Understanding the next() Argument

[00:00:00]
>> Scott Moss: Understanding that, so. Basically you You only wanna call next In my experience, the only time you ever called Next. Is in the in the scenario where your Middleware is only like checking for something or like enhancing the request object, or like doing some side effect unrelated to the request, like doing an analytics log or something like that. That is like the only time you would ever really call next.

[00:00:14]
That is like the only time you would ever really call next. Other than that, you probably won't be calling next because if you're not doing one of those things, you probably like, You know, doing a very Go no, what I call a go no go check, as in like either this value is true or false. And depending on that, I'm gonna stop the request right now, right? So if there's like a go no go check, then You either gonna, you're either gonna do a, you know, a res.json or arrest.send or if it's a goal you'll hit next, and those are like the really only two scenarios that I think you would be doing this.

[00:00:33]
So if there's like a go no go check, then You either gonna, you're either gonna do a, you know, a res.json or arrest.send or if it's a goal you'll hit next, and those are like the really only two scenarios that I think you would be doing this. There's obviously like more complicated things, but for the most part it's like you're either just like hey, I'm just a fly on the wall, checking out the request object. I don't really nothing that I do is ever gonna determine whether or not I call next or not. I'm a side effect.

[00:00:48]
I'm a side effect. I'm only inspecting, I'm only adding to. There's no scenario in which I will never call next and then there's the like. Yeah, depending on this value, I might call next or not.

[00:01:03]
Yeah, depending on this value, I might call next or not. That's it. It's those two scenarios, right? And the scenario of like depending on the value, typically it's like authorization, identification, access control, you know, our box, that type of stuff.

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And the scenario of like depending on the value, typically it's like authorization, identification, access control, you know, our box, that type of stuff. It's usually that or even like, hey, you don't, your credit card is expired and your and your account is past due, so no, you know, it could be that too. So Middleware is really good for that. But like I said, this is key.

[00:01:27]
But like I said, this is key. If you are going to return, as you see right here. Remember, this is you returning on a network level. You are closing the request, but your code is gonna continue to run.

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You are closing the request, but your code is gonna continue to run. This is not synonymous with a return statement. You could think of it as like the return statement for the ACCP request, but it is not the return statement for your code. That's still a return statement.

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That's still a return statement. So if there was no return statement here. Yeah, you're gonna have some problems or whatever's about to run next, you're gonna have some problems Cause you already responded. Right, so.

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Right, so. The best thing I could tell you if you ever respond inside of a Middleware, always just hit return after it unless it's the last thing on the line, but yeah, the best thing I can tell you so. The other thing is, if you don't call next in a middleware, that's the equivalent of not calling a response in a handler, it'll just hang forever. Because I won't know what to do, is I'm like, OK, here's a middleware.

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Because I won't know what to do, is I'm like, OK, here's a middleware. You're not responding, you're not calling next. There's no error being thrown. I'll just sit here and wait.

[00:02:52]
I'll just sit here and wait. It's the same thing, so you need to also make sure you either respond, throw an error, or call next. Otherwise it will hang. And one of the last things about Next is that it does take An optional argument only one, and if you whatever you pass the next, it is assumed that thing is an error.

[00:03:08]
And one of the last things about Next is that it does take An optional argument only one, and if you whatever you pass the next, it is assumed that thing is an error. That is the only thing you can pass the next. If you call next and pass anything in it, even if it's not an error, even if it's just a string that says yes, Express will assume that that's an error. And we'll talk about that when we get into error handling.

[00:03:27]
And we'll talk about that when we get into error handling. It's actually a brilliant system, how it works, but it's basically telling Express like, hey, something bad happened here, I need you to continue, but skip everything that's supposed to come after this and go to the error handler middleware if there's one that's registered, and if there's not one that's registered, use the one that's built in. That's basically what calling next error will do. It's like, yeah, I need you to go next, but skip everything that's not an error handler, and we haven't talked about error handlers yet, so that might be.

[00:03:44]
It's like, yeah, I need you to go next, but skip everything that's not an error handler, and we haven't talked about error handlers yet, so that might be. You know, confusing, but Surprise error handler is also just middleware, so. Everything in Express is middleware. OK, the only one note here that I wanna talk about is just like how dominant this pattern is.

[00:04:03]
OK, the only one note here that I wanna talk about is just like how dominant this pattern is. I really cannot emphasize. How much of a, like, honestly, I think this pattern single-handedly is what made NodeJS blow up. This at the time was significantly.

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This at the time was significantly. Easier to use and understand than its contemporaries in other languages. Like if you ever had to build something in Java or Python or anything like that at this time before this came out. Nah, it was not this simple.

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Nah, it was not this simple. And to this day, even frameworks like Fastify or NJS or any other Framework. If they're not built on XJS, they definitely look at this as inspiration. One so they can remain compatible with all the middleware that I mean there was like you think there's tons of React packages now back in the day there was tons of Express middleware everywhere.

[00:05:01]
One so they can remain compatible with all the middleware that I mean there was like you think there's tons of React packages now back in the day there was tons of Express middleware everywhere. There was Express malware to do anything so people wanted to maintain that backwards compatibility. So when they built new server frameworks or whatever they wanted to keep that so. Yes, lots of, lots of love for this.

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Yes, lots of, lots of love for this. I think it's one of the best developer experiences for the for servers in general, and that's why I still use Node even though I know go, I know Rust, I know all these languages. I will still write a Nojazz API any day until it became very clear that I could no longer use it anymore, but I will always just use it because the experience is so much better and I singlehandedly give kudos to this pattern,