CORS configuration in Node.js - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

What does CORS stand for and why is it important?

Choose the correct explanation of CORS and its purpose in web development.

A Cross-Origin Resource Sharing; it allows web pages to request resources from different domains securely.

B Cross-Origin Request Security; it blocks all requests from other domains by default.

C Cross-Origin Resource Sharing; it disables all security checks for requests between domains.

D Cross-Origin Request Sharing; it encrypts data sent between different domains.

💡 Hint

Think about how browsers handle requests from one website to another domain.

CORS stands for Cross-Origin Resource Sharing. It is a security feature that allows controlled access to resources on a web server from a different domain than the one serving the web page. This prevents malicious sites from reading sensitive data from another domain.

❓ component_behavior

intermediate

What is the output when this Node.js Express server handles a cross-origin request?

Given the following Express server code with CORS configured, what will the browser receive when a request comes from http://example.com?

Node.js

import express from 'express';
import cors from 'cors';

const app = express();

app.use(cors({ origin: 'http://example.com' }));

app.get('/', (req, res) => {
  res.json({ message: 'Hello World' });
});

app.listen(3000);

A The browser receives the JSON response but without any CORS headers, so access is blocked.

B The server throws an error because the origin is hardcoded.

C The browser receives a 404 Not Found error.

D The browser receives the JSON response with CORS headers allowing access from http://example.com.

💡 Hint

Check how the cors middleware is configured and what it does.

The cors middleware adds the appropriate headers to allow requests from http://example.com. The server responds with the JSON and the browser accepts it because the origin matches.

📝 Syntax

advanced

Which option correctly configures CORS to allow multiple specific origins in Express?

Choose the code snippet that correctly allows CORS requests from http://site1.com and http://site2.com only.

A app.use(cors({ origin: '*http://site1.com*|*http://site2.com*' }));

B app.use(cors({ origin: ['http://site1.com', 'http://site2.com'] }));

C app.use(cors({ origin: function(origin, callback) { if(['http://site1.com', 'http://site2.com'].includes(origin)) callback(null, true); else callback(new Error('Not allowed')); } }));

D app.use(cors({ origin: 'http://site1.com,http://site2.com' }));

💡 Hint

The cors package supports arrays for the origin option.

Option B is correct because the cors middleware supports an array of strings directly for the origin option to allow multiple specific origins only. Option B treats the comma-separated value as a single origin. Option B uses Error which propagates an error to the next middleware instead of cleanly denying. Option B uses invalid format.

🔧 Debug

advanced

Why does this CORS configuration cause a runtime error?

Examine the code below. Why does the server crash when starting?

Node.js

import express from 'express';
import cors from 'cors';

const app = express();

app.use(cors({ origin: true }));

app.listen(3000);

A The server crashes because the port 3000 is already in use.

B The server crashes because the cors package is not imported correctly.

C The origin option set to true is valid and does not cause a crash.

D The value true for origin is invalid and causes a TypeError at runtime.

💡 Hint

Check the documentation for the cors package about the origin option.

Setting origin to true in the cors middleware allows requests from any origin. This is valid and does not cause a runtime error. The server crash must be caused by something else, but none of the other options match the code given.

❓ state_output

expert

What is the value of the Access-Control-Allow-Origin header for this request?

Given this Express server code and a request from http://evil.com, what will be the value of the Access-Control-Allow-Origin header in the response?

Node.js

import express from 'express';
import cors from 'cors';

const allowedOrigins = ['http://good.com', 'http://friend.com'];

const app = express();

app.use(cors({
  origin: (origin, callback) => {
    if (!origin) return callback(null, true);
    if (allowedOrigins.includes(origin)) {
      callback(null, origin);
    } else {
      callback(null, false);
    }
  }
}));

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

app.listen(3000);

A undefined (no Access-Control-Allow-Origin header)

B "*"

C "http://evil.com"

D "http://good.com"

💡 Hint

Consider what happens when the origin is not in the allowed list and how the callback is called.

If the origin is not in the allowedOrigins list, the callback is called with false, which means no CORS headers are set. Therefore, the Access-Control-Allow-Origin header is not included in the response.