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