Express.js Mount Event - GeeksforGeeks

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:

You can visit the link to Install express module. You can install this package by using this command.
```
npm install express
```
After installing the express module, you can check your express version in command prompt using the command.
```
npm version express
```
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:

The project structure will look like this:
Make sure you have installed express module using the following command:
```
npm install express
```

Run index.js file using below command:

node index.js

Output:

Admin Mounted
Server listening on PORT 3000

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

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