This post is part of an informal series of small articles focusing on Flow types usages that I’ve discovered and found useful over the past few months. The first article of this series can be found here.

Building generic React components can be a great way to simplify the codebase of a project. By combining overlapping functionalities into a single component, we reduce the entropy of components and thus reduce the amount of testing needed. However, typing generic React components can be a bit of hassle at first, especially when you‘re just getting started with Flow. This post will focus on one example of a generic React component that I was struggling to type a few months ago, and how Flow generics helped me to not only overcome the typing issues but also to build a scalable type for this component.

From multiple typed components to one

In the project I was working on, I had to handle multiple resources (named here Resource1, Resource2, Resource3, … for simplicity). Each resource had its own associatedList component (Resource1List, Resource2List, …), each of which was pretty similar to the others in terms of implementation, as the resources themselves were quite the same outside of a few differences. Below, you can see the types of Resource1 and Resource2 :

Example resource Flow types

1type Resource1 = {
2 id: string,
3 name: string,
6type Resource2 = {
7 Id: string,
8 name: string,

So given these specs, I wanted to build a single ResourceList component to display items of a given resource. Its props would include an onItemClick callback function, which takes a variable of that resource type as an argument. Here’s the first type that I wrote for the props of the ResourceList component:

The first iteration of the “Props” Flow type for ResourceList

1type Props = {
2 // other props
3 onItemClick = (
4 Resource1 |
5 Resource2 |
6 // other resource Flow types fo here
7 ) => void,

Now that the component is typed, the next step consists of trying to use it with one of our Resources. Here’s how ResourceList can be used for Resource1 for example:

Resource1List React component using ResourceList

1import React from 'react';
2import ResourceList from 'components/ResourceList';
3import type { Resource1 } from 'core/types';
6class Resource1List extends React.Component {
7 onResource1Click = (item: Resource1) => {
8 const { name, id } = item;
9 // some action on Resource1 fields
10 };
12 ...
14 render() {
15 return <ResourceList onItemClick={this.onResource1Click(item)} />;
16 }

This is the moment when I ran into the main problem. Given the way I typed this component, running Flow against the code in the example above outputs an error:

Flow error output with the first iteration of the “Props” Flow type for ResourceList
Flow error output with the first iteration of the “Props” Flow type for ResourceList

In the Resource types we declared above ( Resource1, Resource2 , …), you can see that the key of the “id” field of each Resource doesn’t match. Thus, whenever we will use this component and write an onItemClick function for a resource, Flow will give you an error telling us the properties of the other resources are missing.

How to use generics

To overcome this kind of situation, generics can be very practical. This is how we can end up typing this component and fix this issue:

  • we can provide a generic BaseProps type which takes a generic type T
  • use T inside BaseProps for ouronItemClick function which will take an item of type T as argument and thus be of type T => void
  • declare Resource1Props , Resource2Props , etc, based on BaseProps
  • write the type Props as an enum of Resource1Props, Resource2Props, etc.

The resulting code looks like this:

ResourceList “Props” Flow type using a generic BaseProps Flow type

1type BaseProps<T> = {
2 // other props
3 onItemSelect = T => void,
6type Resource1Props = BaseProps<Resource1>;
7type Resource2Props = BaseProps<Resource2>;
9type Props = Resource1Props | Resource2Props;

Running Flow with this type should output the following:

Flow output with no errors thanks to the new ResourceList “Props” Flow type!
Flow output with no errors thanks to the new ResourceList “Props” Flow type!

We have now typed our generic list component properly thanks to Flow. We can see that using generics not only brings flexibility to our type, but also will help in the future when we want to scale up the usage of this component, so it can be used with even more resources.

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Have a wonderful day.

© 2019 Maxime Heckel. Made in SF.