Express.js app.engine() Function

The app.engine() function is used to register the given template engine callback as ext. By default the Express itself will require() the engine based on the file extension.

Syntax:

app.engine(ext, callback)

Parameters: The ext parameter is the extension type like ejs, hbs, etc and callback is the function passed as parameter.

Return Value: It returns on Object.

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 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
```
Create a views folder and put home.html file in it with the following code:
Example 1:
Filename: home.html
<html>

<head>

<title>app.engine() Demo</title>

</head>

<body>

<h2>EJS Engine</h2>

</body>

</html>

For example, to map the EJS template engine to “.html” files:

app.engine('html', require('ejs').renderFile);

Filename: index.js

var express = require('express'); 
var app = express(); 
var PORT = 3000; 
  
app.engine('html', require('ejs').renderFile); 
    
app.get('/', function (req, res) { 
    res.render("home.html") 
}); 
  
app.listen(PORT, function(err){ 
    if (err) console.log(err); 
    console.log("Server listening on PORT", PORT); 
}); 

Steps to run the program:

Make sure you have installed express and ejs module using the following command:
```
npm install express
npm install ejs
```
Run index.js file using below command:
```
node index.js
```
Output:
```
Server listening on PORT 3000
```
Now open your browser and go to http://localhost:3000/, you can see the following output on your browser:
```
EJS Engine
```

Example 2: Filename: home.html

<html> 
<head> 
  <title>app.engine() Demo</title> 
</head> 
<body> 
    <h2>Handlebars Engine</h2> 
</body> 
</html> 

Filename: index.js

var express = require('express'); 
var app = express(); 
var PORT = 3000; 
  
console.log(app.engine('html', require('hbs').renderFile)); 
    
app.get('/', function (req, res) { 
    res.render("home.html") 
}); 
  
app.listen(PORT, function(err){ 
    if (err) console.log(err); 
    console.log("Server listening on PORT", PORT); 
}); 

Steps to run the program:

Make sure you have installed express and hbs module using the following command:
```
npm install express
npm install hbs
```
Run index.js file using below command:
```
node index.js
```
Output:
```
Server listening on PORT 3000
```
Now open your browser and go to http://localhost:3000/, you can see the following output on your browser:
```
Handlebars Engine
```

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

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