CORS configuration in NestJS - Mini Project: Build & Apply

CORS configuration

📖 Scenario: You are building a simple NestJS backend server that will be accessed by a frontend running on a different domain. To allow the frontend to communicate with your backend, you need to configure CORS (Cross-Origin Resource Sharing) properly.

🎯 Goal: Configure CORS in a NestJS application to allow requests only from http://example.com and enable credentials support.

📋 What You'll Learn

Create a basic NestJS application instance

Add a configuration variable for allowed origin

Enable CORS with the allowed origin and credentials option

Complete the main bootstrap function with the CORS setup

💡 Why This Matters

🌍 Real World

Many web applications have frontend and backend on different domains. Configuring CORS correctly allows secure communication between them.

💼 Career

Backend developers often need to configure CORS in NestJS or other frameworks to enable frontend-backend integration without security issues.

1

Create a NestJS application instance

Write code to create a NestJS application instance called app using NestFactory.create(AppModule).

NestJS

// Create a NestJS application instance called app
// Your code here

Hint

Use const app = await NestFactory.create(AppModule); inside an async bootstrap function.

2

Add a configuration variable for allowed origin

Add a constant variable called allowedOrigin and set it to the string 'http://example.com'.

NestJS

import { NestFactory } from '@nestjs/core';
import { AppModule } from './app.module';

async function bootstrap() {
  const app = await NestFactory.create(AppModule);
  // Add allowedOrigin variable here
  // Your code here
}

bootstrap();

Hint

Define const allowedOrigin = 'http://example.com'; inside the bootstrap function.

3

Enable CORS with allowed origin and credentials

Use app.enableCors() with an options object that sets origin to allowedOrigin and credentials to true.

NestJS

import { NestFactory } from '@nestjs/core';
import { AppModule } from './app.module';

async function bootstrap() {
  const app = await NestFactory.create(AppModule);
  const allowedOrigin = 'http://example.com';
  // Enable CORS with origin and credentials
  // Your code here
}

bootstrap();

Hint

Call app.enableCors() with an object: { origin: allowedOrigin, credentials: true }.

4

Complete the bootstrap function by starting the server

Add await app.listen(3000); inside the bootstrap function to start the server on port 3000.

NestJS

import { NestFactory } from '@nestjs/core';
import { AppModule } from './app.module';

async function bootstrap() {
  const app = await NestFactory.create(AppModule);
  const allowedOrigin = 'http://example.com';
  app.enableCors({ origin: allowedOrigin, credentials: true });
  // Start the server on port 3000
  // Your code here
}

bootstrap();

Hint

Use await app.listen(3000); to start the server.

Practice

1. What is the main purpose of enabling CORS in a NestJS application?

easy

A. To encrypt data between client and server

B. To speed up the server response time

C. To manage database connections

D. To allow web browsers from specific origins to access the backend resources

Solution

1. Step 1: Understand CORS purpose

   CORS (Cross-Origin Resource Sharing) controls which websites can access backend resources from a different origin.

2. Step 2: Identify correct purpose in NestJS

   Enabling CORS in NestJS allows specific web origins to access the backend safely.

3. Final Answer:

   To allow web browsers from specific origins to access the backend resources -> Option D

4. Quick Check:

   CORS controls access by origin = C [OK]

Hint: CORS controls which websites can access your backend [OK]

Common Mistakes:

• Confusing CORS with encryption
• Thinking CORS speeds up server
• Mixing CORS with database management

2. Which of the following is the correct way to enable CORS with default settings in a NestJS app?

easy

A. app.setCors(true)

B. app.enableCors({ origin: '*' })

C. app.enableCors()

D. app.useCors()

Solution

1. Step 1: Recall NestJS CORS enabling method

   The official method to enable CORS is app.enableCors() without parameters for default settings.

2. Step 2: Check options for correctness

   Options B, C, and D are not valid NestJS methods or have incorrect syntax.

3. Final Answer:

   app.enableCors() -> Option C

4. Quick Check:

   Default CORS enable = app.enableCors() [OK]

Hint: Use app.enableCors() to enable CORS simply [OK]

Common Mistakes:

• Using incorrect method names like setCors or useCors
• Passing invalid parameters to enableCors
• Confusing CORS enabling with middleware usage

3. Given this NestJS code snippet, what will be the allowed origin for CORS?

app.enableCors({ origin: 'https://example.com' });

medium

A. Only https://example.com can access the backend

B. All websites can access the backend

C. No websites can access the backend

D. Only localhost can access the backend

Solution

1. Step 1: Analyze the origin option in enableCors

   The origin option is set to 'https://example.com', which restricts access to this exact origin.

2. Step 2: Understand CORS behavior with origin setting

   Only requests coming from 'https://example.com' will be allowed by CORS policy.

3. Final Answer:

   Only https://example.com can access the backend -> Option A

4. Quick Check:

   origin restricts access = B [OK]

Hint: Origin option limits access to specified URL only [OK]

Common Mistakes:

• Assuming origin '*' is default
• Thinking multiple origins allowed without array
• Confusing origin with methods or headers

4. Identify the error in this NestJS CORS configuration code:

app.enableCors({ origins: ['https://site1.com', 'https://site2.com'] });

medium

A. The array syntax is invalid for origins

B. The property 'origins' is incorrect; it should be 'origin'

C. enableCors does not accept any options

D. The URLs must be strings without 'https://'

Solution

1. Step 1: Check property name for allowed origins

   The correct property name is 'origin' (singular), not 'origins'.

2. Step 2: Confirm array usage for multiple origins

   Passing an array to 'origin' is valid to allow multiple origins.

3. Final Answer:

   The property 'origins' is incorrect; it should be 'origin' -> Option B

4. Quick Check:

   Correct property name is origin = A [OK]

Hint: Use 'origin' not 'origins' for CORS config [OK]

Common Mistakes:

• Using 'origins' instead of 'origin'
• Thinking enableCors rejects options
• Removing protocol from URLs

5. You want to allow CORS only for GET and POST methods from two specific domains: https://app1.com and https://app2.com. Which is the correct NestJS CORS configuration?

hard

A. app.enableCors({ origin: ['https://app1.com', 'https://app2.com'], methods: ['GET', 'POST'] });

B. app.enableCors({ origins: ['https://app1.com', 'https://app2.com'], methods: 'GET,POST' });

C. app.enableCors({ origin: '*', methods: ['GET', 'POST'] });

D. app.enableCors({ origin: ['https://app1.com', 'https://app2.com'], method: ['GET', 'POST'] });

Solution

1. Step 1: Verify property names and values

   The property for allowed origins is 'origin' (singular), and it accepts an array for multiple domains. The 'methods' property accepts an array of HTTP methods.

2. Step 2: Check each option for correctness

   app.enableCors({ origin: ['https://app1.com', 'https://app2.com'], methods: ['GET', 'POST'] }); uses correct property names and value types. app.enableCors({ origins: ['https://app1.com', 'https://app2.com'], methods: 'GET,POST' }); uses 'origins' (incorrect) and methods as a string (less preferred). app.enableCors({ origin: '*', methods: ['GET', 'POST'] }); allows all origins ('*'), which is not desired. app.enableCors({ origin: ['https://app1.com', 'https://app2.com'], method: ['GET', 'POST'] }); uses 'method' (singular), which is incorrect.

3. Final Answer:

   app.enableCors({ origin: ['https://app1.com', 'https://app2.com'], methods: ['GET', 'POST'] }); -> Option A

4. Quick Check:

   Correct keys and array values = D [OK]

Hint: Use 'origin' and 'methods' with arrays for multiple values [OK]

Common Mistakes:

• Using 'origins' instead of 'origin'
• Passing methods as comma string instead of array
• Using 'method' instead of 'methods'
• Allowing all origins with '*' when not intended