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Scott gives a quick refresher of Async JavaScript, which is used heavily in Node JS.

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Transcript from the "Asynchronous Code in Node.js" Lesson

>> Scott Moss: Async code, so I've been talking about async code. And if you've ever written JavaScript in the browser, you already know that JavaScript is asynchronous. Who here needs a primer? I'm gonna do it anyway. Let's do a primer on async JavaScript and how that works. Without going too deep, cuz I'm sure, I could talk about this for a week, that's how deep it is.

[00:00:20] But I'm just gonna talk about it for a little bit. So if you've ever used any other language like Ruby, or Python, or anything like that, right, it's multi-threaded, right? So you have many threads on your machine, and let's say you have a server in Python, every request that comes in is gonna be its own thread, right?

[00:00:39] And your code is gonna run from top to bottom, right? It's gonna go from this line, to this line, to this line. It's gonna be blocking and with many threads. JavaScript's not that way, JavaScript's one thread with an event loop. Now something you need to figure out if that's bad or not [LAUGH].

[00:00:54] But it enables one thing but it the kind of sucks the life out of something else. So it enables high concurrency, because of that, you can register many things to do at once basically. So because it's asynchronous and you can do things in the background, you can have many operations coming in like, okay, okay.

[00:01:10] You want me to do this, you want me to do this, you want me to do this, you want me to do this, and it will just like cool. I'm going to do all this and continue to go to the next line, while it's doing other stuff in the background, right?

[00:01:18] And whatever is happening in the background it doesn't really matter. It can be JavaScript, it can be a PythonScript, it can be literally whatever. But it's happening in the background outside of the thread, and it's this event loop thing, and that's how JavaScript works. So the way you get notified about these things are callbacks, promises, or async await, all these different things.

[00:01:38] So that's how JavaScript works. And that's been the way in the browser pretty much since Google Maps came out when they invented, they came out with Ajax and stuff like that. So that's been the way forever, and you've always been doing that in a browser. If you ever made a call to the API in the browser you used async code.

[00:01:54] If you ever did a set timeout, you used async code. If you ever did any Listener or any DOM element ever, that was async code. It's the same in Node. The only difference is, and we'll get to it in a little more detail, but it's how your app is being used, right?

[00:02:13] On the browser, if everybody here opened up the same website, you're all gonna have a different instance of the same app. Your JavaScript's not gonna interact with this person's JavaScript, unless you're doing some stuff there. But yeah, it's just its own instances running on your machine. If you've got one server, every client's hitting that one server, right?

[00:02:31] So it's a little different, so now you actually have to consider asynchronous code because it could affect your app. It could kill it, it could cause problems you were not aware of. Yeah, so, node.js is single threaded and event-based, just like the browser. Like I said, unlike the browser, your node.js app will be shared by all clients.

[00:02:51] And this is mostly true for APIs. Even then, at the end of the day you still have one machine that's gonna be handling many requests. And if you have one piece of code that's blocking, like reading a file synchronously, then every request has to wait for that to be done.

[00:03:05] It can't do anything else. It can't register a new task cuz it's waiting on this infinite while loop that you created, that's never-ending. So this is why asynchronous code is very important to Node, because without it kind of fails. Okay, so again you have to consider CPU-intensive tasks that block a single thread, so while loops.