How do you implement authentication in a MERN app?

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In a MERN (MongoDB, Express.js, React, Node.js) app, authentication verifies a user’s identity before granting access to resources. It’s commonly implemented using JWT (JSON Web Tokens) or sessions.

Steps to implement JWT-based authentication:

1. User Registration – React frontend sends user details (username, password) to an Express API. Passwords are hashed using bcrypt before saving in MongoDB.

2. Login – User submits credentials, backend verifies them, and generates a JWT signed with a secret key.

3. Token Storage – JWT is sent to the frontend, stored in HTTP-only cookies (more secure) or localStorage.

4. Protected Routes – Backend middleware checks for a valid token before processing requests.

5. Token Verification – On each request, token is decoded and verified to confirm the user’s identity.

6. Logout – Remove token from storage or invalidate it on the server.

Best practices:

• Use HTTPS to protect tokens in transit.

• Store tokens in HTTP-only cookies to prevent XSS attacks.

• Implement token expiration and refresh tokens for better security.

In short, authentication in MERN involves verifying credentials, issuing a secure token/session, and protecting routes with middleware.

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