JSON request and response patterns 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 that receives a JSON request and sends a JSON response. What will the client receive when sending {"name": "Alice"} in the request body?

Express

app.post('/greet', (req, res) => {
  const { name } = req.body;
  res.json({ message: `Hello, ${name}!` });
});

A {"message":"Hello, Alice!"}

B {"message":"Hello, undefined!"}

C SyntaxError: Unexpected token in JSON

D {"error":"Name not provided"}

💡 Hint

Think about how Express parses JSON request bodies and how the response is formed.

The route extracts the name from the JSON request body and sends back a JSON response with a greeting message including that name.

📝 Syntax

intermediate

Which option correctly parses JSON request body in Express?

You want to handle JSON data sent by clients in Express. Which code snippet correctly enables JSON parsing middleware?

A app.use(express.json());

B app.use(bodyParser.urlencoded());

C app.use(express.text());

D app.use(express.raw());

💡 Hint

Look for the middleware that parses JSON bodies.

express.json() middleware parses incoming JSON requests and makes the data available in req.body.

🔧 Debug

advanced

Why does this Express route always send 'undefined' in the response?

Examine the code below. The client sends {"age":30} in the JSON body, but the response is always {"age":undefined}. What is the cause?

Express

app.post('/age', (req, res) => {
  const age = req.body.age;
  res.json({ age: age });
});

// Middleware setup missing

A res.json() requires a string, not an object

B Incorrect property name; should be req.body.Age

C Missing express.json() middleware to parse JSON body

D Route method should be app.get instead of app.post

💡 Hint

Think about how Express reads the request body.

Without express.json() middleware, req.body is undefined, so req.body.age is undefined.

❓ state_output

advanced

What is the JSON response after multiple requests?

This Express server keeps a count of how many times the '/count' route is called. What JSON does the client receive on the third request?

Express

let count = 0;
app.get('/count', (req, res) => {
  count += 1;
  res.json({ count });
});

A {"count":1}

B {"count":3}

C {"count":0}

D {"count":2}

💡 Hint

The count variable increases with each request.

The variable count starts at 0 and increments by 1 on each request. On the third request, it is 3.

🧠 Conceptual

expert

Which header must be set by the client to send JSON data correctly to an Express server?

When sending JSON data in a POST request to an Express server, which HTTP header should the client include to ensure proper parsing?

A Authorization: Bearer token

B Accept: application/json

C Content-Length: 0

D Content-Type: application/json

💡 Hint

This header tells the server the format of the data being sent.

The Content-Type: application/json header tells the server that the request body contains JSON data.