blog/posts/handler-service-presentation.md

title	description	date	tags	slug	draft
HS(D)P	Describing the architecture of Handlers, Services, (Domains), and Presentation	2024-08-11	development javascript typescript hono	hsdp	true

Handlers, Services, (Domains?), Presentation

In my last blog post I talked about how I have stopped thinking of my Node.js applications as Model-View-Controller and started thinking of the applications in a new, fun acronym: Handlers, Services, (sometimes Domains), and Prfesentation. I thought this would be a good topic for a new blog post. So here it is.

I am going to be talking about this pattern in relationship to JavaScript and why the language lends itself to the pattern when working on applications.

The Application

Like the previous post, I talked about building this blog with Bun and Hono. I am going to do the same here, however the application we are going to build is the all time classic - The TODO Application.

JavaScript

One of the important things about JavaScript that alluded me for far too long was how the modules actually worked with Node.js. A module before ES6 was simply a JavaScript file that declares a module.exports value with the code you want to expose to other modules with the require function. Like so:

function doSomething() {
    // ...
}

module.exports = doSomething;

example.js

Then we can pull in doSomething with require:

const doSomething = require('./example');

index.js

I don't know why I didn't know this, but the code in the example.js file is evaluated and run during the require() call.

In my past life I would try and build a Singleton class for holding onto an application's Connection Pool to a database. But because of how the Modules in Node.js work, we only need to instantiate the Pool and export it. Here's an example of how we can set up a single pool using the MariaDB official package:

const mariadb = require('mariadb');

const config = { ... };
const pool = mariadb.createPool(config);

module.eports = pool;

pool.js

Now the connection pool is only created once and you can require this

Outline:

• Explain what I know about JavaScript
• How does this differ to my experience with PHP
• Describe modules
  • Describe how we can set up singletons thinking in modules
  • Talk about avoiding classes
• Talk about big express applications
  • And how I used to make MVC applications