Introduction

Next.JS has the ability to create its own Node.JS backend server. Any file under pages/api is mapped to /api/[file-name] and could be treated as an API endpoint. Those files are server-side only and won't have any impact on your client-side bundles.

Let's start by creating a sample Next.JS app:

npx create-next-app@latest
# or
yarn create next-app

Then change directory into the newly created project folder. Inside the project folder, you should have a similar project structure like below:

[project name]/
  ├─ 📁 node_modules/
  ├─ 📁 pages/
  ├─ 📁 public/
  ├─ 📁 styles/
  ├─ 📝 .eslintrc.json
  ├─ 📝 .gitignore
  ├─ 📝 next.config.js
  ├─ 📝 package.json
  ├─ 📝 README.md
  └─ 📝 yarn.lock

Inside pages folder you should have:

📁 pages/
  ├─ 📁 api/
  │     └─ 📝 hello.js
  ├─ 📝 _app.js
  └─ 📝 index.js

Now we are going to focus inside pages/api folder.

Create a new file index.js:

📁 api/
   ├─ 📝 hello.js
   └─ 📝 index.js

// pages/api/index.js
export default function handler(req, res) {
  if (req.method === "POST") {
    // Process a POST request
    res.status(200).json("a POST request received");
  } else if (req.method === "GET") {
    // Process a GET request
    res.status(200).json("a GET request received");
    // Handle any other HTTP method
  } else {
    // Handle any other HTTP method
    res.status(200).json(`a ${req.method} request received`);
  }
}

Inside index.js file we have just created a handler function on our /api API endpoint. The next time you create a GET request to /api, the server will return response of "a GET request received".

Now you have 2 existing endpoint: /api and /api/hello.

Source

https://nextjs.org/docs/api-routes/introduction