Validating body fields in Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output when sending a POST request with missing required fields?

Consider this Express route that validates the presence of 'username' and 'password' in the request body. What response will the server send if the client sends a POST request with an empty JSON body?

Express

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

app.post('/login', (req, res) => {
  const { username, password } = req.body;
  if (!username || !password) {
    return res.status(400).json({ error: 'Missing username or password' });
  }
  res.status(200).json({ message: 'Login successful' });
});

A Status 404 with JSON { error: 'Not found' }

B Status 400 with JSON { error: 'Missing username or password' }

C Status 500 with JSON { error: 'Server error' }

D Status 200 with JSON { message: 'Login successful' }

💡 Hint

Think about what happens when required fields are missing in the request body.

The route checks if either 'username' or 'password' is missing. If so, it returns a 400 status with an error message. Since the body is empty, both are missing, so the server responds with status 400 and the error JSON.

📝 Syntax

intermediate

Which option correctly validates that 'email' is a non-empty string in the request body?

Which code snippet correctly checks that the 'email' field exists and is a non-empty string in an Express POST route?

Express

app.post('/subscribe', (req, res) => {
  // validation here
});

A if (req.body.email !== undefined) { res.send('Valid'); } else { res.status(400).send('Invalid email'); }

B if (req.body.email) { res.send('Valid'); } else { res.status(400).send('Invalid email'); }

C if (req.body.email.length > 0) { res.send('Valid'); } else { res.status(400).send('Invalid email'); }

D if (typeof req.body.email === 'string' && req.body.email.trim() !== '') { res.send('Valid'); } else { res.status(400).send('Invalid email'); }

💡 Hint

Check both type and content to ensure 'email' is a non-empty string.

Option D checks that 'email' is a string and not just whitespace. Other options either don't check type or can fail if 'email' is undefined.

🔧 Debug

advanced

What is the response when sending a POST request with missing 'age' field?

Look at this Express route snippet. What response will the server send if the client sends a POST request without the 'age' field?

Express

app.post('/profile', (req, res) => {
  const { age } = req.body;
  if (age && typeof age === 'number' && age > 0) {
    res.send('Valid age');
  } else {
    res.status(400).send('Invalid age');
  }
});

A Because 'age' is undefined, the condition 'age && ...' is skipped, so it returns 200 (Valid age).

B Because 'age' is undefined, the condition 'age && ...' is false, so it returns 200 (Valid age).

C Because 'age' is undefined, the condition 'age && ...' is false, so it returns 400 (Invalid age).

D Because 'age' is undefined, destructuring throws an error, resulting in status 500.

💡 Hint

Think about how JavaScript treats undefined in logical AND expressions.

If 'age' is missing, it is undefined, so 'age && ...' is false, causing the else branch to run and respond with 400 status.

❓ state_output

advanced

What is the response when sending a POST with 'age' as a string '25'?

Given this Express route, what response will the server send if the client sends { "age": "25" } in the JSON body?

Express

app.post('/check-age', (req, res) => {
  const { age } = req.body;
  if (typeof age === 'number' && age >= 18) {
    res.status(200).json({ message: 'Access granted' });
  } else {
    res.status(403).json({ message: 'Access denied' });
  }
});

A Status 403 with JSON { message: 'Access denied' }

B Status 200 with JSON { message: 'Access granted' }

C Status 400 with JSON { error: 'Invalid age type' }

D Status 500 with JSON { error: 'Server error' }

💡 Hint

Check the type of 'age' in the condition.

The 'age' field is a string, not a number, so the condition fails and the server responds with 403 and 'Access denied'.

🧠 Conceptual

expert

Which middleware approach best ensures all required body fields are validated before route handlers?

You want to validate that the request body contains 'name', 'email', and 'password' fields before any route handler runs. Which approach below best achieves this in Express?

A Create a middleware function that checks these fields and calls next() if valid or sends 400 error if not; use app.use() to apply it globally.

B Add validation code inside each route handler separately to check the fields before processing.

C Use a try-catch block inside each route handler to catch missing fields errors.

D Rely on client-side validation only and do not validate on the server.

💡 Hint

Think about reusability and centralizing validation logic.

Middleware applied globally can validate requests before reaching route handlers, avoiding repeated code and ensuring consistent validation.