1. Introduction

In this tutorial, we aim to guide you on how to use TypeScript with Next.js API routes. TypeScript is a strongly typed superset of JavaScript that adds static types to the language. It enhances the API development process by catching errors during development rather than in production and makes your APIs more robust and easy to maintain.

By the end of this tutorial, you will be able to:
- Set up a Next.js project with TypeScript.
- Create API routes with TypeScript in Next.js.
- Understand TypeScript type definitions for Next.js API routes.

Prerequisites:

• Basic knowledge of JavaScript and TypeScript.
• Basic understanding of Next.js.
• Node.js and npm installed on your local development machine.

2. Step-by-Step Guide

To start with, we need to set up a Next.js project and then integrate TypeScript.

Setting up Next.js project

Install create-next-app which is a bootstrap tool for creating Next.js apps. Run the following command in your terminal:

npx create-next-app@latest nextjs-typescript-api

Setting up TypeScript

To set up TypeScript in your Next.js project, run the following command in your terminal:

npm install --save typescript @types/react @types/node

Next, rename the ./pages/index.js file to ./pages/index.tsx. This tells Next.js to enable TypeScript for your project.

Creating API Routes

Next.js has built-in support for API routes. To create a new API route, create a new file in ./pages/api/ directory, for example, ./pages/api/hello.ts.

The API route can look like this:

import type { NextApiRequest, NextApiResponse } from 'next'

interface HelloResponse {
  name: string
}

export default function hello(req: NextApiRequest, res: NextApiResponse<HelloResponse>) {
  res.status(200).json({ name: 'John Doe' })
}

In this code, we define a TypeScript interface HelloResponse for the response data. We then use Next.js types NextApiRequest and NextApiResponse to type our API route.

3. Code Examples

Let's look at a practical example:

Example 1: TypeScript with GET API Route

Create a new file ./pages/api/users.ts. This API route returns a list of users:

import type { NextApiRequest, NextApiResponse } from 'next'

interface User {
  id: number
  name: string
}

const users: User[] = [
  { id: 1, name: 'John Doe' },
  { id: 2, name: 'Jane Doe' },
]

export default function handler(req: NextApiRequest, res: NextApiResponse<User[]>) {
  res.status(200).json(users)
}

In this example, we define a User interface and use it to type our API response. This ensures that the response data matches the structure of the User interface.

4. Summary

In this tutorial, we learned how to set up a Next.js project with TypeScript and create typed API routes. TypeScript helps us write more robust and maintainable code by catching errors during development.

5. Practice Exercises

Now it's time to put your knowledge into practice.

1. Exercise 1: Create a new API route that returns a list of posts. Each post should have an id, title, and content.

2. Exercise 2: Create a new API route that returns a single post by its id. The id should be provided as a query parameter.

3. Exercise 3: Create a new API route that creates a new post. The post data should be provided as the request body.

Remember to define TypeScript interfaces for your data and use them to type your API routes. Good luck!

For further learning, consider exploring more complex examples and Next.js features. You can also check out the Next.js documentation and the TypeScript documentation.