Express Middleware Explained in One Minute

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After months of learning JavaScript, I finally understand what a middleware is, and how it adds value to streamline communication between client and server, under the umbrella of Express.

What Is Middleware in Express Framework

1. Middleware is a software that runs on the HTTP request object to perform certain tasks before it reaches final route callback function
2. A middleware could add additional information to incoming HTTP requests. Subsequent middleware could access those newly added information to perform other operations
3. Express application runs all the middleware according to the order we implement middleware: global comes first and route specific the second

Example

Gist below is a simple Express app with only one route pointing to its root. There are 3 custom middleware declared in this example:

• two of them at global scope, being used by calling app.use(middleware)
• the remaining one is specific to its root route, meaning that middleware will only run when the request is sent to that route
• if we change the order of app.use() , the message logged to message will also changed (try the code yourself)
• a middleware must call next() at the end; otherwise, your app will stuck forever because it never knows when that middleware complete itself

I hope you get the same ah-huh moment as I did, when seeing how middleware and app.use() works together.

Reference

YouTube: User Authentication in Web Apps by FreeCodeCamp & Zach Gollwitzer

I highly recommend to watch Zach’s YouTube Video which is very informative of how middleware works in Express