JSON request and response patterns in Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this Express route?

Consider this Express route handler that receives a JSON request and sends a JSON response. What will the client receive when sending {"name": "Alice"} in the request body?

Express

app.post('/greet', (req, res) => {
  const { name } = req.body;
  res.json({ message: `Hello, ${name}!` });
});

A {"message":"Hello, Alice!"}

B {"message":"Hello, undefined!"}

C SyntaxError: Unexpected token in JSON

D {"error":"Name not provided"}

💡 Hint

Think about how Express parses JSON request bodies and how the response is formed.

The route extracts the name from the JSON request body and sends back a JSON response with a greeting message including that name.

📝 Syntax

intermediate

Which option correctly parses JSON request body in Express?

You want to handle JSON data sent by clients in Express. Which code snippet correctly enables JSON parsing middleware?

A app.use(express.json());

B app.use(bodyParser.urlencoded());

C app.use(express.text());

D app.use(express.raw());

💡 Hint

Look for the middleware that parses JSON bodies.

express.json() middleware parses incoming JSON requests and makes the data available in req.body.

🔧 Debug

advanced

Why does this Express route always send 'undefined' in the response?

Examine the code below. The client sends {"age":30} in the JSON body, but the response is always {"age":undefined}. What is the cause?

Express

app.post('/age', (req, res) => {
  const age = req.body.age;
  res.json({ age: age });
});

// Middleware setup missing

A res.json() requires a string, not an object

B Incorrect property name; should be req.body.Age

C Missing express.json() middleware to parse JSON body

D Route method should be app.get instead of app.post

💡 Hint

Think about how Express reads the request body.

Without express.json() middleware, req.body is undefined, so req.body.age is undefined.

❓ state_output

advanced

What is the JSON response after multiple requests?

This Express server keeps a count of how many times the '/count' route is called. What JSON does the client receive on the third request?

Express

let count = 0;
app.get('/count', (req, res) => {
  count += 1;
  res.json({ count });
});

A {"count":1}

B {"count":3}

C {"count":0}

D {"count":2}

💡 Hint

The count variable increases with each request.

The variable count starts at 0 and increments by 1 on each request. On the third request, it is 3.

🧠 Conceptual

expert

Which header must be set by the client to send JSON data correctly to an Express server?

When sending JSON data in a POST request to an Express server, which HTTP header should the client include to ensure proper parsing?

A Authorization: Bearer token

B Accept: application/json

C Content-Length: 0

D Content-Type: application/json

💡 Hint

This header tells the server the format of the data being sent.

The Content-Type: application/json header tells the server that the request body contains JSON data.

Practice

1. What does the express.json() middleware do in an Express app?

easy

A. It sends JSON responses to the client.

B. It parses incoming JSON request bodies automatically.

C. It logs JSON data to the console.

D. It validates JSON schema in requests.

Solution

1. Step 1: Understand middleware purpose

   The express.json() middleware is designed to parse JSON data sent in the request body.

2. Step 2: Differentiate from response methods

   Sending JSON responses is done by res.json(), not express.json().

3. Final Answer:

   It parses incoming JSON request bodies automatically. -> Option B

4. Quick Check:

   express.json() parses JSON requests [OK]

Hint: express.json() parses JSON request bodies [OK]

Common Mistakes:

• Confusing express.json() with res.json()
• Thinking it sends JSON responses
• Assuming it validates JSON schema

2. Which of the following is the correct way to send a JSON response with Express?

easy

A. res.sendJson({ message: 'Hello' })

B. res.send({ json: 'Hello' })

C. res.json({ message: 'Hello' })

D. res.jsonify({ message: 'Hello' })

Solution

1. Step 1: Identify Express response methods

   Express provides res.json() to send JSON responses with proper headers.

2. Step 2: Check method names

   Methods like sendJson or jsonify do not exist in Express.

3. Final Answer:

   res.json({ message: 'Hello' }) -> Option C

4. Quick Check:

   Use res.json() to send JSON responses [OK]

Hint: Use res.json() to send JSON responses [OK]

Common Mistakes:

• Using non-existent methods like sendJson or jsonify
• Using res.send() without JSON formatting
• Confusing method names

3. Given this Express route, what will be the JSON response when a POST request with body { "name": "Alice" } is sent?

app.use(express.json());
app.post('/greet', (req, res) => {
  const user = req.body.name;
  res.json({ greeting: `Hello, ${user}!` });
});

medium

A. Empty response

B. { "greeting": "Hello, undefined!" }

C. SyntaxError

D. { "greeting": "Hello, Alice!" }

Solution

1. Step 1: Parse JSON body with express.json()

   The middleware parses the JSON body, so req.body.name is 'Alice'.

2. Step 2: Construct JSON response

   The response sends { greeting: `Hello, Alice!` } as JSON.

3. Final Answer:

   { "greeting": "Hello, Alice!" } -> Option D

4. Quick Check:

   Parsed JSON body used in res.json() response [OK]

Hint: express.json() parses body; res.json() sends response [OK]

Common Mistakes:

• Forgetting to use express.json() middleware
• Accessing req.body before parsing
• Expecting undefined instead of 'Alice'

4. What is wrong with this Express code snippet for handling JSON requests?

app.post('/data', (req, res) => {
  const data = req.body;
  res.json({ received: data });
});

medium

A. Missing express.json() middleware to parse JSON body.

B. Using res.json() instead of res.send().

C. Incorrect route method; should be app.get.

D. No error; code works fine.

Solution

1. Step 1: Check JSON parsing middleware

   The code accesses req.body but does not show express.json() middleware usage.

2. Step 2: Understand middleware necessity

   Without express.json(), req.body will be undefined for JSON requests.

3. Final Answer:

   Missing express.json() middleware to parse JSON body. -> Option A

4. Quick Check:

   express.json() needed to parse JSON requests [OK]

Hint: Always add express.json() to parse JSON bodies [OK]

Common Mistakes:

• Assuming req.body is parsed automatically
• Confusing res.json() with res.send()
• Using wrong HTTP method for JSON POST

5. You want to create an Express route that accepts a JSON array of numbers in the request body and responds with a JSON object containing the sum of those numbers. Which code snippet correctly implements this?

hard

A. app.post('/sum', express.json(), (req, res) => {
const numbers = req.body;
const sum = numbers.reduce((a, b) => a + b, 0);
res.json({ sum });
});

B. app.post('/sum', (req, res) => {
const numbers = JSON.parse(req.body);
const sum = numbers.reduce((a, b) => a + b, 0);
res.json({ sum });
});

C. app.post('/sum', express.urlencoded(), (req, res) => {
const numbers = req.body;
const sum = numbers.reduce((a, b) => a + b, 0);
res.json({ sum });
});

D. app.post('/sum', (req, res) => {
const numbers = req.body.numbers;
const sum = numbers.reduce((a, b) => a + b, 0);
res.send({ sum });
});

Solution

1. Step 1: Use express.json() middleware to parse JSON array

   app.post('/sum', express.json(), (req, res) => { const numbers = req.body; const sum = numbers.reduce((a, b) => a + b, 0); res.json({ sum }); }); correctly uses express.json() middleware inline to parse the JSON array in the request body.

2. Step 2: Sum array and send JSON response

   It sums the numbers with reduce and sends the result with res.json(), which sets correct headers.

3. Step 3: Identify errors in other options

   app.post('/sum', (req, res) => { const numbers = JSON.parse(req.body); const sum = numbers.reduce((a, b) => a + b, 0); res.json({ sum }); }); tries to parse req.body manually, which is already parsed by middleware. app.post('/sum', express.urlencoded(), (req, res) => { const numbers = req.body; const sum = numbers.reduce((a, b) => a + b, 0); res.json({ sum }); }); uses express.urlencoded() which is for form data, not JSON. app.post('/sum', (req, res) => { const numbers = req.body.numbers; const sum = numbers.reduce((a, b) => a + b, 0); res.send({ sum }); }); accesses req.body.numbers but expects array directly in body; also uses res.send() which may not set JSON headers properly.

4. Final Answer:

   app.post('/sum', express.json(), (req, res) => { const numbers = req.body; const sum = numbers.reduce((a, b) => a + b, 0); res.json({ sum }); }); -> Option A

5. Quick Check:

   express.json() + res.json() for JSON array sum [OK]

Hint: Use express.json() middleware and res.json() for JSON data [OK]

Common Mistakes:

• Not using express.json() to parse JSON body
• Using express.urlencoded() for JSON data
• Manually parsing req.body with JSON.parse
• Using res.send() instead of res.json() for JSON response