Schema validation in Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when a request body misses a required field in this Express schema validation?

Consider this Express middleware using express-validator to validate a POST request body:

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

app.post('/user', [
  body('username').isString().notEmpty(),
  body('age').isInt({ min: 18 })
], (req, res) => {
  const errors = validationResult(req);
  if (!errors.isEmpty()) {
    return res.status(400).json({ errors: errors.array() });
  }
  res.send('User created');
});

What will the server respond if the request body is { "username": "alice" } (missing age)?

Express

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

app.post('/user', [
  body('username').isString().notEmpty(),
  body('age').isInt({ min: 18 })
], (req, res) => {
  const errors = validationResult(req);
  if (!errors.isEmpty()) {
    return res.status(400).json({ errors: errors.array() });
  }
  res.send('User created');
});

A Responds with status 400 and an error about missing 'age' field

B Responds with status 200 and message 'User created'

C Responds with status 500 due to server error

D Responds with status 400 and an error about missing 'username' field

💡 Hint

Think about what happens when a required field is not present and the validation rule expects an integer.

The body('age').isInt({ min: 18 }) rule requires the age field to be an integer at least 18. If age is missing, validation fails and validationResult(req).isEmpty() returns false. The server responds with status 400 and an error message about the missing or invalid age field.

📝 Syntax

intermediate

Identify the syntax error in this Express schema validation middleware

Look at this Express route with schema validation using express-validator:

app.post('/login', [
  body('email').isEmail(),
  body('password').isLength({ min: 6 })
], (req, res) => {
  const errors = validationResult(req)
  if (!errors.isEmpty()) {
    return res.status(400).json({ errors: errors.array() })
  }
  res.send('Login successful')
})

What is the syntax error in this code?

Express

app.post('/login', [
  body('email').isEmail(),
  body('password').isLength({ min: 6 })
], (req, res) => {
  const errors = validationResult(req)
  if (!errors.isEmpty()) {
    return res.status(400).json({ errors: errors.array() })
  }
  res.send('Login successful')
})

A Missing semicolons after statements inside the route handler

B Missing parentheses after 'validationResult' call

C Missing import of 'body' and 'validationResult' from 'express-validator'

D Missing closing bracket for the array of validation middlewares

💡 Hint

Check if all required functions are imported before usage.

The code uses body and validationResult but does not show importing them from express-validator. Without importing, the code will throw a ReferenceError. Missing semicolons or parentheses are not syntax errors in JavaScript here, and the array brackets are properly closed.

🔧 Debug

advanced

Why does this Express schema validation not catch invalid email input?

Given this Express route:

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

app.post('/register', [
  body('email').isEmail(),
  body('password').isLength({ min: 8 })
], (req, res) => {
  const errors = validationResult(req);
  if (!errors.isEmpty()) {
    return res.status(400).json({ errors: errors.array() });
  }
  res.send('Registration successful');
});

But when sending { "email": "not-an-email", "password": "12345678" }, the server responds with 'Registration successful'. Why?

Express

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

app.post('/register', [
  body('email').isEmail(),
  body('password').isLength({ min: 8 })
], (req, res) => {
  const errors = validationResult(req);
  if (!errors.isEmpty()) {
    return res.status(400).json({ errors: errors.array() });
  }
  res.send('Registration successful');
});

A The isEmail() validator is not working because the password is valid

B The route handler does not return after sending the error response

C The validationResult is not called correctly

D The request body is not parsed as JSON before validation, so fields are undefined

💡 Hint

Think about how Express reads the request body before validation.

If the Express app does not use middleware like express.json() to parse JSON bodies, req.body will be undefined or empty. Validators like isEmail() then receive undefined and do not fail as expected. This causes validation to pass incorrectly.

❓ state_output

advanced

What is the output of this Express validation error response?

Consider this Express route with validation:

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

app.post('/submit', [
  body('title').isLength({ min: 5 }),
  body('year').isInt({ min: 2000, max: 2025 })
], (req, res) => {
  const errors = validationResult(req);
  if (!errors.isEmpty()) {
    return res.status(422).json({ errors: errors.array() });
  }
  res.send('Submission accepted');
});

If the request body is { "title": "abc", "year": 1999 }, what is the JSON response?

Express

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

app.post('/submit', [
  body('title').isLength({ min: 5 }),
  body('year').isInt({ min: 2000, max: 2025 })
], (req, res) => {
  const errors = validationResult(req);
  if (!errors.isEmpty()) {
    return res.status(422).json({ errors: errors.array() });
  }
  res.send('Submission accepted');
});

A {"errors":[{"value":"abc","msg":"Title must be at least 5 characters long","param":"title","location":"body"},{"value":1999,"msg":"Year must be between 2000 and 2025","param":"year","location":"body"}]}

B {"errors":[{"value":"abc","msg":"Invalid value","param":"title","location":"body"},{"value":1999,"msg":"Invalid value","param":"year","location":"body"}]}

C {"errors":[{"value":"abc","msg":"Too short","param":"title","location":"body"}]}

D {"errors":[]}

💡 Hint

Check the default error messages from express-validator when no custom message is provided.

Without custom error messages, express-validator returns a default message 'Invalid value' for failed validations. Both title and year fail their checks, so the response includes errors for both with the default message.

🧠 Conceptual

expert

Which option best explains why schema validation middleware must run before route handlers in Express?

In Express, why is it important to place schema validation middleware before the main route handler?

A Because validation middleware checks and rejects invalid input early, preventing the handler from processing bad data

B Because validation middleware modifies the request body to match the schema before the handler uses it

C Because route handlers cannot access <code>req.body</code> unless validation middleware runs first

D Because validation middleware automatically sends success responses if validation passes, so handlers are skipped

💡 Hint

Think about the flow of request processing and error handling.

Validation middleware runs before the route handler to check input early. If input is invalid, it sends an error response immediately, stopping further processing. This prevents the route handler from running with bad data. Validation middleware does not modify data or send success responses automatically.