Configuring allowed origins in Express - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the CORS middleware.

Express

const cors = require('[1]');

Drag options to blanks, or click blank then click option'

Acors

Bexpress

Chttp

Dpath

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'express' instead of 'cors' for CORS middleware.

Forgetting to install the 'cors' package before requiring it.

The cors package is used to configure allowed origins in Express apps.

2fill in blank

medium

Complete the code to allow all origins using CORS middleware.

Express

app.use(cors({ origin: [1] }));

Drag options to blanks, or click blank then click option'

A'*'

Btrue

Cnull

Dfalse

Attempts:

3 left

💡 Hint

Common Mistakes

Using boolean true instead of '*' to allow all origins.

Setting origin to false disables CORS.

Setting origin: '*' allows requests from any origin.

3fill in blank

hard

Fix the error in the code to allow only 'http://example.com' origin.

Express

app.use(cors({ origin: [1] }));

Drag options to blanks, or click blank then click option'

Ahttp://example.com

B'http://example.com'

C['http://example.com']

Dtrue

Attempts:

3 left

💡 Hint

Common Mistakes

Using an array instead of a string for a single origin.

Not quoting the URL string.

The origin value must be a string for a single allowed origin.

4fill in blank

hard

Fill both blanks to allow only 'http://site1.com' and 'http://site2.com' origins.

Express

const allowedOrigins = [[1], [2]];
app.use(cors({ origin: function(origin, callback) {
  if (!origin || allowedOrigins.indexOf(origin) !== -1) {
    callback(null, true);
  } else {
    callback(new Error('Not allowed by CORS'));
  }
}}));

Drag options to blanks, or click blank then click option'

A'http://site1.com'

B'http://site2.com'

C'http://example.com'

D'http://localhost:3000'

Attempts:

3 left

💡 Hint

Common Mistakes

Using unquoted URLs in the array.

Including origins not specified in the allowed list.

We list allowed origins as strings in an array, then check if the request origin is in that list.

5fill in blank

hard

Fill all three blanks to configure CORS with multiple allowed origins and enable credentials.

Express

const allowed = [[1], [2]];
app.use(cors({
  origin: function(origin, callback) {
    if (!origin || allowed.indexOf(origin) !== -1) {
      callback(null, true);
    } else {
      callback(new Error('CORS not allowed'));
    }
  },
  [3]: true
}));

Drag options to blanks, or click blank then click option'

A'http://myapp.com'

B'http://yourapp.com'

Ccredentials

Dmethods

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'methods' instead of 'credentials' to enable credentials.

Not quoting the URLs in the array.

We list allowed origins as strings, then set credentials: true to allow cookies and auth headers.

Practice

1. What is the main purpose of configuring allowed origins in an Express app using cors middleware?

easy

A. To encrypt data sent between client and server

B. To speed up the server response time

C. To control which websites can access your server resources

D. To log all incoming requests

Solution

1. Step 1: Understand what allowed origins mean

   Allowed origins specify which websites are permitted to make requests to your server.

2. Step 2: Identify the role of cors middleware

   The cors middleware in Express helps set these allowed origins to control access.

3. Final Answer:

   To control which websites can access your server resources -> Option C

4. Quick Check:

   Allowed origins = control access [OK]

Hint: Allowed origins control access, not speed or encryption [OK]

Common Mistakes:

• Confusing allowed origins with encryption
• Thinking it speeds up server
• Assuming it logs requests

2. Which of the following is the correct way to allow only 'https://example.com' as an origin using the cors middleware in Express?

easy

A. app.use(cors({ origin: [https://example.com] }));

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

C. app.use(cors({ origins: 'https://example.com' }));

D. app.use(cors(https://example.com));

Solution

1. Step 1: Check the correct option name for allowed origins

   The correct option is origin, not origins.

2. Step 2: Verify the value type for origin

   It accepts a string for a single allowed origin, so 'https://example.com' is correct.

3. Final Answer:

   app.use(cors({ origin: 'https://example.com' })); -> Option B

4. Quick Check:

   Option name is origin, value is string [OK]

Hint: Use 'origin' option with string for single allowed site [OK]

Common Mistakes:

• Using 'origins' instead of 'origin'
• Passing array for single origin string
• Calling cors without options

3. Given this Express code snippet, what will be the result when a request comes from 'https://allowed.com'?

const cors = require('cors');
app.use(cors({ origin: ['https://allowed.com', 'https://other.com'] }));

medium

A. The request will be allowed because 'https://allowed.com' is in the list

B. The request will be blocked due to invalid origin format

C. The request will be allowed only if it uses POST method

D. The request will be blocked because origin must be a string

Solution

1. Step 1: Understand the origin option accepts an array

   The origin option can accept an array of allowed origins to permit multiple sites.

2. Step 2: Check if 'https://allowed.com' is in the array

   Since 'https://allowed.com' is listed, requests from it will be allowed.

3. Final Answer:

   The request will be allowed because 'https://allowed.com' is in the list -> Option A

4. Quick Check:

   Array of origins allows listed sites [OK]

Hint: Array of origins lets listed sites access [OK]

Common Mistakes:

• Thinking origin must be string only
• Assuming method affects origin check
• Believing array format causes error

4. Identify the error in this Express CORS setup that aims to allow only 'https://site.com':

app.use(cors({ origin: 'https://site.com', methods: ['GET', 'POST'] }));
app.use(cors());

medium

A. Calling cors() twice causes conflict and overrides settings

B. The methods option is invalid in cors middleware

C. The origin value should be an array, not a string

D. Missing next() call in middleware

Solution

1. Step 1: Check middleware usage order

   Calling cors() twice means the second call overrides the first, ignoring origin restrictions.

2. Step 2: Confirm methods option is valid

   The methods option is valid to restrict HTTP methods, so no error there.

3. Final Answer:

   Calling cors() twice causes conflict and overrides settings -> Option A

4. Quick Check:

   Multiple cors calls override previous config [OK]

Hint: Only call cors once with all options [OK]

Common Mistakes:

• Calling cors middleware multiple times
• Thinking origin must be array always
• Ignoring middleware order effects

5. You want to allow requests only from origins that end with '.trusted.com' dynamically in Express. Which cors configuration correctly implements this?

hard

A. app.use(cors({ origin: ['*.trusted.com'] }));

B. app.use(cors({ origin: (origin, callback) => { if (origin.includes('.trusted.com')) callback(null, true); else callback(new Error('Not allowed')); } }));

C. app.use(cors({ origin: '/^https:\/\/.*\.trusted\.com$/' }));

D. app.use(cors({ origin: (origin, callback) => { if (origin.endsWith('.trusted.com')) callback(null, true); else callback(new Error('Not allowed')); } }));

Solution

1. Step 1: Understand dynamic origin checking

   To allow origins ending with '.trusted.com', a function can check the origin string dynamically.

2. Step 2: Evaluate each option's approach

   app.use(cors({ origin: (origin, callback) => { if (origin.endsWith('.trusted.com')) callback(null, true); else callback(new Error('Not allowed')); } })); uses a function with endsWith to precisely match the domain ending, which is correct. app.use(cors({ origin: ['*.trusted.com'] })); uses wildcard string which is not supported. app.use(cors({ origin: '/^https:\/\/.*\.trusted\.com$/' })); uses regex but cors does not accept regex directly. app.use(cors({ origin: (origin, callback) => { if (origin.includes('.trusted.com')) callback(null, true); else callback(new Error('Not allowed')); } })); uses includes which may allow unwanted matches.

3. Final Answer:

   app.use(cors({ origin: (origin, callback) => { if (origin.endsWith('.trusted.com')) callback(null, true); else callback(new Error('Not allowed')); } })); -> Option D

4. Quick Check:

   Use function with endsWith for dynamic origin [OK]

Hint: Use function with endsWith() to allow domain patterns [OK]

Common Mistakes:

• Using wildcard strings in origin array
• Passing regex directly as origin
• Using includes() instead of endsWith()