CORS configuration in NestJS

Introduction

CORS helps your NestJS app control who can access its resources from other websites. It keeps your app safe by blocking unwanted requests.

When your frontend app runs on a different domain than your NestJS backend.

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

When you want to enable your API to be accessed by mobile apps or other services.

When you want to prevent unauthorized websites from calling your backend.

Syntax

NestJS

const app = await NestFactory.create(AppModule);
app.enableCors({
  origin: 'http://example.com',
  methods: ['GET', 'HEAD', 'PUT', 'PATCH', 'POST', 'DELETE'],
  credentials: true,
});
await app.listen(3000);

The origin option controls which website URLs can access your backend.

Use credentials: true if you want to allow cookies or authorization headers.

Examples

Enable CORS with default settings (allows all origins).

NestJS

app.enableCors();

Allow only requests from https://myfrontend.com.

NestJS

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

Allow multiple specific websites to access your API.

NestJS

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

Allow all origins but only GET and POST methods with specific headers.

NestJS

app.enableCors({
  origin: '*',
  methods: ['GET', 'POST'],
  allowedHeaders: ['Content-Type', 'Authorization']
});

Sample Program

This NestJS app allows CORS requests only from http://localhost:4200 with GET and POST methods. It also allows credentials like cookies.

NestJS

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

async function bootstrap() {
  const app = await NestFactory.create(AppModule);
  app.enableCors({
    origin: 'http://localhost:4200',
    methods: ['GET', 'POST'],
    credentials: true,
  });
  await app.listen(3000);
  console.log('Server running on http://localhost:3000');
}

bootstrap();

Important Notes

Be careful with origin: '*' when using credentials; browsers block this combination.

You can also configure CORS globally in the main.ts file or per route using decorators.

Use browser DevTools Network tab to check CORS headers and errors when testing.

Summary

CORS controls which websites can access your NestJS backend.

Use app.enableCors() to set CORS options simply.

Always specify allowed origins and methods to keep your app secure.

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