Gain insights into integrating TypeScript with React to improve productivity. Learn about strongly-typed props, state, and events, and set up projects using Create React App, Webpack, and Babel.

If you are a React developer and want to build your React apps faster and more robustly, this course is for you. Learn to integrate TypeScript with React, boost your productivity, and create apps that are easier to maintain.

In this course, you will learn how to utilize TypeScript’s sophisticated type system to make React development faster and your code more readable. You will learn how to create component props, state, and events that are strongly-typed, allowing you to quickly understand what you can pass and helping you avoid mistakes.

Lastly, you will set up React projects with TypeScript. First, you will learn how to do this with the Create React App tool, then how to create a React and TypeScript project with Webpack and Babel.

Using TypeScript with React

What is the inferred type for the `name` state from the `useState` hook?

```
const [name, setName] = React.useState("");
```

What is the inferred type for the <code>name</code> state from the <code>useState</code> hook?
<pre><code>const [name, setName] = React.useState(&quot;&quot;);
</code></pre>

```
const [email, setEmail] = React.useState(null);
```

```
const [email, setEmail] = React.useState<string | null>(null);
```

```
const [email, setEmail] = React.useState<string>(null);
```

How can we define a state variable called `email` with the `useState` hook that can be a `string` or `null`. Its initial value should be `null`.

How can we define a state variable called <code>email</code> with the <code>useState</code> hook that can be a <code>string</code> or <code>null</code>. Its initial value should be <code>null</code>.

```
type Loaded = {
  readonly type: 'loaded';
  readonly data: readonly string[];
};
```

```
type Loaded = {
  type: 'loaded';
  data: string[];
};
```

```
type Loaded = {
  readonly type: string;
  readonly data: readonly string[];
};
```

We are implementing state using the `useReducer` hook. One of the pieces of state, called `data`, is an array of strings. An action object like the one below will start the process of updating the `data` state:

```
{
  type: "loaded",
  data: ["Bob", "Tom", "Jane"]
}
```

What type could best represent this action object? We want the object to be immutable.

We are implementing state using the <code>useReducer</code> hook. One of the pieces of state, called <code>data</code>, is an array of strings. An action object like the one below will start the process of updating the <code>data</code> state:
<pre><code>{
 type: &quot;loaded&quot;,
 data: [&quot;Bob&quot;, &quot;Tom&quot;, &quot;Jane&quot;]
}
</code></pre>
What type could best represent this action object? We want the object to be immutable.

We can't improve the implementation - it will already throw an error if there are any types in the `Actions` union are unhandled.

Add a default branch that calls a function with a parameter of type `never`:

```
function reducer (state: State, action: Actions): State {
  switch (action.type) {
    case 'loading':
      return { ...state, loading: true };
    case 'loaded':
      return { ...state, loading: false, data: action.data };
    default:
      neverReached(action);
  }
};

function neverReached (never: never) {};
```

Add a default branch that throws an error:

```
function reducer (state: State, action: Actions): State {
  switch (action.type) {
    case 'loading':
      return { ...state, loading: true };
    case 'loaded':
      return { ...state, loading: false, data: action.data };
    default:
      throw 'Unhandled type'
  }
};
```

We have the following reducer function that we want to use in the `useReducer` hook:

```
function reducer (state: State, action: Actions): State {
  switch (action.type) {
    case 'loading':
      return { ...state, loading: true };
    case 'loaded':
      return { ...state, loading: false, data: action.data };
  }
};
```

How can we improve the implementation so that the type checking process ensures all the types in the `Actions` union are handled in the `switch` statement?

We have the following reducer function that we want to use in the <code>useReducer</code> hook:
<pre><code>function reducer (state: State, action: Actions): State {
 switch (action.type) {
 case 'loading':
 return { ...state, loading: true };
 case 'loaded':
 return { ...state, loading: false, data: action.data };
 }
};
</code></pre>
How can we improve the implementation so that the type checking process ensures all the types in the <code>Actions</code> union are handled in the <code>switch</code> statement?

Use the generic type parameter for `useReducer` to explicitly define the type for the state:

```
const [state, dispatch] = React.useReducer<React.Reducer<State, Actions>>(...);
```

Explicitly specify a return type of `State` for the reducer:

```
(state: State, action: Actions): State => {
  switch (action.type) {
    case "add":
      return { total: state.total + action.amount };
    case "subtract":
      return { total: state.total + action.amount };
  }
}

Return the `state` parameter at the end of the reducer:

```
(state: State, action: Actions) => {
  switch (action.type) {
    case "add":
      return { total: state.total + action.amount };
    case "subtract":
      return { total: state.total + action.amount };
  }
  return state;
},
```

Below is a `useReducer` hook:

```
const [state, dispatch] = React.useReducer(
  (state: State, action: Actions) => {
    switch (action.type) {
      case "add":
        return { total: state.total + action.amount };
      case "subtract":
        return { total: state.total + action.amount };
    }
  },
  initialState
);
```

The inferred type for `state` isn't `State` - it is `{total: number}` instead.

What could we change in the implementation so that the type of `state` is `State`? (There is more than one correct answer).
  

Below is a <code>useReducer</code> hook:
<pre><code>const [state, dispatch] = React.useReducer(
 (state: State, action: Actions) =&gt; {
 switch (action.type) {
 case &quot;add&quot;:
 return { total: state.total + action.amount };
 case &quot;subtract&quot;:
 return { total: state.total + action.amount };
 }
 },
 initialState
);
</code></pre>
The inferred type for <code>state</code> isn’t <code>State</code> - it is <code>{total: number}</code> instead.
What could we change in the implementation so that the type of <code>state</code> is <code>State</code>? (There is more than one correct answer).

Creating strongly-typed function component state

Creating strongly-typed function component state

This quiz will test your knowledge on creating strongly-typed function component state.

Why TypeScript?

Using basic types

Creating types

Working with generic types

Creating strongly-typed function component props

Creating strongly-typed function component state

Creating strongly-typed class components

Creating strongly-typed component events and event handlers

Creating strongly-typed context

Creating strongly-typed refs

Creating React projects with TypeScript using Create React App

Creating a React and TypeScript project manually

Putting everything together

Quiz