CORS configuration in Node.js

Introduction

CORS lets your server control which websites can talk to it. It helps keep your app safe by blocking unwanted access.

When your frontend and backend are on different websites or ports.

When you want to allow only certain websites to use your API.

When you want to prevent other sites from reading your server data.

When building a public API that needs to control who can access it.

When debugging frontend errors related to blocked requests.

Syntax

Node.js

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

const app = express();

app.use(cors({
  origin: 'https://example.com',
  methods: ['GET', 'POST'],
  allowedHeaders: ['Content-Type', 'Authorization']
}));

The origin option sets which website can access your server.

You can list allowed HTTP methods and headers to be more specific.

Examples

Allow all websites to access your server (not secure for private APIs).

Node.js

app.use(cors());

Only allow requests from https://myfrontend.com.

Node.js

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

Allow multiple specific websites to access your server.

Node.js

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

Allow only GET and POST requests with Content-Type header from example.com.

Node.js

app.use(cors({
  origin: 'https://example.com',
  methods: ['GET', 'POST'],
  allowedHeaders: ['Content-Type']
}));

Sample Program

This server only accepts requests from https://myfrontend.com. If you try from another site, the browser will block the request.

Node.js

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

const app = express();

// Allow only https://myfrontend.com to access
app.use(cors({ origin: 'https://myfrontend.com' }));

app.get('/data', (req, res) => {
  res.json({ message: 'Hello from server!' });
});

app.listen(3000, () => {
  console.log('Server running on http://localhost:3000');
});

Important Notes

Browsers enforce CORS, so server settings alone don't block direct calls (like curl).

Always test CORS with your frontend to avoid blocked requests.

You can customize CORS more with options like credentials, exposedHeaders, and preflightContinue.

Summary

CORS controls which websites can use your server resources.

Use the cors package in Node.js to set rules easily.

Be careful to allow only trusted sites to keep your app safe.

Practice

1. What is the main purpose of configuring CORS in a Node.js server?

easy

A. To speed up server response time

B. To control which websites can access server resources

C. To encrypt data sent between client and server

D. To manage database connections

Solution

1. Step 1: Understand CORS role

   CORS (Cross-Origin Resource Sharing) controls which external websites can request resources from your server.

2. Step 2: Identify main purpose

   It is not about speed, encryption, or database management but about access control from different origins.

3. Final Answer:

   To control which websites can access server resources -> Option B

4. Quick Check:

   CORS controls access = A [OK]

Hint: Remember CORS = Cross-Origin Resource Sharing control [OK]

Common Mistakes:

• Confusing CORS with encryption or speed optimization
• Thinking CORS manages databases
• Assuming CORS is a security firewall

2. Which of the following is the correct way to import and use the cors middleware in an Express app?

easy

A. import cors from 'cors'; app.use(cors());

B. import cors from 'cors'; app.use(cors);

C. const cors = require('cors'); app.use(cors);

D. const cors = require('cors'); app.use(cors());

Solution

1. Step 1: Identify modern import syntax

   Using ES modules, the correct import is import cors from 'cors';.

2. Step 2: Use middleware correctly

   Middleware must be called as a function: app.use(cors()); not just cors.

3. Final Answer:

   import cors from 'cors'; app.use(cors()); -> Option A

4. Quick Check:

   Import + call middleware = D [OK]

Hint: Use import + call cors() as function [OK]

Common Mistakes:

• Forgetting parentheses when using middleware
• Mixing require and import syntax
• Using app.use(cors) without parentheses

3. Given this code snippet, what will be the CORS behavior?

import express from 'express';
import cors from 'cors';
const app = express();
app.use(cors({ origin: 'https://example.com' }));
app.get('/', (req, res) => res.send('Hello'));
app.listen(3000);

medium

A. Only requests from https://example.com will be allowed

B. Requests from any origin will be allowed

C. No requests will be allowed due to missing headers

D. Requests from localhost will be allowed only

Solution

1. Step 1: Analyze the origin option

   The cors middleware is configured with origin: 'https://example.com', which restricts access to that origin only.

2. Step 2: Understand effect on requests

   Requests from other origins will be blocked by CORS policy, so only https://example.com is allowed.

3. Final Answer:

   Only requests from https://example.com will be allowed -> Option A

4. Quick Check:

   origin restricts access = C [OK]

Hint: Check origin value to know allowed sites [OK]

Common Mistakes:

• Assuming all origins allowed by default
• Confusing origin with methods or headers
• Thinking localhost is allowed without explicit mention

4. Regarding this CORS setup code, which statement is accurate?

import express from 'express';
import cors from 'cors';
const app = express();
app.use(cors({ origin: ['https://site1.com', 'https://site2.com'] }));
app.listen(3000);

medium

A. Missing call to cors() function

B. The app.listen() call is missing a callback

C. The origin option can be an array

D. The cors package is not imported correctly

Solution

1. Step 1: Check origin option type

   The origin option in cors accepts an array directly for multiple static origins; it also supports string, boolean, regex, or function.

2. Step 2: Confirm other parts are correct

   The cors() function is called correctly, app.listen() works without callback, and import syntax is valid.

3. Final Answer:

   The origin option can be an array -> Option C

4. Quick Check:

   origin array valid = B [OK]

Hint: origin can be string, array, regex, or function [OK]

Common Mistakes:

• Thinking origin cannot accept an array
• Forgetting to call cors() as a function
• Thinking app.listen needs a callback always

5. You want to allow CORS requests only from https://trusted.com and http://localhost:3000. Which is the best way to configure the cors middleware in your Node.js app?

hard

A. Use origin as a function to check allowed origins dynamically

B. Set origin to '*' to allow all origins

C. Disable CORS middleware and handle manually

D. Set origin to an array of allowed origins directly

Solution

1. Step 1: Understand multiple origins handling

   The cors package accepts an array directly for origin with a static list of allowed origins. This is the simplest and best approach here.

2. Step 2: Evaluate options

   Use function for dynamic logic (e.g., regex or env-based). '*' allows all (insecure). Manual handling is error-prone and not recommended.

3. Final Answer:

   Set origin to an array of allowed origins directly -> Option D

4. Quick Check:

   Static multiple origins use array = C [OK]

Hint: Use array for static multiple origins [OK]

Common Mistakes:

• Using '*' to allow all origins insecurely
• Using a single string for multiple origins
• Disabling middleware instead of configuring properly