The `props.children` pattern in React allows you to pass child components to a parent component, then have those child components appear beneath the parent in the tree.

```typescript
export const Button = (props: {}) => {
  return <button>{props.children}</button>;
};

const Parent = () => {
  return (
    <>
      {/* @ts-expect-error */}
      <Button></Button>
      <Button>Hello world!</Button>
    </>
  );
};
```

Many teams type `children` as `any`:

```typescript
export const Button = (props: {
  children: any;
}) => {
  ...
```

This is somewhat acceptable, but there is actually a more appropriate way to define this in TypeScript.

## Challenge

Your challenge is to reference the provided resources to find a proper type definition for `props.children`.

Keep in mind that you can pass various types of as children. Elements, strings, numbers, and `null` are all available.

### Resources

<a href="https://react-typescript-cheatsheet.netlify.app/docs/basic/getting-started/basic_type_example#useful-react-prop-type-examples" target="_blank" rel="noopener">Useful React Props Examples</a>

Many teams type React's children as any, but there is a better option.

The correct way to handle this is by adding `children` into the props, and have it be typed as `React.ReactNode`.

```typescript
export const Button = (props: { children: React.ReactNode }) => {
  return <button>{props.children}</button>;
};
```

The `React.ReactNode` covers all of the different cases where you can pass `children` in.

You might have been tempted to use `React.ReactChild`, but this has been deprecated.

## Checking `React.ReactNode`'s Type Definitions

Using `CMD + Click` to explore `React.ReactNode`'s type definitions will show us all of the different things that can potentially go into it. We have `React.ReactElement`, `string`, `number`, `React.ReactFragment`, `React.ReactPortal`, `boolean`, `null`, or `undefined`.

It's kind of acting like a helper type.

Don't worry about the composition of `React.ReactElement` for now. The important thing to know is that `React.ReactNode` is a great way to express all of the different things that you can pass into that slot.

By specifying that `children` is of type `React.ReactNode`, the `children` property is now required to be there. If you don't pass any in, TypeScript will give an error. However, adding a single empty space will count.

`React.ReactNode` is incredibly useful and really good for avoiding the `any` problem when it comes to children.

Typing React's Children

[0:00] The correct way to handle this is by adding children into the props. This children property is going to be typed as React.ReactNode. Now, this React.ReactNode, what it does is it covers all of the different cases where you can pass children in. It's perfect for this case. You might be tempted to use React.ReactChild or something. This is deprecated.  
  
[0:23] React.ReactNode is the one. If we command-click on it and we can actually see all of the different things that can potentially go into React.Node. It's like a helper type really. We've got ReactElement, string, number, React Fragment, React Portal, boolean, , or undefined.  
  
[0:39] We don't need to worry about the composition of ReactElement. I'm going to get into that in a future video. You should know that React.Node is a great way to express all of the different things that you can pass in into that slot. Now, what this means then is that actually this children is enforced to be there. We have to pass children now.  
  
[1:00] If I don't pass children, what you're going to see is it's actually going to yell at me because property children is missing in typed empty object. This can be worked around a little bit. You notice if I leave a space there, then technically that space is a child, which is a little bit funky. If I do this as well then technically I am passing in children.  
  
[1:18] It works a little bit funky with Prettier as well. You get the idea. Now we have to pass a set of children into there, which is super duper useful. React.ReactNode incredibly useful, really good for avoiding the any problem when it comes to children.

Solving the any Problem with Children

[0:00] In React, there's a pattern called props.children, which is where you're able to pass children, like child components, to this Button here. Then it flows through that component to appear beneath it in the tree.  
  
[0:15] How do we type this, though, in TypeScript? I see a lot of teams going children any, like this, which I guess is acceptable, but there is actually a legit way to do this and a proper way to define this.  
  
[0:28] Your job is to go and look at some of the resources that I've got below and to try to find a proper definition for this that will let you pass anything as a child.  
  
[0:39] There is various things that you can pass. You can pass strings. You can pass numbers. If I do this, I think I can even pass things like here. Of course I can pass other elements too. I can pass divs and all sorts of stuff.  
  
[0:53] That's your job, is to try to find the correct definition for how to type these children. Good luck.

TypeScript's features and developer experience make a great combination with React, but it can be confusing to get started.

What’s the best way to type component props? What are these weird `useRef` errors?

That's where this React with TypeScript tutorial comes in!

Starting from the very beginning of bringing TS support to a React project, you'll soon find yourself properly typing hooks and mastering components. You’ll learn everything you need to know to get productive with React and TypeScript.

Tutorial topics include:

* Reading React's type definitions to debug errors and ensure correct usage
* Defining props for custom and `React.FC` function components.
* Putting TypeScript's inference to work for you
* Supporting component children
* Typing and overriding event handlers
* Utilizing `ComponentProps` to extract properties from imported components or HTML DOM elements
* Understanding the APIs and typing for `useState`, `useCallback`, `useEffect`, and other React Hooks. By the end of this workshop, you'll be confident when using React with TypeScript in your own projects!

<Testimonial author={{name: "Francisko de Moraes Rezende", image: "https://cdn.sanity.io/images/z9io1e0u/production/8e2b0b48599527885121568e19c761edd1c3e2ee-460x460.jpg"}}>Went through it and I think it's great! I wish this tutorial existed when I started my first gig 11 months ago. It teaches all the things I use on a daily basis and some that I didn't know; I was using the verbose way of getting HTML props in types until I went through the tutorial. This is my new go-to recommendation for people starting to work with TS+React. Thanks for creating this and making it free, Matt.</Testimonial>

## Instructions

This React with TypeScript tutorial is split into several exercises. Each features a problem that encourages you to take an active, exploratory approach to finding a solution.

You'll need to:

* Watch and read the problem introduction.
* Check out the TypeScript or React documentation or dive into type definitions to look for help
* Try a solution and check your work. There may be more than one way to solve the challenge! Whether you succeed or get stuck, watch and read the solution and check out the included code.

## Prerequisites

You'll need a working knowledge of React, as well as foundational TypeScript knowledge. Working through the <a href="https://www.totaltypescript.com/tutorials/beginners-typescript" target="_blank" rel="noopener">Beginner's TypeScript Tutorial</a> is recommended.

## Running the Exercises

Exercises can be worked on using the inline Stackblitz editor, or by <a href="https://gitpod.io/#https://github.com/total-typescript/react-typescript-tutorial" target="_blank" rel="noopener">opening the tutorial repo in Gitpod</a>. Online editors do not support the full feature set of a local VS Code installation (e.g. quickly jumping to type definitions).

To work locally, clone the <a href="https://github.com/total-typescript/react-typescript-tutorial" target="_blank" rel="noopener">React with TypeScript Tutorial repo</a>, and use a package manager to install dependencies. From there, check out the README for further instructions.

Get productive building applications with React and TypeScript with our interactive guide on React's types - from component props to useRef.

React TypeScript Tutorial 

There are a few steps to follow in order to add React to a TypeScript project.

Adding React to a TypeScript Project

Most frontend frameworks include optional TypeScript support, meaning you won't have to manually set it up.

TypeScript in React Frameworks

VS Code has a variety of features for working around your codebase when writing JSX.

Three Helpful Tools for Writing JSX

Navigating JSX Types

Introduction

It's important to understand how to type props since you'll work with them every day in React.

Four Approaches to Typing Props

Ensure Props are Present and Defined

React's Function Component React.FC has its own considerations for enabling proper type checking.

Specifying Props for React.FC 

Typing a Component as a Function

Instead of guessing a handler's type, you should be specific.

Choose the Correct Event Handler Type

Typing Event Handlers

When building React components, there are a couple of options for proper typing based on the underlying HTML element.

Add HTML Props to React Components

Using HTML Props in React Components

Components can have a lot of built-in props for things like event handlers that you don't always want, but there are a few techniques for overriding them.

Techniques for Overriding Native Props

Overriding and Removing Component Props 

Use a Type Helper to Extract Component Props

Extracting Props from Custom Components

Components

It's common for the useState hook to be called with an empty array, which TypeScript has issues inferring. Follow this approach for proper typing.

Fix useState's never[] Error

Properly Typing useState

The useState hook uses function overloads to handle cases where you may or may not want to pass an intial value.

Specifying types for useState

Typing useState with undefined

There's a strange property of the useState API that TypeScript treats in a specific way.

Going from Function Comparisons to Object Comparisons

Calling useState with Excess Properties

The useEffect Hook isn't generic, so you need to follow its API to use it properly.

Properly use the useEffect API

Solving Errors with useEffect

By examining a function's type definitions, you can properly type React's useCallBack Hook

Have TypeScript Infer the Callback Function Type

Typing the useCallback Hook

Not all hooks use the same type signature. Dive in to fix errors and specify the return type of a memoized value.

Specify or Infer useMemo's Return Type

Typing the useMemo Hook

There are three type definitions for useRef. Let's start by looking at the most basic.

Typing the useRef Hook

Basic useRef Typing

There are a few different ways to use refs in React, along with some rules to follow.

Refs in React

Specify the Initial Value with useRef

useRef with HTML Elements

Targeting a useRef Overload

Making a Read-Only Ref Mutable

Leveraging one of TypeScript's built-in features allows you to specifically type the state and actions for a reducer function in React.

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Solving the any Problem with Children

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