Validation error response formatting in Express - Mini Project: Build & Apply

Validation error response formatting

📖 Scenario: You are building a simple Express server that accepts user data. You want to send clear error messages back to the client when the data is invalid.

🎯 Goal: Create an Express route that validates incoming JSON data and sends a formatted error response if validation fails.

📋 What You'll Learn

Create an Express app with a POST route at /submit

Check if the request body has a username field

If username is missing, respond with status 400 and a JSON error message

Format the error response as { error: 'Username is required' }

If username exists, respond with status 200 and { message: 'Success' }

💡 Why This Matters

🌍 Real World

APIs often need to validate user input and send clear error messages so clients know what went wrong.

💼 Career

Backend developers frequently write validation logic and format error responses to improve API usability and reliability.

1

Set up Express app and JSON parsing

Create an Express app by requiring express and calling express(). Use app.use(express.json()) to parse JSON bodies.

Express

const express = require('express');
const app = express();
// Your code here

Need a hint?

Remember to import Express and enable JSON body parsing.

2

Create POST route at /submit

Add a POST route handler for /submit using app.post('/submit', (req, res) => { }).

Express

const express = require('express');
const app = express();
app.use(express.json());

// Your code here

Need a hint?

Use app.post to create the route.

3

Check for username and send error response

Inside the /submit route, check if req.body.username is missing. If missing, respond with res.status(400).json({ error: 'Username is required' }).

Express

const express = require('express');
const app = express();
app.use(express.json());

app.post('/submit', (req, res) => {
  // Your code here
});

Need a hint?

Use an if statement to check req.body.username.

4

Send success response if username exists

If username exists, respond with res.status(200).json({ message: 'Success' }) inside the /submit route.

Express

const express = require('express');
const app = express();
app.use(express.json());

app.post('/submit', (req, res) => {
  if (!req.body.username) {
    return res.status(400).json({ error: 'Username is required' });
  }
  // Your code here
});

Need a hint?

Send a success JSON response with status 200.