Creating Custom Middleware

Middleware functions are the backbone of an Express.js application. They allow us to intercept requests, modify data, and control the flow between incoming and outgoing responses.
While built-in and third-party middleware provide useful functionality, we often need to create custom middleware to address specific application needs, including:

• Logging

• Authentication

• Validation

• Error handling

• Data transformation

• Response modification

• Security enforcement

In this lesson, we’ll focus on writing custom middleware, organizing middleware for reusability, and implementing effective error handling.

Designing and writing custom middleware

Custom middleware makes writing reusable code easier, simplifying requests and adding features tailored to your app. For instance, this middleware function converts all response object keys to uppercase before sending them back to the client.