---
layout: page
title: Developing template engines for Express
description: Learn how to develop custom template engines for Express.js using app.engine(), with examples on creating and integrating your own template rendering logic.
menu: advanced
redirect_from: "/advanced/developing-template-engines.html"
---

# Developing template engines for Express

Use the `app.engine(ext, callback)` method to create your own template engine. `ext` refers to the file extension, and `callback` is the template engine function, which accepts the following items as parameters: the location of the file, the options object, and the callback function.

The following code is an example of implementing a very simple template engine for rendering `.ntl` files.

```js
const fs = require('fs') // this engine requires the fs module
app.engine('ntl', (filePath, options, callback) => { // define the template engine
  fs.readFile(filePath, (err, content) => {
    if (err) return callback(err)
    // this is an extremely simple template engine
    const rendered = content.toString()
      .replace('#title#', `<title>${options.title}</title>`)
      .replace('#message#', `<h1>${options.message}</h1>`)
    return callback(null, rendered)
  })
})
app.set('views', './views') // specify the views directory
app.set('view engine', 'ntl') // register the template engine
```

Your app will now be able to render `.ntl` files. Create a file named `index.ntl` in the `views` directory with the following content.

```pug
#title#
#message#
```
Then, create the following route in your app.

```js
app.get('/', (req, res) => {
  res.render('index', { title: 'Hey', message: 'Hello there!' })
})
```
When you make a request to the home page, `index.ntl` will be rendered as HTML.