flow & React - This recipe demonstrates how to use Flow effectively when creating React components. Including type parameters in functional and class components provides an alternative to propTypes that can provide better safety and modularity.

(Coming soon) Flow & Redux - Flow and Redux could have been made for each other. This recipe demonstrates several patterns that are useful for building Redux action creators and reducers. This is a companion to the post on React.


Extras are not about practical patterns. In these articles we explore ideas just because they are interesting. Read these if you want to dig deeper into type theory, or to learn about Flow’s lesser-known capabilities.

(Coming soon) What are types? The short answer is, types are sets of possible values. This post gets into what that means, and shows that Flow takes more of a purist approach to types compared to most object-oriented languages.

(Coming soon) The “algebra” in “algebraic data types” - In the recipe Uses for union types I mentioned that union types are also called “sum types” or “algebraic data types”. This post gives a brief background on type algebra so that you can understand where those terms come from.

(Coming soon) Advanced algebraic data types - Union types are great, but not perfect. This post introduces an alternative formulation for sum types that allows Flow to check for missing pattern matches. It also shows that GADTs are almost possible in Flow.

More resources

Flow has inspired many programmers to put bits to screen. Here are some articles that I found to particularly helpful:

Getting started with Flow is a tutorial from the official documentation. If you don’t know where to start, start there.

Why use type-checking? And if you do, Why Use Flow? Follow that link for the answers, and learn some things that you might not know about Flow. Aria Fallah covers a lot of background, and also introduces some interesting work from Giulio Canti.

Authoring and publishing JavaScript modules with Flow is a detailed guide on publishing an NPM module with Flow type annotations included, so that anyone who uses the library can benefit from those annotations if they choose to use Flow as well.


  • 2017-01-03: Added recipe, Flow & React
  • 2017-01-07: Added “More resources” section