Request and response schemas in Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this Express route when sending a valid JSON body?

Consider this Express route that expects a JSON body with a name property and responds with a greeting.

app.post('/greet', (req, res) => {
  const { name } = req.body;
  if (!name) {
    return res.status(400).json({ error: 'Name is required' });
  }
  res.json({ message: `Hello, ${name}!` });
});

What will the response be if the client sends { "name": "Alice" } as JSON?

Express

app.post('/greet', (req, res) => {
  const { name } = req.body;
  if (!name) {
    return res.status(400).json({ error: 'Name is required' });
  }
  res.json({ message: `Hello, ${name}!` });
});

A {"error":"Name is required"}

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

C Status 500 Internal Server Error

D Empty response with status 200

💡 Hint

Think about what the route does when name is present in the request body.

The route checks if name exists in req.body. If it does, it sends a JSON response with a greeting message including the name. Since the client sends { "name": "Alice" }, the response will be {"message":"Hello, Alice!"}.

📝 Syntax

intermediate

Which option correctly defines a JSON schema middleware for validating a request body in Express?

You want to validate that the request body contains a username (string) and age (number) before processing it. Which middleware definition is correct?

A

const schema = {
  username: 'string',
  age: 'number',
  required: ['username', 'age']
};

B

const schema = {
  type: 'array',
  items: {
    username: 'string',
    age: 'number'
  }
};

C

const schema = {
  type: 'object',
  properties: {
    username: { type: 'string' },
    age: { type: 'number' }
  },
  required: ['username', 'age'],
  additionalProperties: false
};

D

const schema = {
  type: 'object',
  properties: {
    username: 'string',
    age: 'number'
  },
  required: ['username', 'age']
};

💡 Hint

JSON schema requires type and properties keys with proper types.

Option C correctly defines a JSON schema object with type as 'object', properties specifying types, required fields, and disallows extra properties. Other options misuse types or structure.

🔧 Debug

advanced

What error does this Express route produce when the request body is missing the required field?

Given this route using a JSON schema validator middleware:

app.post('/user', validateSchema(schema), (req, res) => {
  res.json({ success: true });
});

If the client sends {} (empty body) and schema requires a username, what error will occur?

Express

const schema = {
  type: 'object',
  properties: {
    username: { type: 'string' }
  },
  required: ['username'],
  additionalProperties: false
};

A SyntaxError thrown by Express

B 500 Internal Server Error due to missing property

C 200 OK with empty JSON response

D 400 Bad Request with validation error message

💡 Hint

Think about how validation middleware handles missing required fields.

The validation middleware detects the missing username and responds with a 400 status and an error message. It does not cause a server error or syntax error.

❓ state_output

advanced

What is the response body after this Express route processes a request with extra fields?

Consider this route that validates request body against a schema disallowing extra fields:

const schema = {
  type: 'object',
  properties: {
    email: { type: 'string' }
  },
  required: ['email'],
  additionalProperties: false
};

app.post('/subscribe', validateSchema(schema), (req, res) => {
  res.json({ subscribed: true });
});

If the client sends { "email": "[e-mail]", "age": 30 }, what will the response be?

A 400 Bad Request with error about additional properties

B 200 OK with {"subscribed": true}

C 500 Internal Server Error

D 200 OK with empty JSON {}

💡 Hint

Check what additionalProperties: false means in JSON schema.

The schema forbids any properties not listed in properties. Since age is extra, the validation fails and the middleware returns a 400 error.

🧠 Conceptual

expert

Which statement best describes the role of request and response schemas in Express applications?

Choose the most accurate description of why request and response schemas are used in Express apps.

A They ensure data sent and received matches expected formats, improving reliability and security.

B They allow Express to run faster by skipping data parsing.

C They replace the need for any manual error handling in routes.

D They automatically generate HTML forms for user input based on schema definitions.

💡 Hint

Think about the purpose of validating data in web applications.

Request and response schemas help verify that data matches expected formats, which prevents errors and security issues. They do not generate forms, replace all error handling, or improve speed by skipping parsing.