Request validation in Node.js - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when a required field is missing in this Express.js validation middleware?

Consider this Express.js middleware that validates a JSON request body for a 'username' field:

function validateUser(req, res, next) {
  if (!req.body.username) {
    return res.status(400).json({ error: 'Username is required' });
  }
  next();
}

What will the server respond if a POST request is sent without the 'username' field?

Node.js

function validateUser(req, res, next) {
  if (!req.body.username) {
    return res.status(400).json({ error: 'Username is required' });
  }
  next();
}

A The server responds with status 200 and proceeds to the next middleware

B The server responds with status 400 and JSON { error: 'Username is required' }

C The server crashes with a TypeError because req.body is undefined

D The server responds with status 500 and a generic error message

💡 Hint

Think about what happens when the condition !req.body.username is true.

If the 'username' field is missing, the condition !req.body.username is true, so the middleware sends a 400 status with an error message and does not call next(). This stops further processing.

📝 Syntax

intermediate

Which option correctly validates that 'age' is a number using Joi schema?

Given the Joi validation library, which schema correctly requires 'age' to be a number?

Node.js

const Joi = require('joi');

const schema = Joi.object({
  age: ???
});

A Joi.array().required()

B Joi.string().required()

C Joi.number().required()

D Joi.boolean().required()

💡 Hint

Think about the data type for 'age'.

To validate that 'age' is a number and required, use Joi.number().required(). Other options validate wrong types.

🔧 Debug

advanced

Why does this Express.js validation middleware not catch missing fields?

Look at this middleware:

function validate(req, res, next) {
  if (req.body.email === undefined || req.body.password === undefined) {
    res.status(400).send('Missing fields');
  }
  next();
}

Why might it fail to stop requests missing 'email' or 'password'?

Node.js

function validate(req, res, next) {
  if (req.body.email === undefined || req.body.password === undefined) {
    res.status(400).send('Missing fields');
  }
  next();
}

A Because next() is called even after sending a response, allowing the request to continue

B Because req.body is always undefined in Express.js

C Because the condition uses OR instead of AND

D Because res.status(400).send() does not send a response

💡 Hint

What happens after res.status(400).send() is called?

Calling next() after sending a response causes the request to continue processing, which can lead to unexpected behavior. The middleware should return after sending the response.

❓ state_output

advanced

What is the value of 'errors' after running this validation with express-validator?

Given this code snippet:

const { validationResult, check } = require('express-validator');

app.post('/login', [
  check('email').isEmail(),
  check('password').isLength({ min: 6 })
], (req, res) => {
  const errors = validationResult(req);
  res.json({ errors: errors.array() });
});

If the request body is { "email": "not-an-email", "password": "123" }, what will be the JSON response?

Node.js

const { validationResult, check } = require('express-validator');

app.post('/login', [
  check('email').isEmail(),
  check('password').isLength({ min: 6 })
], (req, res) => {
  const errors = validationResult(req);
  res.json({ errors: errors.array() });
});

A [{ msg: 'Invalid value', param: 'password', location: 'body' }]

B []

C [{ msg: 'Invalid value', param: 'email', location: 'body' }]

D [{ msg: 'Invalid value', param: 'email', location: 'body' }, { msg: 'Invalid value', param: 'password', location: 'body' }]

💡 Hint

Both 'email' and 'password' fail their validations.

The 'email' is not a valid email format and 'password' is shorter than 6 characters, so both produce errors in the array.

🧠 Conceptual

expert

Which statement best describes the role of middleware in request validation in Node.js frameworks?

Choose the most accurate description of how middleware functions in request validation.

A Middleware intercepts requests before reaching route handlers to check and reject invalid data early, improving security and performance.

B Middleware automatically fixes invalid request data and modifies it to pass validation without notifying the client.

C Middleware delays request processing until all asynchronous database validations complete, blocking the event loop.

D Middleware only logs validation errors but does not affect request flow or responses.

💡 Hint

Think about when middleware runs and what it can do with invalid data.

Middleware runs before route handlers and can stop invalid requests early by sending error responses, which helps keep the app secure and efficient.