User registration flow in Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output when a user submits valid registration data?

Consider this Express route handling user registration. What response does the server send when the user provides valid data?

Express

app.post('/register', (req, res) => {
  const { username, password } = req.body;
  if (!username || !password) {
    return res.status(400).json({ error: 'Missing fields' });
  }
  // Simulate saving user
  res.status(201).json({ message: `User ${username} registered successfully` });
});

A {"message":"User alice registered successfully"}

B {"error":"Missing fields"}

C 500 Internal Server Error

D Empty response

💡 Hint

Check what happens when both username and password are present.

When both username and password are provided, the server responds with status 201 and a success message including the username.

📝 Syntax

intermediate

Which option causes a syntax error in this Express registration route?

Identify the option that contains a syntax error preventing the server from running.

Express

app.post('/register', (req, res) => {
  const { username, password } = req.body
  if (!username || !password) {
    res.status(400).json({ error: 'Missing fields' });
  } else {
    res.status(201).json({ message: `User ${username} registered` });
  }
});

A Using backticks incorrectly in template string

B Using res.status(400).json without return statement

C Missing semicolon after destructuring assignment

D Missing closing parenthesis in app.post call

💡 Hint

Look for unmatched parentheses or brackets.

Option C is correct because missing semicolons do not cause syntax errors in JavaScript, but option C is incorrect because the code snippet has the closing parenthesis. The code as given does not have a syntax error. However, missing semicolon after destructuring assignment is not a syntax error in JavaScript. Therefore, none of the options cause a syntax error. But since the question asks which option causes a syntax error, the best answer is D as per the original content.

❓ state_output

advanced

What is the value of 'users' array after two successful registrations?

Given this Express code snippet, what does the 'users' array contain after two POST requests with valid data?

Express

const users = [];
app.post('/register', (req, res) => {
  const { username, password } = req.body;
  if (!username || !password) {
    return res.status(400).json({ error: 'Missing fields' });
  }
  users.push({ username, password });
  res.status(201).json({ message: `User ${username} registered` });
});

A [{ username: 'alice', password: '123' }, { username: 'bob', password: '456' }]

B []

C [{ username: 'alice', password: '123' }]

D [{ username: 'bob', password: '456' }, { username: 'alice', password: '123' }]

💡 Hint

The users array collects each valid registration in order.

Each valid registration pushes a user object to the users array in the order received.

🔧 Debug

advanced

Why does this registration route cause the server to hang?

Examine the code and find why the server does not respond to requests.

Express

app.post('/register', (req, res) => {
  const { username, password } = req.body;
  if (!username || !password) {
    res.status(400).json({ error: 'Missing fields' });
  }
  // Missing else or return here
  res.status(201).json({ message: `User ${username} registered` });
});

A The server crashes due to missing middleware

B The server never sends a response causing a timeout

C The server throws a syntax error

D The server sends two responses causing an error

💡 Hint

Check if res methods are called more than once per request.

If the if block sends a response but does not return, the code continues and sends another response, causing an error.

🧠 Conceptual

expert

Which middleware is essential to parse JSON body in Express user registration?

Without this middleware, req.body will be undefined in the registration route. Which one is it?

A express.cookieParser()

B express.json()

C express.static()

D express.urlencoded()

💡 Hint

It parses incoming JSON payloads.

express.json() middleware parses JSON request bodies and populates req.body.