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

## Handlers, Services, (Domains?), Presentation

In [my last blog post](https://davesmithhayes.com/posts/fullstack-hono) 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:

```js
function doSomething() {
    // ...
}

module.exports = doSomething;
```
_`example.js`_

Then we can pull in `doSomething` with `require`:

```js
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:

```js
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