res.json for JSON responses 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:

app.get('/data', (req, res) => { res.json({ message: 'Hello World' }); });

What will the client receive when requesting '/data'?

Express

app.get('/data', (req, res) => { res.json({ message: 'Hello World' }); });

A A 404 Not Found error

B A JSON response with Content-Type 'application/json' and body {"message":"Hello World"}

C An HTML page displaying the text 'Hello World'

D A plain text response with body 'Hello World'

💡 Hint

res.json sends a JSON response with the correct content type.

The res.json method sends the JavaScript object as a JSON string with the Content-Type header set to 'application/json'.

📝 Syntax

intermediate

Which option correctly sends a JSON response with a status code 201?

You want to send a JSON response with status code 201 and body { success: true }. Which code is correct?

A res.status(201).json({ success: true });

B res.sendStatus(201).json({ success: true });

C res.json(201, { success: true });

D res.json({ success: true }).status(201);

💡 Hint

Set status before sending JSON.

The correct pattern is to call res.status(code) first, then res.json(body). Other options either use invalid method signatures or chain incorrectly.

🔧 Debug

advanced

What error does this code cause?

Examine this Express route:

app.get('/test', (req, res) => { res.json('Hello'); res.json({ done: true }); });

What happens when a client requests '/test'?

Express

app.get('/test', (req, res) => { res.json('Hello'); res.json({ done: true }); });

A Sends only the first JSON response 'Hello' and ignores the second

B Sends a combined JSON response with both values

C Throws an error because res.json is called twice on the same response

D Sends 'Hello' as JSON and then sends { done: true } as JSON successfully

💡 Hint

You cannot send multiple responses for one request.

Calling res.json twice tries to send two responses, which is not allowed and causes an error.

🧠 Conceptual

advanced

How does res.json handle JavaScript objects?

When you pass a JavaScript object to res.json, what does Express do internally before sending the response?

A It converts the object to a JSON string and sets Content-Type to 'application/json'

B It sends the object as is without conversion

C It converts the object to XML format

D It converts the object to a plain text string

💡 Hint

Think about how JSON responses are sent over HTTP.

res.json converts the object to a JSON string and sets the correct content type header so the client knows it is JSON.

❓ state_output

expert

What is the final response body and status code?

Given this Express route:

app.get('/status', (req, res) => { const data = { ok: true }; res.status(202); res.json(data); });

What will the client receive?

Express

app.get('/status', (req, res) => { const data = { ok: true }; res.status(202); res.json(data); });

A Status code 500 Internal Server Error

B Status code 200 with JSON body {"ok":true}

C Status code 202 with empty body

D Status code 202 with JSON body {"ok":true}

💡 Hint

Calling res.status sets the status for the next response method.

res.status(202) sets the status code, and res.json sends the JSON body with that status code.

Practice

1. What does res.json() do in an Express app?

easy

A. Parses JSON data from a file

B. Reads JSON data from the client request

C. Sends a JSON response to the client with proper headers

D. Logs JSON data to the server console

Solution

1. Step 1: Understand the purpose of res.json()

   This method is used to send JSON data back to the client in an HTTP response.

2. Step 2: Recognize header setting and formatting

   It automatically sets the Content-Type header to 'application/json' and converts the JavaScript object to JSON format.

3. Final Answer:

   Sends a JSON response to the client with proper headers -> Option C

4. Quick Check:

   res.json() sends JSON response [OK]

Hint: Remember: res.json sends data, not reads it [OK]

Common Mistakes:

• Confusing res.json with reading client data
• Thinking it logs data instead of sending
• Assuming it parses files automatically

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

easy

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

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

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

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

Solution

1. Step 1: Identify the correct method name

   The Express method to send JSON is res.json(), not res.sendJson() or res.jsonify().

2. Step 2: Check the argument format

   The argument should be a JavaScript object, like { message: 'Hello' }.

3. Final Answer:

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

4. Quick Check:

   Correct method is res.json() [OK]

Hint: Use res.json() exactly, no extra letters [OK]

Common Mistakes:

• Using res.sendJson or res.jsonify which don't exist
• Passing string instead of object
• Mixing res.send with JSON method

3. What will the client receive when this Express route runs?

app.get('/data', (req, res) => {
  res.json({ success: true, count: 5 });
});

medium

A. success: true, count: 5

B. ["success", true, "count", 5]

C. An error because res.json needs a string

D. {"success":true,"count":5}

Solution

1. Step 1: Understand what res.json sends

   It converts the JavaScript object into a JSON string with keys and values properly quoted.

2. Step 2: Check the exact JSON format

   The object { success: true, count: 5 } becomes the JSON string '{"success":true,"count":5}'.

3. Final Answer:

   {"success":true,"count":5} -> Option D

4. Quick Check:

   res.json sends JSON string [OK]

Hint: res.json sends JSON string with quotes around keys [OK]

Common Mistakes:

• Thinking it sends array format
• Assuming it sends plain text without quotes
• Believing res.json requires string input

4. Identify the error in this Express code snippet:

app.get('/info', (req, res) => {
  res.json = { name: 'Alice' };
});

medium

A. Missing parentheses after res.json()

B. res.json is assigned an object instead of called as a function

C. res.json cannot send objects, only strings

D. Route path should be '/info/' with trailing slash

Solution

1. Step 1: Check how res.json is used

   The code assigns an object to res.json instead of calling it as a function.

2. Step 2: Understand correct usage

   res.json is a method and should be called like res.json({ name: 'Alice' });

3. Final Answer:

   res.json is assigned an object instead of called as a function -> Option B

4. Quick Check:

   res.json must be called, not assigned [OK]

Hint: res.json is a function, always call it with () [OK]

Common Mistakes:

• Assigning to res.json instead of calling it
• Forgetting parentheses after res.json
• Thinking res.json only accepts strings

5. You want to send a JSON response with a list of users but only include users with active status true. Which code correctly uses res.json to do this?

const users = [
  { id: 1, active: true },
  { id: 2, active: false },
  { id: 3, active: true }
];

app.get('/active-users', (req, res) => {
  // Fill in here
});

hard

A. res.json(users.filter(user => user.active));

B. res.json(users.map(user => user.active));

C. res.json(users);

D. res.json(users.filter(user => user.id));

Solution

1. Step 1: Filter users by active status

   Use users.filter(user => user.active) to get only users where active is true.

2. Step 2: Send filtered list as JSON response

   Pass the filtered array to res.json() to send it properly formatted.

3. Final Answer:

   res.json(users.filter(user => user.active)); -> Option A

4. Quick Check:

   Filter active users, then send JSON [OK]

Hint: Filter first, then send with res.json() [OK]

Common Mistakes:

• Using map instead of filter, sending booleans only
• Sending full users list without filtering
• Filtering by id instead of active status