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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 associated List 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

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

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

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

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

type which takes a generic type use T inside BaseProps for our onItemClick function which will take an item of type T as argument and thus be of type T => void

inside for our function which will take an item of type as argument and thus be of type declare Resource1Props , Resource2Props , etc, based on BaseProps

, , etc, based on 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

Running Flow with this type should output the following:

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.