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.