Creating TypeScript typings for existing React components

There are a lot of React components available on-line, and if you're using TypeScript with React, it can't sometimes be a little disappointing because many of those components don't have their type definitions.

Fear not, creating a type definition file is easy, and because you have already saved some time at work using a component built by somebody else, maybe you can use that time and collaborate creating the type definition file for the component.

In my case, I was working with the react-selectize component.

I will explain how to add typings for an existing OSS project in case you'd like to contribute to a project, and also, in the end, I'll explain what is the difference if you just want to add the typings to your project.

The package.json file

When adding typings, the easiest way for TypeScript to recognize it, is by adding a key to the package.json file. Like so:

{
	// ....
	"typings": "./dist/index.d.ts"
}

In this case, I'm assuming that when the package is deployed there will be a folder with an index.d.ts file in it.

If TypeScript does not find a typings key in the package.json file, it will look for an index.d.ts file in the root of the package's folder.

The type definition file

Now, on to the d.ts file itself. Here's how we define a type for a React component.

import * as React from "react";

declare class SimpleSelect extends React.Component<SimpleSelectProps, any> {}

// ...

First of, what is the declare keyword? Well, inside a d.ts file we cannot have actual code, just type definitions, so, we can't have a class, but we have to declare that a class exists and that's how we do it.

Of course, a component extends from React.Component, and using TypeScript's generics we declare what is the type of the component's props (in this case SimpleSelectProps) and the component's state type (in this case any).

As users of the component, we don't care about its state. Just the props which is the interface we'll be using, so we can declare the state as any.

Now the SimpleSelectProps is an interface with all the possibilities of the component. If you're lucky, the component you're using will have documentation you can use as a guide for creating the interface. Otherwise, you'll have to check the code.

export interface SimpleSelectProps {
  autofocus?: boolean;
  cancelKeyboardEventOnSelection?: boolean;
  className?: string;
  createFromSearch?(items: OptionValue[], search: string): OptionValue;
  defaultValue?: OptionValue;
  delimiters?: [any];
  disabled?: boolean;
  // ...
}

Don't forget that if an attribute is not required it should have an ? to indicate that it's optional.

In the case of react-selectize, it has two components, a SimpleSelect and MultipleSelect, that's why there's no export default. If there were only one component it could be exported by default like this:

declare class SimpleSelect extends React.Component<SimpleSelectProps, any> {}
// ...
export default SimpleSelect;

And that's it for this case.

Creating an internal (to your project) type definition file

Now, if you just want to add some types for a component you're using in your project and can't or don't want to modify the original package, you can add a .d.ts file anywhere in your project, as long as that file is not excluded by the tsconfig.json file configuration.

In this file, you will create a module with the same name as the package you're using. Let's use the same example, the react-selectize.

So, if you'd use the component like this:

import { SimpleSelect } from "react-selectize";

You'll be creating a module with the same name:

declare module "react-selectize" {
	// type definitions goes here
}

All the type definitions will be the same as before, with one exception the class declaration. Here we don't need the keyword declare.

// ...
class SimpleSelect extends React.Component<SimpleSelectProps, any> {}
// ...

And that's it for today.