Using and understanding middleware

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. Since every request passes through each filter, each filter can perform a specific task against the request before it passes through to the next filter. 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 and you can use hundreds of third-party modules as well as simply write your own.

The order that the middleware is called in is very important. Again, using the concept of filters, as a request passes through each filter, you want to be sure that they are performing their responsibilities in the correct order. A great example of this is implementing a cookie parser before a session handler—since sessions typically rely on cookies...