Configuring allowed origins in Express

Introduction

Configuring allowed origins helps control which websites can talk to your server. It keeps your app safe by blocking unwanted access.

When you want only your website to access your API.

When you have multiple websites and want to allow some but not all to connect.

When you want to prevent other sites from making requests to your server.

When you are building a public API and want to limit usage to trusted domains.

Syntax

Express

const cors = require('cors');

const corsOptions = {
  origin: 'https://example.com',
  optionsSuccessStatus: 200
};

app.use(cors(corsOptions));

The origin option sets which website is allowed.

You can use a string for one origin or a function/array for multiple origins.

Examples

Allow only https://mywebsite.com to access your server.

Express

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

Allow multiple specific websites by checking the origin dynamically.

Express

const allowedOrigins = ['https://site1.com', 'https://site2.com'];

app.use(cors({
  origin: function(origin, callback) {
    if (!origin || allowedOrigins.includes(origin)) {
      callback(null, true);
    } else {
      callback(new Error('Not allowed by CORS'));
    }
  }
}));

Allow all origins (not recommended for private APIs).

Express

app.use(cors());

Sample Program

This Express server allows requests only from https://trusted.com and https://partner.com. Others get blocked by CORS.

Express

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

const app = express();

const allowedOrigins = ['https://trusted.com', 'https://partner.com'];

const corsOptions = {
  origin: (origin, callback) => {
    if (!origin || allowedOrigins.includes(origin)) {
      callback(null, true);
    } else {
      callback(new Error('Not allowed by CORS'));
    }
  }
};

app.use(cors(corsOptions));

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

app.listen(3000, () => {
  console.log('Server running on port 3000');
});

Important Notes

Remember that CORS only affects browsers. Other clients like Postman are not blocked by CORS.

Always test your allowed origins carefully to avoid blocking your own app.

Summary

Configuring allowed origins controls which websites can access your server.

Use the cors middleware with the origin option to set allowed sites.

Test your settings to keep your app safe and working well.

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()