Middleware is like a middle man in a business transaction, the only difference is middleware saves us time and money unlike middle men. It's a term coined to refer code that executes in between sending a request to a server and receiving a response. It is one way to adhere to the DRY(Don't Repeat Yourself) software development principle.

In express we use app.use ( ) to leverage on middleware function calls. We pass an arrow function to .use ( ) that takes three parameters req, res and next where next is a callback function. The callback function fires the next operation and passes the data from the arrow function.

app.use((req, res, next) => {});

So now any redundant code can be bundled up in the arrow function body, and executed in between processing the request.