Here we have a `Component` that uses `useRef`.

Inside the component is a mutable ref object called `id`, along with a `useEffect` that assigns a random value to `id.current`:

```typescript
export const Component = () => {
  const id = useRef();

  useEffect(() => {
    // error under id.current
    id.current = "Random value!";
  }, []);

  return <div></div>;
};
```

However, there is an error that `id.current` is not assignable to type `undefined`.

## Challenge

Your job is to find a solution that fixes this error without changing any of the runtime code.

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

Let's start by using `CMD + Click` to check out `useRef`'s type definitions.

There are actually three definitions for it, but for this exercise we'll be focusing on the one where we don't pass anything to it:

```typescript
function useRef<T = undefined>(): MutableRefObject<T | undefined>; 
```

As we can see, there's a generic type `T` which defaults to `undefined`, and the function returns a `MutableRefObject` with `T` or `undefined`.

## Back to Our Code

Back in our exercise code, we aren't passing anything to `useRef`:

```typescript
const id = useRef(); 
```

When we hover over `useRef`, it's typed as `undefined`:

```typescript
// hovering over useRef()
(alias) useRef<undefined>(): React.MutableRefObject<undefined>
```

Let's update `useRef` to pass `string` as a type argument:

```typescript
export const Component = () => {
  const id = useRef<string>();

  useEffect(() => {
    id.current = "Random value!";
  }, []);

  return <div></div>;
};
```

Now when we hover over `id`, it's typed as:

```typescript
// hovering over id
React.Ref<MutableRefObject<string | undefined>>
```

Hovering over `current` shows us:

```typescript
// hovering over current
React.Ref<MutableRefObject<string | undefined>.current: string
```

Our test passes successfully!

Typing the useRef Hook

[0:00] Let's take a look at useRef in terms of its type definitions. We can Command-click on useRef. Whoops, there we go, and now zoom out a little bit so we can see it.  
  
[0:09] You'll see that there are actually several definitions. There are three definitions for it, and over the next few exercises we're going to get to know them. This one, we're not passing anything to it. We've got a T that's just here, and we get back a MutableRefObject with T or undefined.  
  
[0:26] What we can see, then, is if we hover over useRef, we've got a type argument there, and it's currently typed as undefined. If we pass it something, then I assume it's going to start behaving properly.  
  
[0:39] Yes, we can pass a string to it. Now the id is going to be typed as React.MutableRefObject string or undefined, and id.current is now string or undefined.  
  
[0:51] This is good. This means that if we, for instance, were to pass a number here, then this would error properly because string is not assignable to type number. It means, then, that we can get this working and id.current can be assigned its proper value.

Basic useRef Typing

[0:00] Let's take a look at useRef here. We have a component that inside it has an ID. In a useEffect, we're assigning that ID to a random value. The way that this works is we have id, which is a MutableRefObject, and id.current is basically the API, so id.current, current is the only property on this id-ref.  
  
[0:23] We're having an error here, which is id.current is not assignable to type undefined. Your job is to figure out how we can fix it without actually changing any of the runtime code. 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

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

Typing React's Children

Solving the any Problem with Children

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 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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Basic useRef Typing

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