res.send for general responses in Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What does this Express code send to the client?

Consider this Express route handler:

app.get('/hello', (req, res) => { res.send('Hello World!'); });

What will the client receive when requesting /hello?

Express

app.get('/hello', (req, res) => { res.send('Hello World!'); });

A A redirect to '/hello/world'

B The string 'Hello World!' as the response body with status 200

C An empty response with status 204

D A JSON object { message: 'Hello World!' }

💡 Hint

res.send sends the exact data you give it as the response body.

The res.send method sends the string 'Hello World!' as the response body with HTTP status 200 by default.

📝 Syntax

intermediate

Which option correctly sends a JSON response using res.send?

You want to send this JSON object as a response:

{ success: true, data: [1, 2, 3] }

Which code correctly sends it using res.send?

A res.send('success: true, data: [1, 2, 3]');

B res.send(JSON.stringify({ success: true, data: [1, 2, 3] }));

C res.send({ success: true, data: [1, 2, 3] });

D res.send({ 'success': 'true', 'data': '[1, 2, 3]' });

💡 Hint

res.send can accept an object and automatically sends JSON.

Passing an object to res.send automatically converts it to JSON and sets the Content-Type header.

🔧 Debug

advanced

Why does this code cause an error?

Look at this Express handler:

app.get('/test', (req, res) => { res.send(123); res.send('Done'); });

What error or behavior will happen when a client requests /test?

Express

app.get('/test', (req, res) => { res.send(123); res.send('Done'); });

A Error: Cannot set headers after they are sent to the client

B The client receives '123Done' as a combined response

C The client receives only 'Done' as the response

D The server crashes with a syntax error

💡 Hint

You can only send one response per request.

Calling res.send twice causes an error because headers and body can only be sent once per request.

❓ state_output

advanced

What is the Content-Type header when sending a number with res.send?

Given this code:

app.get('/number', (req, res) => { res.send(42); });

What Content-Type header will the client receive?

Express

app.get('/number', (req, res) => { res.send(42); });

A No Content-Type header is sent

B application/json; charset=utf-8

C text/plain; charset=utf-8

D text/html; charset=utf-8

💡 Hint

Numbers are converted to strings when sent.

Sending a number with res.send converts it to a string and sets Content-Type to text/html; charset=utf-8.

🧠 Conceptual

expert

Which statement about res.send behavior is true?

Choose the correct statement about how res.send works in Express:

A res.send ends the response process and cannot be called multiple times per request

B res.send buffers the entire response before sending it to the client

C res.send automatically sets Content-Type to application/json when sending objects

D res.send can only send strings and buffers, not objects or numbers

💡 Hint

Think about what happens after res.send is called.

res.send ends the response and calling it again causes an error; it can send strings, buffers, numbers, and objects (converted to JSON).

Practice

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

easy

A. Logs information to the console

B. Sends a response back to the client and ends the request

C. Reads data from the client request

D. Starts a new server

Solution

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

   res.send() is used to send a response back to the client in Express.

2. Step 2: Recognize that it ends the response

   After sending data, res.send() automatically ends the response, so no more data is sent.

3. Final Answer:

   Sends a response back to the client and ends the request -> Option B

4. Quick Check:

   res.send() = sends response [OK]

Hint: Remember: res.send sends and finishes response [OK]

Common Mistakes:

• Thinking res.send starts the server
• Confusing res.send with reading request data
• Assuming res.send logs to console

2. Which of the following is the correct syntax to send a plain text response using res.send?

easy

A. res.send('Hello');

B. res.send[Hello];

C. res.send({ message: 'Hello' });

D. res.send = 'Hello';

Solution

1. Step 1: Identify correct method call syntax

   The correct way to call res.send is as a function with parentheses and the data inside.

2. Step 2: Check the options for valid syntax

   res.send('Hello'); uses res.send('Hello'); which is correct. res.send({ message: 'Hello' }); sends an object, which is valid but not plain text. Options B and D have invalid syntax.

3. Final Answer:

   res.send('Hello'); -> Option A

4. Quick Check:

   Correct function call = res.send('text') [OK]

Hint: Use parentheses and quotes: res.send('text') [OK]

Common Mistakes:

• Using square brackets instead of parentheses
• Assigning value instead of calling function
• Sending object when plain text is needed

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

app.get('/test', (req, res) => {
  res.send({ success: true });
});

medium

A. A plain text: '[object Object]'

B. Nothing, response is empty

C. An error because object can't be sent

D. A JSON string: '{"success":true}'

Solution

1. Step 1: Understand res.send with an object

   When res.send() receives an object, Express converts it to JSON automatically.

2. Step 2: Predict the client response

   The client will get a JSON string representing the object: '{"success":true}'.

3. Final Answer:

   A JSON string: '{"success":true}' -> Option D

4. Quick Check:

   res.send(object) = JSON string [OK]

Hint: Objects sent with res.send become JSON strings [OK]

Common Mistakes:

• Expecting plain text '[object Object]'
• Thinking it causes an error
• Assuming empty response

4. Identify the error in this Express route:

app.get('/hello', (req, res) => {
  res.send('Hello');
  res.send('Again');
});

medium

A. Calling res.send twice causes an error

B. Missing semicolon after res.send

C. res.send cannot send strings

D. Route path is invalid

Solution

1. Step 1: Check multiple res.send calls in one route

   Once res.send() is called, the response is finished and cannot be sent again.

2. Step 2: Understand the error caused

   Calling res.send() a second time causes an error because the response is already closed.

3. Final Answer:

   Calling res.send twice causes an error -> Option A

4. Quick Check:

   Only one res.send per response [OK]

Hint: Send response once; multiple res.send calls cause errors [OK]

Common Mistakes:

• Ignoring that response ends after first send
• Thinking multiple sends append data
• Blaming syntax or route path

5. You want to send a JSON response with a status code 201 and a message 'Created' using res.send. Which code correctly does this?

hard

A. res.send({ status: 201, message: 'Created' });

B. res.send(201, { message: 'Created' });

C. res.status(201).send({ message: 'Created' });

D. res.status(201).send('Created');

Solution

1. Step 1: Set status code before sending response

   Use res.status(201) to set the HTTP status code to 201.

2. Step 2: Send JSON object with message

   Chain .send({ message: 'Created' }) to send the JSON response.

3. Final Answer:

   res.status(201).send({ message: 'Created' }); -> Option C

4. Quick Check:

   Status then send JSON = res.status(201).send({ message: 'Created' }); [OK]

Hint: Chain res.status(code).send(data) to set status and send [OK]

Common Mistakes:

• Passing status code as first argument to send
• Sending status inside JSON instead of HTTP status
• Sending plain text when JSON is needed