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.

Practice

1. What is the main purpose of using app.post('/register', ...) in an Express user registration flow?

easy

A. To update user details after registration

B. To serve the registration form HTML page

C. To handle incoming registration data sent by the client

D. To delete a user from the database

Solution

1. Step 1: Understand HTTP methods in Express

   app.post is used to handle POST requests, which usually send data to the server.

2. Step 2: Identify the role of the '/register' route

   The '/register' route is commonly used to receive new user data for account creation.

3. Final Answer:

   To handle incoming registration data sent by the client -> Option C

4. Quick Check:

   POST route = handle registration data [OK]

Hint: POST routes handle data sent from forms or clients [OK]

Common Mistakes:

• Confusing GET with POST for data submission
• Thinking app.post serves HTML pages
• Mixing update/delete operations with registration

2. Which of the following is the correct way to parse JSON data sent in a POST request in Express?

easy

A. app.use(express.urlencoded())

B. app.use(express.json())

C. app.get(express.json())

D. app.post(express.text())

Solution

1. Step 1: Identify middleware for JSON parsing

   Express provides express.json() middleware to parse JSON request bodies.

2. Step 2: Confirm correct usage

   It should be used with app.use() to apply globally or on specific routes.

3. Final Answer:

   app.use(express.json()) -> Option B

4. Quick Check:

   Use express.json() middleware for JSON data [OK]

Hint: Use express.json() to parse JSON POST data [OK]

Common Mistakes:

• Using express.urlencoded() for JSON data
• Calling express.json() inside app.get()
• Using express.text() for JSON parsing

3. Given the following Express route code for user registration, what will be the response if the user already exists?

app.post('/register', (req, res) => {
  const { username } = req.body;
  if (users.includes(username)) {
    res.status(400).send('User exists');
  } else {
    users.push(username);
    res.status(201).send('User created');
  }
});

medium

A. Status 201 with message 'User created'

B. Status 500 with message 'Server error'

C. Status 404 with message 'Not found'

D. Status 400 with message 'User exists'

Solution

1. Step 1: Check the condition for existing user

   The code checks if username is in users array and sends status 400 with 'User exists' if true.

2. Step 2: Understand response for existing user

   If the user exists, the response is a 400 status code with the message 'User exists'.

3. Final Answer:

   Status 400 with message 'User exists' -> Option D

4. Quick Check:

   User exists triggers 400 'User exists' [OK]

Hint: Check if user exists before adding to respond 400 [OK]

Common Mistakes:

• Assuming new user response for existing user
• Confusing status codes 400 and 201
• Ignoring the if condition logic

4. Identify the error in this Express user registration route:

app.post('/register', (req, res) => {
  const { username, password } = req.body;
  if (!username || !password) {
    res.status(400).send('Missing fields');
  }
  users.push({ username, password });
  res.status(201).send('User registered');
});

medium

A. Missing return after sending 400 response causes multiple responses

B. Incorrect HTTP method used; should be GET instead of POST

C. users.push should be outside the route handler

D. No error; code works correctly

Solution

1. Step 1: Analyze the if condition handling missing fields

   The code sends a 400 response if username or password is missing but does not stop execution.

2. Step 2: Identify consequence of missing return

   Without return, the code continues and tries to push user and send another response, causing an error.

3. Final Answer:

   Missing return after sending 400 response causes multiple responses -> Option A

4. Quick Check:

   Return after res.send to stop execution [OK]

Hint: Always return after sending response to avoid errors [OK]

Common Mistakes:

• Not returning after res.send in condition
• Using GET instead of POST for data submission
• Trying to push users outside route handler

5. You want to improve the user registration flow to prevent duplicate usernames and ensure passwords are at least 8 characters. Which code snippet correctly implements these checks before saving the user?

app.post('/register', (req, res) => {
  const { username, password } = req.body;
  // Your code here
});

hard

A. if (users.includes(username)) { res.status(400).send('User exists'); return; }
if (password.length < 8) { res.status(400).send('Password too short'); return; }
users.push({ username, password });
res.status(201).send('User registered');

B. if (!users.includes(username)) { res.status(400).send('User exists'); return; }
if (password.length > 8) { res.status(400).send('Password too short'); return; }
users.push({ username, password });
res.status(201).send('User registered');

C. if (users.includes(username)) { res.status(201).send('User exists'); return; }
if (password.length < 8) { res.status(201).send('Password too short'); return; }
users.push({ username, password });
res.status(400).send('User registered');

D. if (users.includes(username)) { res.status(400).send('User exists'); }
if (password.length < 8) { res.status(400).send('Password too short'); }
users.push({ username, password });
res.status(201).send('User registered');

Solution

1. Step 1: Check for existing username

   The code checks users.includes(username). If true, it responds with status 400 'User exists' and returns to prevent duplicates.

2. Step 2: Validate password length

   The code checks password.length < 8. If true, responds with 400 'Password too short' and returns.

3. Step 3: Add user and send success response

   After validations, users.push({ username, password }) and sends 201 'User registered'.

4. Final Answer:

   if (users.includes(username)) { res.status(400).send('User exists'); return; } if (password.length < 8) { res.status(400).send('Password too short'); return; } users.push({ username, password }); res.status(201).send('User registered'); -> Option A

5. Quick Check:

   Check user exists + password length, return on error [OK]

Hint: Return immediately after validation errors to stop flow [OK]

Common Mistakes:

• Not returning after sending error response
• Using wrong status codes for errors
• Adding user before validation checks