TypeScript support in Next.js

Introduction

TypeScript helps catch mistakes early by checking your code. Next.js supports TypeScript to make building web apps safer and easier.

You want to avoid bugs by checking your code as you write.

You are building a web app with Next.js and want better code hints.

You want to use modern JavaScript plus type safety.

You want your team to understand code better with clear types.

You want to catch errors before running your app.

Syntax

NextJS

1. Rename your files from .js to .tsx for components or .ts for other files.
2. Add a tsconfig.json file to configure TypeScript.
3. Next.js will automatically detect TypeScript and install needed packages.
4. Use types for props and state in your components.

Example component:

import React from 'react';

type Props = { message: string };

export default function Hello({ message }: Props) {
  return <h1>{message}</h1>;
}

Next.js auto-creates a default tsconfig.json if you don't have one.

Use .tsx for React components to allow JSX syntax with TypeScript.

Examples

Simple typed component with a number prop.

NextJS

import React from 'react';

type Props = { count: number };

export default function Counter({ count }: Props) {
  return <p>Count is {count}</p>;
}

Using an interface to type a function return value.

NextJS

interface User {
  id: number;
  name: string;
}

export function getUser(): User {
  return { id: 1, name: 'Alice' };
}

Basic component without props, still using TypeScript file.

NextJS

export default function Page() {
  return <div>Hello from TypeScript in Next.js!</div>;
}

Sample Program

This is a simple Next.js component using TypeScript. It takes a name prop typed as a string and displays a greeting.

NextJS

import React from 'react';

type GreetingProps = {
  name: string;
};

export default function Greeting({ name }: GreetingProps) {
  return <h2>Hello, {name}!</h2>;
}

Important Notes

TypeScript helps you find errors before running your app.

Next.js automatically reloads when you change TypeScript files.

Use the Next.js docs for advanced TypeScript features like API routes and getStaticProps.

Summary

Next.js supports TypeScript out of the box for safer code.

Rename files to .ts or .tsx and add types for props and data.

Next.js auto-configures TypeScript and reloads on changes.

Practice

1. What happens when you add a tsconfig.json file to a new Next.js project?

easy

A. You must manually configure TypeScript settings in Next.js.

B. Next.js automatically detects TypeScript and configures the project.

C. Next.js disables JavaScript support.

D. The project will not run until you install extra plugins.

Solution

1. Step 1: Understand Next.js TypeScript auto-configuration

   Next.js detects tsconfig.json and sets up TypeScript automatically without extra steps.

2. Step 2: Confirm no manual setup needed

   Developers do not need to manually configure TypeScript; Next.js handles it.

3. Final Answer:

   Next.js automatically detects TypeScript and configures the project. -> Option B

4. Quick Check:

   TypeScript auto-config = A [OK]

Hint: Add tsconfig.json and Next.js auto-configures TypeScript [OK]

Common Mistakes:

• Thinking manual config is required
• Believing JavaScript support is disabled
• Assuming extra plugins are mandatory

2. Which file extension should you use for a React component with TypeScript in Next.js?

easy

A. .js

B. .ts

C. .tsx

D. .jsx

Solution

1. Step 1: Identify file extension for React components with TypeScript

   React components with JSX syntax and TypeScript use the .tsx extension.

2. Step 2: Differentiate from plain TypeScript files

   Files without JSX use .ts, but components need .tsx.

3. Final Answer:

   .tsx -> Option C

4. Quick Check:

   React + TypeScript = .tsx [OK]

Hint: Use .tsx for React components with TypeScript [OK]

Common Mistakes:

• Using .ts for components with JSX
• Using .js or .jsx without TypeScript
• Confusing .tsx with .jsx

3. Given this Next.js TypeScript component, what is the rendered output?

type Props = { name: string };

export default function Greeting({ name }: Props) {
  return <h1>Hello, {name}!</h1>
}

Used as <Greeting name="Alice" />

medium

A. Hello, Alice!

B. Hello, {name}!

C. Error: Missing prop type

D. Hello, !

Solution

1. Step 1: Understand prop typing and usage

   The component expects a prop name of type string and uses it inside JSX.

2. Step 2: Substitute the prop value in JSX

   When called with name="Alice", it renders Hello, Alice! inside an h1.

3. Final Answer:

   <h1>Hello, Alice!</h1> -> Option A

4. Quick Check:

   Prop value replaces {name} = C [OK]

Hint: Props typed and passed render as expected in JSX [OK]

Common Mistakes:

• Thinking JSX shows literal {name}
• Assuming missing props cause error here
• Ignoring prop substitution

4. What is wrong with this Next.js TypeScript page component?

export default function Page({ id }: { id: number }) {
  return <p>ID: {id.toUpperCase()}</p>
}

medium

A. Next.js does not support inline prop typing.

B. Missing explicit return type causes a compile error.

C. Props object should be typed as any.

D. Using toUpperCase() on a number causes a compile-time error.

Solution

1. Step 1: Analyze prop type and method usage

   The prop id is a number, but toUpperCase() is a string method.

2. Step 2: Identify compile-time error cause

   Calling toUpperCase() on a number causes a compile-time error because numbers don't have this method.

3. Final Answer:

   Using toUpperCase() on a number causes a compile-time error. -> Option D

4. Quick Check:

   Number.toUpperCase() = error [OK]

Hint: Check method matches prop type to avoid compile-time errors [OK]

Common Mistakes:

• Assuming missing return type causes error
• Thinking inline typing is unsupported
• Using any type unnecessarily

5. You want to create a Next.js API route with TypeScript that returns JSON with a typed message. Which is the correct way to type the request and response parameters?

import type { NextApiRequest, NextApiResponse } from 'next';

export default function handler(req: ???, res: ???) {
  res.status(200).json({ message: 'Success' });
}

hard

A. req: NextApiRequest, res: NextApiResponse<{ message: string }>

B. req: any, res: any

C. req: Request, res: Response

D. req: NextApiRequest, res: NextApiResponse

Solution

1. Step 1: Import correct types from Next.js

   Use NextApiRequest for request and NextApiResponse with a generic type for response data.

2. Step 2: Specify response JSON shape

   Typing NextApiResponse<{ message: string }> ensures the response JSON matches the expected structure.

3. Final Answer:

   req: NextApiRequest, res: NextApiResponse<{ message: string }> -> Option A

4. Quick Check:

   Typed API response = D [OK]

Hint: Use NextApiResponse with generic for typed JSON response [OK]

Common Mistakes:

• Using generic Request/Response from Node or browser
• Not typing response JSON shape
• Using any type losing type safety