Resolving session is undefined Issue in Express with MongoDB and TypeScript

Learn how to resolve the `session is undefined` problem in your Express application with MongoDB and TypeScript. Discover best practices for session handling, middleware configuration, and frontend adjustments.
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How to Fix the session is undefined Issue in Express with MongoDB and TypeScript

When working with web applications that involve user authentication, managing user sessions is critical. An issue many developers face is an undefined session after a user logs in successfully. In this guide, we will explore a common scenario concerning session management using Express, MongoDB, and TypeScript, and we will provide a step-by-step guide on how to solve it effectively.

The Problem

In a typical Express application, you might encounter a situation where the session data becomes undefined after a user logs in. This issue can manifest in various ways:

You successfully save user session data during login.

Upon making a subsequent GET request, the session data is missing or not recognized.

Let's break down the situation described by the developer who was using TypeScript alongside Express and MongoDB. On sending login requests correctly, they observed that session data was saved in MongoDB but it was not accessible in later requests.

Symptoms

During login, the session data is properly initialized.

On a different route (GET request), the session shows no saved data, resulting in undefined values.

Solution: Troubleshooting Session Management

The developer’s issue was rooted in the session handling and frontend setup. Below are the solutions that resolved the problem.

Step 1: Identify the Cause

After troubleshooting, the developer discovered that the frontend was running on a Live Server in Visual Studio Code. This configuration was blocking cookies, which are necessary for maintaining sessions.

Step 2: Move Frontend to Express

To fix the cookie issue, the developer migrated the frontend to run on an Express server rather than relying on the Live Server. Here’s a simplified breakdown of actions taken:

Set up an Express server to serve frontend files.

Ensure that both backend and frontend are under the same origin to manage cookies seamlessly.

Step 3: Middleware Configuration

The middleware in the Express app plays a crucial role in how sessions are managed. Here's a brief guide on setting it up correctly:

[[See Video to Reveal this Text or Code Snippet]]

Key Points:

saveUninitialized: false prevents the creation of sessions for users that do not log in.

Ensure credentials: true is set in the CORS configuration to allow credentials (cookies) to be sent.

Step 4: Testing Login Functionality

After setting up the Express server, you can refactor the login functionality. Here’s an example of how the login function looks when it's appropriately set:

[[See Video to Reveal this Text or Code Snippet]]

Step 5: Final Adjustments

If the application were using frameworks like Vue or React, implementing sessions might have been more straightforward, but with simple Vanilla JavaScript, ensuring proper configurations and origins becomes even more crucial.

Conclusion

By transitioning the frontend server to Express and properly configuring middleware settings, the developer resolved issues relating to session management. The key takeaway is to always ensure that your front and back ends are correctly aligned regarding session management.

If you're still experiencing issues, consider checking your browser's cookie settings, session storage options, and ensuring that both servers are communicating correctly under the same origin.

Happy coding!