res.json for JSON responses in Express - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does res.json() do in Express?

res.json() sends a JSON response to the client. It converts a JavaScript object or array into JSON format and sets the correct headers automatically.

beginner

How do you send a JSON object { message: 'Hello' } using res.json()?

Use res.json({ message: 'Hello' }). This sends the JSON { "message": "Hello" } to the client.

beginner

What HTTP header does res.json() set automatically?

It sets Content-Type: application/json so the client knows the response is JSON.

beginner

Can res.json() send arrays as well as objects?

Yes! You can send arrays like res.json([1, 2, 3]) and Express will convert it to JSON format.

intermediate

What happens if you pass a non-serializable value to res.json()?

Express will throw an error because it cannot convert functions or undefined values to JSON.

What does res.json() do in Express?

A Reads JSON from the client

B Redirects to another URL

C Sends a JSON response to the client

D Sends plain text response

res.json() sends JSON data as a response to the client.

Which header does res.json() set automatically?

A Content-Type: text/html

B Content-Type: text/plain

C Content-Type: multipart/form-data

D Content-Type: application/json

It sets Content-Type: application/json so the client knows the response is JSON.

How would you send the JSON object { success: true } in Express?

A res.json({ success: true })

B res.send({ success: true })

C res.redirect({ success: true })

D res.render({ success: true })

res.json() is the correct method to send JSON responses.

Can res.json() send an array as a response?

A No, only objects

B Yes, arrays and objects

C Only strings

D Only numbers

res.json() can send arrays or objects as JSON.

What happens if you pass a function to res.json()?

A It throws an error

B It converts the function to JSON

C It ignores the function

D It sends the function as text

Functions cannot be converted to JSON, so Express throws an error.

Explain how res.json() works in Express and why it is useful.

Think about how servers send data to browsers in a format they understand.

Describe a situation where you would use res.json() in a web app.

Imagine building a simple app that shows user info fetched from the server.

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