How to do it...

We will write a middleware function that allows access to the root path "/" only when the query parameter allowme is present:

1. Create a new file named middleware-functions.js
2. Initialize a new ExpressJS application:

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

1. Write a middleware function that will add a property allowed to the request object:

      app.use((request, response, next) => { 
          request.allowed = Reflect.has(request.query, 'allowme') 
          next() 
      }) 

1. Add a request method to handle requests for path "/":

      app.get('/', (request, response, next) => { 
          if (request.allowed) { 
              response.send('Hello secret world!') 
          } else { 
              response.send('You are not allowed to enter') 
          } 
      }) 

1. Listen on port 1337 for new connections:

      app.listen( 
          1337, 
          () => console.log('Web Server running on port 1337'), 
      ) 

1. Save the file
2. Open a terminal and run:

      node middleware-functions.js
  

1. To see the results, in your web browser, navigate to:

 http://localhost:1337/
      http://localhost:1337/?allowme