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Error Handling
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00:00:02 Let's create a global error handling middleware that'll nicely alert us whenever something goes wrong.
00:00:08 We can do that easily by creating a new folder called middlewares.
00:00:13 And within it, we can create a new file called error.middleware.js.
00:00:20 And within it, we can define it just as a regular JavaScript function const errorMiddleware is equal to a function that looks like this.
00:00:30 Now, not just errorMiddleware, but any kind of middleware in Node.js or any other backend software typically looks like this.
00:00:40 Alongside the request and the response, you get two additional things.
00:00:44 The information that happened before the request, and then what happens after, when you're ready to proceed to the next step.
00:00:52 Let me explain what I mean by this.
00:00:54 Like let's say that we have the ability to create a new user.
00:00:59 In a way, when we had a subscription model in the previous lesson, this subscription schema that pre was actually in a way a middleware because it happened
00:01:10 before something and then told us, hey, now we are ready to move to the next step.
00:01:14 So let's say that you're trying to create a subscription, create a subscription.
00:01:20 Then we have a new middleware that maybe checks something, like check for renewal date.
00:01:27 And then you can have as many middlewares as you want.
00:01:30 We can have another middleware, maybe this one will actually check for errors.
00:01:36 And then only when both of these middlewares call their next status, we are actually navigated over to the controller, which handles the actual logic of
00:01:46 creating a subscription.
00:01:47 Hopefully that makes sense.
00:01:49 Basically, some blocks of code that are executed before or after something allowing us to intercept what is happening.
00:01:57 So let's open up a try and catch block.
00:02:00 And in the catch, we get the error and we'll simply send that error over to the next step to let us know that the error actually happened.
00:02:08 But in the try block, we'll actually try to decipher that error to let us know exactly what the issue is.
00:02:14 So I'll say let error is equal to, and I'll create a new object and destructure the error that we're getting through props.
00:02:24 Then I'll set the error message to be equal to err.message.
00:02:30 And I'll console log the error to the dev.
00:02:34 So console.error, and then we pass the error right here.
00:02:39 Just so we know what's happening.
00:02:42 After that, we can try to figure out a type of error.
00:02:46 Maybe it is a Mongoose bad object ID.
00:02:50 This is a very common error that we get when working with Mongoose and MongoDB.
00:02:55 This one would happen if we have a cast error, if that is the name.
00:02:59 If that is the name, we can create a new message, const message is equal to resource not found.
00:03:10 And then we can set the error to be equal to new error, and we can pass this message right in.
00:03:17 We can also set the status code to 404, so we know exactly what happened.
00:03:22 Resource not found.
00:03:24 Another type of error is a Mongoose duplicate key.
00:03:27 That is when we're trying to create something with the same key.
00:03:30 And this one happens if the error code is 11,000. Don't ask me how I know that I've encountered this error one too many times.
00:03:39 So if that is the case, we can again, form a message equal to something like duplicate field value entered.
00:03:46 And we can set that error to be equal to new error message and the status code of 400. Another type of common error is a mongoose validation error.
00:03:57 So when we're trying to create a document and we don't pass all the right props, so we can check if error.name is equal to validation error.
00:04:06 In this case, we'll form the message by mapping over the values of the object.
00:04:12 because we might have many validation errors and we'll show a message for each one.
00:04:17 Once we do that, we can simply set that message to the error and join it based on commas and spaces.
00:04:24 So for each one of these validation errors, we'll add it to the error message.
00:04:29 And finally, We can return the response from this error middleware by saying res.status is equal to the error.status code.
00:04:38 It can be either 404 or 400. Or if it doesn't exist, we can make it 500, which is a general server error.
00:04:47 And we can return a JSON success is false, as well as an error equal to error.message or just a string of server error.
00:05:00 So in simple words, what we're doing here is we're intercepting the error and trying to find a bit more information about it,
00:05:08 so we much more quickly know what went wrong.
00:05:11 Now we can export that error middleware by saying export default error middleware, and we can use it within the app.
00:05:20 So head over to app.js and then let's add it right here below the routes.
00:05:25 That'll be app.use and you can call it error middleware and make sure to import it from errormiddleware.js.
00:05:34 Alongside this custom piece of middleware that we created, Express also has many built-in middlewares that we can use.
00:05:42 Let's actually add them right here at the top.
00:05:45 The first one you can see auto recommended to me is app.use express.json.
00:05:53 This allows your app to handle JSON data sent in requests or API calls.
00:05:58 Something that is super common.
00:06:00 Another one is app.use express.url encoded extended is set to false.
00:06:08 This helps us to process the form data sent via HTML forms in a simple format.
00:06:14 And another one you'll often see used is app.use and that is cookie parser.
00:06:21 This one has to be imported at the top.
00:06:23 So make sure to say import cookie parser coming from cookie-parser.
00:06:31 This one, as the name says, reads cookies from incoming requests so your app can store user data.
00:06:37 If you want to see how all of these works, you can just click into them and immediately see the code.
00:06:43 And since this is the middleware, you know what it has to have.
00:06:46 It has to modify the request, right?
00:06:49 Do something with it.
00:06:50 There's a reason why we had to have it in the first place.
00:06:53 Intercepted it, did something with the request, modified it, and then, check this out, we're calling the next function, letting the request pass through.
00:07:02 And it's similar with all the other middlewares.
00:07:04 Later on, once we start working with our database, we'll make one or two of these rookie mistakes and we'll get a very nice error message saying resource
00:07:13 not found or duplicate field value entered, or maybe a validation error.
00:07:18 Either way, creating this global error handling middleware will save us hours of debugging later on, and it'll make our app that much more scalable.
00:07:27 Great work.