Register a User and Test API using Postman in Next.js with MongoDB

When building a full stack application, one common feature is user registration.

This means allowing users to create an account using email and password.

In this blog we will understand how to:

✔ Create a register API in Next.js

✔ Connect the API with MongoDB database

✔ Save a user in the database

✔ Test the API using Postman

This example uses Next.js App Router, MongoDB and Mongoose.

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📁 Project API Structure

In a Next.js App Router project the API route can look like this:

app
 └ api
   └ auth
     └ register
       └ route.ts

This creates the API endpoint:

http://localhost:3000/api/auth/register

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⚙️ Register API Code

Below is the API used to register a user.

import { NextRequest, NextResponse } from "next/server";
import { connectToDatabase } from "@/lib/db";
import User from "@/models/User";

export async function POST(request: NextRequest) {
  try {
    const { email, password } = await request.json();

    if (!email || !password) {
      return NextResponse.json(
        { error: "Email and password are required" },
        { status: 400 },
      );
    }

    await connectToDatabase();

    const existingUser = await User.findOne({ email });

    if (existingUser) {
      return NextResponse.json(
        { error: "Email is already registered" },
        { status: 400 },
      );
    }

    await User.create({
      email,
      password,
    });

    return NextResponse.json(
      { message: "User registered successfully" },
      { status: 201 },
    );
  } catch (error) {
    return NextResponse.json(
      { error: "Failed to register User" },
      { status: 500 },
    );
  }
}

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🧠 Understanding the Code Step by Step

1️⃣ Import Required Modules

import { NextRequest, NextResponse } from "next/server";
import { connectToDatabase } from "@/lib/db";
import User from "@/models/User";

These imports help us:

• handle HTTP requests

• connect to MongoDB

• access the User model

---

2️⃣ Create POST API Function

export async function POST(request: NextRequest)

This means the API will run when we send a POST request.

Example API URL

/api/auth/register

---

3️⃣ Read Data from Request

const { email, password } = await request.json();

This reads data sent from the client.

Example request body

{
 "email": "[e-mail]",
 "password": "123456"
}

---

4️⃣ Validate Input Data

if (!email || !password)

If the user does not send email or password the API returns an error.

Example response

{
 "error": "Email and password are required"
}

Status code

400 Bad Request

---

5️⃣ Connect to MongoDB

await connectToDatabase();

This connects the application to the MongoDB database.

The connection uses the MongoDB Atlas connection string stored in:

.env.local

Example

MONGODB_URI=mongodb+srv://username:[e-mail]/reels-pro

---

6️⃣ Check if User Already Exists

const existingUser = await User.findOne({ email });

This checks if the email already exists in the database.

Example stored user

{
 email: "[e-mail]",
 password: "hashedpassword"
}

If the email already exists the API returns an error.

Example response

{
 "error": "Email is already registered"
}

---

7️⃣ Create New User

If the user does not exist the API creates a new user.

await User.create({
 email,
 password,
});

The password is stored as a hashed password using bcrypt in the User model.

Example saved user in MongoDB

{
 _id: ObjectId("..."),
 email: "[e-mail]",
 password: "$2a$10$FJH78S...",
 createdAt: "2025",
 updatedAt: "2025"
}

---

8️⃣ Send Success Response

return NextResponse.json(
 { message: "User registered successfully" },
 { status: 201 }
);

Status 201 means

Created successfully

---

🗄 MongoDB Atlas Setup

Before the API works you must connect your project to MongoDB Atlas.

Steps include

✔ create a cluster

✔ create a database user

✔ whitelist your IP address

✔ copy MongoDB connection string

Example connection string

mongodb+srv://username:[e-mail]/reels-pro

This connection string is stored inside

.env.local

---

📬 Testing the API with Postman

After creating the API we test it using Postman.

Open Postman and create a request.

Request Method

POST

API URL

http://localhost:3000/api/auth/register

---

Request Body

Select:

Body → raw → JSON

Example request

{
 "email": "[e-mail]",
 "password": "123456"
}

Click Send.

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✅ Successful Response

If everything works correctly Postman returns

{
 "message": "User registered successfully"
}

Status code

201 Created

---

📊 Check User in MongoDB Atlas

You can verify the saved user in MongoDB Atlas.

Steps

1. Open MongoDB Atlas dashboard
2. Click your cluster
3. Click Browse Collections
4. Open your database
5. Open the users collection

You will see a document like

{
 email: "[e-mail]",
 password: "$2a$10$...",
 createdAt: "...",
 updatedAt: "..."
}

This means the API worked correctly.

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🎯 Summary

In this project we learned how to:

✔ Create a register API in Next.js

✔ Connect the application to MongoDB Atlas

✔ Save user data in the database

✔ Test the API using Postman

This is an important step when building authentication systems in full stack applications.

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✨ Building APIs and testing them with tools like Postman helps developers understand how the frontend, backend and database work together.

Happy coding 💻🚀