Custom validation rules in Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this Express middleware with custom validation?

Consider this Express middleware that validates a request body property 'age' to be a number greater than 18. What response will the server send if the request body is {"age": 16}?

Express

app.post('/check-age', (req, res, next) => {
  const age = req.body.age;
  if (typeof age !== 'number' || age <= 18) {
    return res.status(400).json({ error: 'Age must be a number greater than 18' });
  }
  next();
}, (req, res) => {
  res.json({ message: 'Age is valid' });
});

A Status 400 with JSON { error: 'Invalid request body' }

B Status 200 with JSON { message: 'Age is valid' }

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

D Status 400 with JSON { error: 'Age must be a number greater than 18' }

💡 Hint

Think about the condition that checks the age value and what happens when it fails.

The middleware checks if age is not a number or less than or equal to 18. Since 16 is less than 18, it returns a 400 status with the error message.

📝 Syntax

intermediate

Which option correctly defines a custom validation function in Express?

You want to create a custom validation function to check if a string is uppercase. Which code snippet correctly defines and uses this function in Express middleware?

A function isUpperCase(str) { return str === str.toUpperCase(); } app.use((req, res, next) => { if (!isUpperCase(req.body.name)) return res.status(400).send('Name must be uppercase'); next(); });

B const isUpperCase = (str) => { str.toUpperCase(); }; app.use((req, res, next) => { if (!isUpperCase(req.body.name)) return res.status(400).send('Name must be uppercase'); next(); });

C function isUpperCase(str) { return str.toLowerCase() === str; } app.use((req, res, next) => { if (!isUpperCase(req.body.name)) return res.status(400).send('Name must be uppercase'); next(); });

D const isUpperCase = str => { return str.toUpperCase === str; }; app.use((req, res, next) => { if (!isUpperCase(req.body.name)) return res.status(400).send('Name must be uppercase'); next(); });

💡 Hint

Check if the function returns a boolean and compares correctly.

Option A correctly returns true if the string equals its uppercase version. Other options either don't return a value or compare incorrectly.

🔧 Debug

advanced

Why does this custom validation middleware always pass even with invalid input?

This middleware is supposed to reject requests where 'email' is missing or invalid. Why does it allow invalid emails?

Express

app.use((req, res, next) => {
  const email = req.body.email;
  if (!email && !email.includes('@')) {
    return res.status(400).send('Invalid email');
  }
  next();
});

A Because next() is called before the condition, so validation is skipped.

B Because email.includes('@') throws an error if email is undefined, so middleware crashes silently.

C Because the condition uses && instead of ||, so it only fails if email is falsy AND includes '@', which never happens.

D Because res.status(400).send() is asynchronous and does not stop middleware execution.

💡 Hint

Look carefully at the if condition combining !email and !email.includes('@').

The condition uses &&, so it only triggers if email is falsy AND email.includes('@') is false. But if email is falsy, email.includes('@') is not checked, so the condition fails to catch missing or invalid emails.

❓ state_output

advanced

What is the value of req.customData after this validation middleware runs?

This middleware adds a customData property to req if validation passes. What is req.customData after a request with body { username: 'User123' }?

Express

app.use((req, res, next) => {
  const username = req.body.username;
  if (typeof username === 'string' && username.length >= 5) {
    req.customData = { valid: true, user: username.toLowerCase() };
  } else {
    req.customData = { valid: false };
  }
  next();
});

A { valid: false }

B { valid: true, user: 'user123' }

C undefined

D null

💡 Hint

Check the username length and type, then see what customData is set to.

The username 'User123' is a string and length is 7, which is >= 5, so customData is set with valid true and user lowercase.

🧠 Conceptual

expert

Which option best explains why custom validation middleware should call next() only once?

In Express, why must custom validation middleware call next() exactly once and not multiple times or skip it?

A Calling next() multiple times causes the request to be processed by multiple handlers repeatedly, leading to errors or crashes.

B Calling next() multiple times improves performance by parallelizing middleware execution.

C Skipping next() causes the server to automatically retry the request, causing infinite loops.

D Skipping next() causes the response to be sent twice, which is allowed but not recommended.

💡 Hint

Think about how Express middleware chains work and what happens if next() is called more than once.

Calling next() multiple times causes Express to continue processing middleware multiple times for the same request, which can cause errors or crashes. Skipping next() stops the chain and can cause the request to hang.