Express.js Mount Event

The mount event is fired on a sub-app when it is mounted on a parent app and the parent app is basically passed to the callback function.

Syntax:

app.on('mount', callback(parent))

Parameter: It is an event named ‘mount’, and callback function is called when this event is called.

Return Value: Since its an event so it doesn’t have any return value.

Installation of express module:

1. You can visit the link to Install express module. You can install this package by using this command.

   npm install express

2. After installing the express module, you can check your express version in command prompt using the command.

   npm version express

3. After that, you can just create a folder and add a file for example, index.js. To run this file you need to run the following command.

   node index.js

Example 1: Filename: index.js

var express = require('express');
var app = express();  // The main app
var admin = express();
var PORT = 3000;

admin.on('mount', function (parent) {
  console.log('Admin Mounted');
});

admin.get('/', function (req, res) {
  res.send('Admin Homepage');
});

app.use('/admin', admin);

app.listen(PORT, function(err){
    if (err) console.log(err);
    console.log("Server listening on PORT", PORT);
});

Steps to run the program:

1. The project structure will look like this:
   
2. Make sure you have installed express module using the following command:

   npm install express

3. Run index.js file using below command:

   node index.js

   Output:

   Admin Mounted
   Server listening on PORT 3000
   

4. Now open your browser and go to http://localhost:3000/admin, now you can see the following output on your screen:

   Admin Homepage

Example 2:
Filename: index.js

var express = require('express');
var app = express();  // The main app
var student = express();
var teacher = express();
var PORT = 3000;

// Multiple mounting
teacher.on('mount', function (parent) {
    console.log('Teacher Mounted');
});

student.on('mount', function (parent) {
  console.log('Student Mounted');
});

app.use('/student', student);
app.use('/teacher', teacher);

app.listen(PORT, function(err){
    if (err) console.log(err);
    console.log("Server listening on PORT", PORT);
});

Run index.js file using below command:

node index.js

Output: Now open your browser and make GET request to http://localhost:3000, now you can see the following output on your console:

Student Mounted
Teacher Mounted
Server listening on PORT 3000

Reference: https://expressjs.com/en/4x/api.html#app.onmount