Express - Middleware

What is a middleware?

• Middleware is used extensively in Express apps, for tasks from serving static files to error handling, to compressing HTTP responses.
• middleware functions typically perform some operation on the request or response and then call the next function in the "stack", which might be more middleware or a route handler.
• Most apps will use third-party middleware in order to simplify common web development tasks like working with cookies, sessions, user authentication, accessing request POST and JSON data, logging
• You can write your own middleware functions, and you are likely to have to do so (if only to create error handling code).

Hello World Middleware (f. morgan)

1 Install HTTP request logger middleware morgan

$ npm install morgan

2 Import middleware

const express = require('express');
const logger = require('morgan');
const app = express();
app.use(logger('dev'));
...

3 Result

Writing your own middleware

const express = require('express');
const app = express();

// An example middleware function
let a_middleware_function = function(req, res, next) {
  // ... perform some operations
  next(); // Call next() so Express will call the next middleware function in the chain.
}

// Function added with use() for all routes and verbs
app.use(a_middleware_function);

// Function added with use() for a specific route
app.use('/someroute', a_middleware_function);

// A middleware function added for a specific HTTP verb and route
app.get('/', a_middleware_function);

app.listen(3000);

REF

https://developer.mozilla.org/en-US/docs/Learn/Server-side/Express_Nodejs/Introduction