One of the most powerful features available with Express is the concept of middleware. The idea behind middleware is that it acts like a stack of filters that every request to your server passes through. Every request passes through each filter and each filter can perform a specific task against the request before it passes through to the next filter.

To understand it better, here is a diagrammatic view of middlewares:

Typically, these filters are used for tasks such as cookie parsing, form-field handling, session handling, authentication, and error handling and logging. The list goes on and on. You can use hundreds of third-party modules as well as simply write your own custom middleware.