What is CORS? Why is it Important? How to Use IT in a MEAN App?

CORS (Cross-Origin Resource Sharing) is a security feature that is essential in modern web development. It allows web pages to send requests to domains other than the one from which they originated. This feature is critical because it enables web applications to access resources from other domains, such as APIs or third-party services. In this article, we’ll go over what is CORS , why it’s important, how it works, and how to implement it in an Angular and Node.js application.

Contents

What is CORS?

CORS (Cross-Origin Resource Sharing) is a security mechanism used by web browsers to limit access to resources across domains. Browsers enforce the same-origin policy, which prevents web pages from making requests to domains other than the one that supplied the web page. This policy prevents harmful scripts from accessing sensitive information or conducting unauthorized actions.

However, there are legitimate scenarios where web applications need to communicate with resources hosted on different domains. An Angular front-end application, for example, may require data from a RESTful API developed with Express.js and Node.js, even if they are hosted on different servers or domains. CORS enables cross-domain communication by loosening the same-origin policy in a regulated and secure manner.

Why is CORS important?

CORS plays a critical role in modern web development for several reasons:

• Cross-Domain Data Access: Many web applications must consume data or communicate with APIs located on various domains. CORS enables the front-end application to securely access and consume these resources, allowing for seamless integration and a better user experience.
• Enhanced Security: CORS adds an extra layer of protection by requiring limited access to resources across domains. Server administrators can specify which domains are permitted to access their resources, lowering the risk of unauthorised access and safeguarding sensitive data.
• Integrations with Third-Party Services: Web applications frequently rely on third-party services such as payment gateways or social network APIs. CORS supports these integrations by allowing the web application to communicate with trustworthy external domains, hence enhancing the application’s functionality and capabilities.
• Single-Page Applications: APIs are frequently used in single-page applications (SPAs) created with frameworks such as Angular or React. CORS enables SPAs to securely conduct cross-domain API calls, improving the application’s interactivity and responsiveness.

How CORS Works?

CORS works by adding specific HTTP headers to requests and responses. These headers provide information that allows the browser and server to agree on whether the request should be permitted or denied. When a browser makes a cross-origin request, it first sends a preflight request to the server (an HTTP OPTIONS request) to see whether the actual request (e.g., GET, POST) is permitted. The server responds with the necessary CORS headers, indicating whether the request should be allowed or denied.

The CORS headers include:

• Access-Control-Allow-Origin: This property specifies which domains are permitted to access the resource. It can be a specific domain, a “*” to enable access from any domain, or a list of trusted domains.
• Access-Control-Allow-Methods: Specifies which HTTP methods (e.g., GET, POST, PUT) are permitted for the resource.
• Access-Control-Allow-Headers: Specifies which additional HTTP headers are permitted in the request.
• Access-Control-Allow-Credentials: Indicates whether the browser should include credentials in the request (e.g., cookies, authorization headers).
• Access-Control-Max-Age: The maximum amount of time in seconds that the preflight response can be cached.

By understanding and properly implementing CORS, web developers can create robust and secure applications that can seamlessly interact with resources hosted on different domains while maintaining strict control over access permissions.

How to Implement CORS in a MEAN Stack App

To implement CORS in a MEAN stack application, You can configure CORS in the Express.js backend and ensuring the relevant headers are included in the Angular frontend requests to implement CORS in a MEAN stack application.

Step 1: Install the CORS Middleware in Express.js (Back-End)

1. Open your terminal and navigate to the root directory of your Express.js project.

2. Run the following command to install the cors middleware package:

npm install cors

Step 2: Configure CORS in Express.js (Back-End)

1. Open your Express.js server file (e.g., server.js or app.js).

2. Import the cors middleware at the top of the file:

const cors = require('cors');

3. Use the cors middleware as a global middleware before defining your routes:

app.use(cors());

Step 3: Configure CORS in Angular (Front-End)

1. Open the proxy.conf.json file in the root directory of your Angular project. If the file doesn’t exist, create it.

2. Add the following configuration to the file:

{
  "/api/*": {
    "target": "http://localhost:3000",
    "secure": false,
    "logLevel": "debug"
  }
}

This configuration sets up a proxy that redirects requests starting with /api/ to the Express.js server running on http://localhost:3000. Adjust the target URL according to your server configuration.

3. Open your Angular package.json file.

4. Inside the "scripts" section, add the following script:

"start": "ng serve --proxy-config proxy.conf.json"

Step 4: Run the MEAN Stack App

1. In your terminal, navigate to the root directory of your Angular project.

2. Run the following command to start the Angular development server:

npm start

3. Open a new terminal window or tab.

4. Navigate to the root directory of your Express.js project.

5. Start the Express.js server by running the following command:

node server.js

CORS is now enabled in your MEAN stack application, allowing cross-origin communication between the Angular front-end and the Express.js back-end.

Conclusion

Enabling CORS in a MEAN stack application is critical for allowing safe communication between front-end and back-end components. You have successfully deployed CORS in your MEAN stack app by following this step-by-step method, ensuring regulated cross-origin resource access.

Remember to define the necessary CORS settings for production environments, including specifying authorised origins, methods, headers, and credentials based on the requirements and security concerns of your application.

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