You can switch the file type in your editor or you from the command line. Whatever makes you happy.

git mv src/components/controls.jsx src/components/controls.tsx

And now, TypeScript is angry with us. Luckily, we've seen this error before, so we shouldn't be too surprised.

Our props are missing type information. That's information that we're more than happy to provide.

Let's start by creating a type with the name and then we'll figure out what to do with that event handler.

type ControlPanelProps = {
  name: string;
  onChange: any;
};

const ControlPanel = ({ name, onChange }: ControlPanelProps) => {
  // …
};

That'll work, but using any is missing the point. We're doing all of this work for the reassurances that TypeScript gives us. It doesn't make sense to opt out so soon.

The only thing we're doing with our onChange handler is we're passing it to the <input /> element. So, it just needs to be the same type the input field.

If we hover of the onChange property on the <input /> element, we see something interesting.

Let's break down that's happening:

• React.InputHTMLAttributes<HTMLInputElement> is the <input /> element. We're getting that information from React's own type definitions.
• .onChange? is an optional property that <input /> elements take, but we knew that already. That ? means it's an optional prop.
• React.ChangeEventHandler<HTMLInputElement> | undefined is the type it's expecting.
  • The | undefined means that it could be undefined, which meanes sense since it's optional.
• This means, that the type is React.ChangeEventHandler<HTMLInputElement>.

If you're curious what other event handles React includes, you can see a list of them in here.

type ControlPanelProps = {
  name: string;
  onChange: React.ChangeEventHandler<HTMLInputElement>;
};

We can shorted this a bit if we want.

import { ChangeEventHandler } from 'react';

type ControlPanelProps = {
  name: string;
  onChange: ChangeEventHandler<HTMLInputElement>;
};

const ControlPanel = ({ name, onChange }: ControlPanelProps) => {
  // …
};

export default ControlPanel;

Adding an another prop to NameBadge

Adding another prop is fairly straight-forward:

type NameBadgeProps = {
  name: string;
  greeting: string;
};

const NameBadge = ({ name, greeting }: NameBadgeProps) => {
  return (
    <section className="badge">
      <header className="badge-header">
        <h1 className="text-5xl">{greeting}</h1>
        <p>My name is…</p>
      </header>
      <div className="badge-body">
        <p className="badge-name">{name}</p>
      </div>
      <footer className="badge-footer" />
    </section>
  );
};

export default NameBadge;

And we can update Application accordingly:

import NameBadge from './name-badge';

const Application = () => (
  <main className="application">
    <NameBadge name={'Brendan Kelly'} greeting="Salutations" />
  </main>
);

export default Application;

Making props option

It would be cool if it would default to "hello" and then could omit greeting if we didn't want to customize it. We can assign it a sensible default.

const NameBadge = ({ name, greeting = 'Yo' }: NameBadgeProps) => {
  // …
};

But, we still get yelled at if we try to remove the property in src/components/application.tsx.

I will say: That is a pretty helpful error message. Even better, the fix is simple. A few seconds ago, I casually mentioned that React used a ? to denote that a property was optional. We can do the same thing.

type NameBadgeProps = {
  name: string;
  greeting?: string;
};

And now everything should compile as expected. You can check my work on the exercise-solution branch.

For reference, I've written up some of the other props that you might come across in your day to day life, but we're going to be moving along and get into more React-specific stuff like what are we going do about child components?