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.