Error Handler Middleware

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>> Scott Moss: Let's make one, so what we'll do. Is we'll make an Error Handler and then we'll register it in our Express server. So the simplest way to do that is go to our middleware. Like a new one here, we'll call it error, Handler.

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Like a new one here, we'll call it error, Handler. That's yes, like this. We'll import our types from Express. Requests and response.

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Requests and response. We will export a. Error Handler, middleware, so what's called Error Handler. It's got to take those 4 arguments, so error for now you could just put error class if you wanted to type this out.

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It's got to take those 4 arguments, so error for now you could just put error class if you wanted to type this out. Our request is our request response. And then Next Is the Next function, there we go. Cool, so we got all those.

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Cool, so we got all those. First thing is if you want you can just log the entire stack trace of this error if you wanted to. That's just something that's helpful. I mean this is not something you would do in Production.

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I mean this is not something you would do in Production. I think you would have like a logger that you're logging to, but this is just something I'm doing in Development. And then There's no wrong answer here. Like what we're doing right now at this point is all opinion based, right?

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Like what we're doing right now at this point is all opinion based, right? This is like how do you handle your errors? Do you report to entry right here? Do you have something like a bug snag?

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Do you have something like a bug snag? Do you have, are you logging with something like Pino? Are you writing to a file? Are you sending off a Slack to alert somebody?

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Are you sending off a Slack to alert somebody? Like what are you doing here? There's no wrong answer. You do whatever you want.

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You do whatever you want. This is saying, hey, there was an error in one of the middlewares or the Controllers somewhere. And I'm letting you know it also depends on what does your errors, you know, what do they look like? Do you have like custom errors that you've created that have different you know, error codes on them that do specific things and inside of here you do a switch statement on those error codes and depending on that error code you do something else, there's no wrong answer here.

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Do you have like custom errors that you've created that have different you know, error codes on them that do specific things and inside of here you do a switch statement on those error codes and depending on that error code you do something else, there's no wrong answer here. So everything I have right here is just me showing you what's possible. It's not a right or wrong here it's really just based on opinions. So what I have here is just like.

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So what I have here is just like. Oh, I'll look at the name of the error. Depending on the name of the error, I'll set a different status and a message, and that's why I send back to the clients. That's a very simple way of doing it.

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That's a very simple way of doing it. You could do, you can literally do whatever you want here. There's no wrong answer. Does that make sense?

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Does that make sense? It's mostly the fact that the error was handled, right, cause if you don't handle the error, then your server will break. What you do with the error is up to you. The lesson is handling the error to begin with and how to handle the error.

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The lesson is handling the error to begin with and how to handle the error. So in this case, I'm just setting a status. I'll default to, if there is an error. Dot status, I'll default to that otherwise I'm just gonna assume we messed up somewhere and I'm gonna do 500.

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Dot status, I'll default to that otherwise I'm just gonna assume we messed up somewhere and I'm gonna do 500. And then I'll default to a message. Hey, if there is a message on the error, you know, I'll use that one, otherwise I'll just say, yeah, you know, internal. Server error.

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Server error. And then if error.name. Equals validation error. So in this case, something happened on the validation like input validation, and I'll say the status equals 400 and the message equals.

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So in this case, something happened on the validation like input validation, and I'll say the status equals 400 and the message equals. Validation error, right, so this would free up our Validation middleware for like, for instance, we go look at that Validation middleware right now. They automatically do that themselves, right? They're like, oh I know what the error is.

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They're like, oh I know what the error is. I'm just gonna go ahead and respond with the error handling that we're gonna set globally in our app this middleware no longer has to do this. All it has to do is throw an error now. It doesn't even, I'm sorry, it doesn't, it has to call Next now.

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It doesn't even, I'm sorry, it doesn't, it has to call Next now. I'm sorry, so it would make an error and it would call Next. So instead of returning here, it would say, oh snap, there's an error, I'll just call Next. With that with that error object that I want to pass in that satisfies this so in this case I'm making like a custom error object that can have a name and a status and a message.

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With that with that error object that I want to pass in that satisfies this so in this case I'm making like a custom error object that can have a name and a status and a message. Right, so I would have to make that, which I do believe I go over making. I know, I got rid of that because I thought it was silly making a custom error one, but like you could we maybe we can make one right now just to show you what it looks like, but. There there's just no wrong way of doing it.

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There there's just no wrong way of doing it. Like I'm trying to explain like it's very, there's a million ways to how you wanna handle your errors. It's mostly just that like you have a system. Is it custom errors?

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Is it custom errors? Is it? Based off messages is it based off of codes that you do, special use cases, things from the database, you know, how you wanna do it is totally up to you. You have the freedom you can check whatever you wanna check here, right?

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You have the freedom you can check whatever you wanna check here, right? But in that case that's what would happen. So if I said that, you know, this is what will happen, let's say there was a. Let's say the name was.

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Let's say the name was. If error.name. Was unauthorized error, so this is an error that might be thrown. If you try to Access to API and didn't have it you had it invalid JSON Web token or something like that so I would say status is 404 or 401 and the message is unauthorized.

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If you try to Access to API and didn't have it you had it invalid JSON Web token or something like that so I would say status is 404 or 401 and the message is unauthorized. So I can do something like that. And then ultimately, once I'm done handling all the different types of error messages or names that I support, I can then finally set the status. I start status to whatever status is.

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I start status to whatever status is. And then I can finally send back a message here. And I can say, hey, the error in this case is the message. And then if I'm in Development mode, maybe I wanna do something special to help me debug this.

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And then if I'm in Development mode, maybe I wanna do something special to help me debug this. I can say, OK, if env. What did you just import, OK, not from there. There we go from there, so I can say ENV.

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There we go from there, so I can say ENV. App stage equals. Dev, so we're in Development. Then I might wanna Go ahead and add some extra stuff here.

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Then I might wanna Go ahead and add some extra stuff here. Show me the error stack. Which will be, you know, error. Stack, show me some more details here.

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Stack, show me some more details here. I wanna see the error. Message. Something extra while I'm in Development mode maybe.

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Something extra while I'm in Development mode maybe. And we haven't incorporated this anywhere. I'm not throwing any errors that satisfies these constraints so like this is none of these statements are gonna get triggered because I'm not throwing anything that matches this. And this is why you gotta figure out what is your system because whatever you decide here you didn't have to go back and implement this everywhere there's a potential error.

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And this is why you gotta figure out what is your system because whatever you decide here you didn't have to go back and implement this everywhere there's a potential error. Right, so in this scenario, I would make like a custom error class, right, let's say I call this a. Class, you know, API. Error and it extends.

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Error and it extends. The error class, right? I could do that. I can say super.

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I can say super. Oops, I'm sorry. I can do that and then. You know, whatever Error whatever like message you pass in here.

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You know, whatever Error whatever like message you pass in here. I could, I could do whatever I want. So in this case, I can say that like, yeah, these things have like a status, you know, it's gonna be a number, they have a. What else did I have yeah, I was just like a name, I guess.

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What else did I have yeah, I was just like a name, I guess. That's a stream and then you can just pass all those in here, right? Like it also has a message which is a stream and you just pass those in you can say here's a message, here's a name. Here's the status.

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Here's the status. Then you would just find those just that message. Do all that. The name and then status.

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The name and then status. So you do that and then this would be like my custom error class, right? So then down here. Assuming everywhere there was an error, I would be throwing this API error down here.

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Assuming everywhere there was an error, I would be throwing this API error down here. I will be catching that AI API error and then in that case right, if I change this to API error. I do have an error that status now right? I do have.

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I do have. Error message now, so, but then now I would, I would have to be instead of throwing a new error like I might be doing like. Somewhere like here, am I even throwing one somewhere? Let me see.

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Let me see. I don't I don't even throwing one anywhere, but instead of throwing an error, a regular error, I would throw an API error and I would pass in its status, its name, and its message. And that's just one of many ways to do this. There's literally no.

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There's literally no. Yeah, there's like I don't have a strong opinion on how to do this because I really think it comes down to. How is your debug how is your Production Debugging? Like how do you debug when things are live?

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Like how do you debug when things are live? Do you have like an on call system and pager duty? Do you have alerts? All that is gonna matter here, so it's really hard to walk you through what that might look like, but this is just an idea of what it would be like if that makes sense, right?

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All that is gonna matter here, so it's really hard to walk you through what that might look like, but this is just an idea of what it would be like if that makes sense, right? So Now that you have this, all you gotta do is just add it, To our server, so we'll Go down to our server. And we just need to add this at the bottom. So we'll just say, cool, let's just do a app.use and then we can just say Error Handler Boom, done.

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So we'll just say, cool, let's just do a app.use and then we can just say Error Handler Boom, done. Long as it is at the bottom, this is a global Error Handler so if anywhere we call Next and we throw this will work so we can test this by. I'll go here. I'll say app.use.

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I'll say app.use. I'll make a middleware function in line. And I'm just gonna Do that. I want this 3rd argument here, which is Next.

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I want this 3rd argument here, which is Next. And then I'll say Next new. API Error, oh, did I export that? Let me see, let me export this.

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Let me see, let me export this. New API error. And I gotta give it a message like this is, or I guess I should just. Get this thing to trigger, so I'll give it a message.

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Get this thing to trigger, so I'll give it a message. I'll give it a name of validation error. And I can give the message like whatever I want, so. I'll say For the message is like, validation error, and then for the name is literally that.

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I'll say For the message is like, validation error, and then for the name is literally that. And for the status. I'll put a 400. Cool.

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Cool. So I'm just gonna put that error there, right, and this is gonna happen no matter what I hit. This is this is gonna throw cause I just, I just put this here. Now this should handle it.

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Now this should handle it. So now if I go. And just literally run anything. Let's see what happens.

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Let's see what happens. Boom, Validation error. Right, that's the error that I just threw, and here's the stack and the details because we're in dev mode and I said show me the stack in details in dev mode. So you can see it's skipped over.

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So you can see it's skipped over. Where did I put it? Oh yeah, here we go. It skipped over all of this.

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It skipped over all of this. It didn't care. I didn't even try to process this because I called Next with an error. Everything got skipped and it found the closest Error Handler, which is here.

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Everything got skipped and it found the closest Error Handler, which is here. Right, so as you see, you can get pretty creative of what you want to put here. What if your error handler throws an error? If your Error Handler throws an error, which is actually very common, This is why what you wanna do is.

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If your Error Handler throws an error, which is actually very common, This is why what you wanna do is. You wanna put a try catch in here if you're doing anything asynchronous. If you're doing anything synchronous, it's gonna break. On every you like you you'll find it, but it's doing something asynchronous like I'm reporting the century or whatever, yeah, you wanna put a try catch in here and then you wanna catch that error.

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On every you like you you'll find it, but it's doing something asynchronous like I'm reporting the century or whatever, yeah, you wanna put a try catch in here and then you wanna catch that error. And you know, Do whatever you need to do to be notified about that, but yes, your Error Handler can also throw an error. There is something you know that you can do. To potentially catch unhandled errors, and that's process Dot, process that.

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To potentially catch unhandled errors, and that's process Dot, process that. On No, what is it? Processed dots. Oh, I forgot what it is.

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Oh, I forgot what it is. I see. Remove all now, processed dots, maybe you don't have this in a later version of Node anymore, but there was something where you can like. Handle on or process that on.

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Handle on or process that on. On there it is, OK, I'm like, I know it's something there it is, unhandled rejection, or unhandled or uncaught exception. These would be errors that are thrown on your server that you've never caught. But errors.

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But errors. Inside of Express. Might not bubble up to this because Express itself might be doing some catching and doing stuff, so. Yeah, that's one thing, but.

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Yeah, that's one thing, but. It won't harm you to put this here. Unhandled. Or what is it?

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Or what is it? Uncaught exception and things like that you can run a call back here and you could do a thing. It just can't be an asynchronous thing cause it's shutting down at this point, I think, so this is where you can like restart the server or, you know, do something else, so.

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So there really is no 100% safe way of stopping your server from crashing if it was gonna break other than writing good code, and then you know when you deploy this you wanna make sure you have something that restarts the server if it ever crashes if the thing is recoverable and that's the other thing like is this a recoverable error or not or is this like yeah this is a note this is a error that we can't recover from and even restarting it it's just gonna crash it again then you gotta fix your code