77 lines
2.3 KiB
Markdown
77 lines
2.3 KiB
Markdown
|
---
|
||
|
|
title: JavaScript Scoping
|
||
|
|
description: A blog post talking about how JavaScript handles scoping
|
||
|
|
date: 2024-08-11
|
||
|
|
tags:
|
||
|
|
- development
|
||
|
|
- javascript
|
||
|
|
- typescript
|
||
|
|
slug: javascript-scoping
|
||
|
|
draft: true
|
||
|
|
---
|
||
|
|
|
||
|
|
## The EventEmitter Class
|
||
|
|
|
||
|
|
The other day I was reading through the Types file of the MariaDB JavaScript
|
||
|
|
library (since its the better MySQL library for Node.js), and I noticed that
|
||
|
|
the `PoolConnection` class _actually_ extends the `EventEmitter` class from
|
||
|
|
Node.js. That in and of itself isn't that exciting, but it had me thinking
|
||
|
|
about the utility of extending that particular class, and how the scopeing of
|
||
|
|
the code used in that class would actually work.
|
||
|
|
|
||
|
|
I never put a lot of thought into it, but if you have used the `EventEmitter`
|
||
|
|
class in JavaScript at any point in your life, you know that you can attach
|
||
|
|
callbacks to events that will be run when that particular event is emitted.
|
||
|
|
|
||
|
|
```javascript
|
||
|
|
import EventEmitter from "node:events";
|
||
|
|
|
||
|
|
const em = new EventEmitter();
|
||
|
|
em.on("home-event", function () {
|
||
|
|
console.log("Something happened.");
|
||
|
|
});
|
||
|
|
|
||
|
|
em.emit("some-event");
|
||
|
|
```
|
||
|
|
|
||
|
|
When you call `em.emit("some-event")` anywhere this `EventEmitter` exists in
|
||
|
|
code, you will get a console log from inside that function. Okay, makes sense,
|
||
|
|
so what's interesting about this?
|
||
|
|
|
||
|
|
Well the scope of the callback function actually ineherits the called method's
|
||
|
|
properties and methods. This can get really interesting if you are extending
|
||
|
|
the functionality of the `EventEmitter` class - like in the MariaDB module's
|
||
|
|
`PoolConnection` class.
|
||
|
|
|
||
|
|
## Getting In-Depth with Scope
|
||
|
|
|
||
|
|
So say we had a class that extends the `EventEmitter` class and had some
|
||
|
|
methods with some state we cared about in the callback. In the MariaDB context
|
||
|
|
that could be anything that has to do with the connection pool - like maybe
|
||
|
|
how many open connections are available. We can sketch this out in a very
|
||
|
|
trivial class example in TypeScript.
|
||
|
|
|
||
|
|
```typescript
|
||
|
|
import EventEmitter from "node:events";
|
||
|
|
|
||
|
|
class SomePool extends EventEmitter {
|
||
|
|
private freeConnections: number;
|
||
|
|
|
||
|
|
public constructor() {
|
||
|
|
super();
|
||
|
|
this.freeConnections = 4;
|
||
|
|
}
|
||
|
|
|
||
|
|
public getFreeConnectionCount(): number {
|
||
|
|
return this.freeConnections;
|
||
|
|
}
|
||
|
|
}
|
||
|
|
```
|
||
|
|
|
||
|
|
So now, in any callback function* we can get the total number of free
|
||
|
|
connections in the `SomePool` class.
|
||
|
|
|
||
|
|
```typescript
|
||
|
|
title=
|
||
|
|
```
|