1 00:00:01,320 --> 00:00:03,670 ‫So, up until this point in the course 2 00:00:03,670 --> 00:00:05,430 ‫we haven't really handled errors 3 00:00:05,430 --> 00:00:09,420 ‫in a good way or in a central place in our application. 4 00:00:09,420 --> 00:00:11,690 ‫What we did was to simply send back 5 00:00:11,690 --> 00:00:15,180 ‫an error message as JSON in each route handler 6 00:00:15,180 --> 00:00:16,750 ‫in case something went wrong. 7 00:00:16,750 --> 00:00:17,760 ‫Right? 8 00:00:17,760 --> 00:00:21,250 ‫So that's basically what we're gonna fix in this section. 9 00:00:21,250 --> 00:00:22,860 ‫But for now, let's take a minute 10 00:00:22,860 --> 00:00:26,653 ‫to just get a brief overview of error handling in Express. 11 00:00:27,490 --> 00:00:29,700 ‫And we start by actually distinguishing 12 00:00:29,700 --> 00:00:32,550 ‫between two types of errors that can occur. 13 00:00:32,550 --> 00:00:35,660 ‫Operational errors and programming errors. 14 00:00:35,660 --> 00:00:38,060 ‫So, operational errors are problems 15 00:00:38,060 --> 00:00:40,760 ‫that we can predict will inevitably happen 16 00:00:40,760 --> 00:00:42,480 ‫at some point in the future. 17 00:00:42,480 --> 00:00:45,500 ‫And so we just need to handle them in advance. 18 00:00:45,500 --> 00:00:48,430 ‫They have nothing to do with bugs in our code. 19 00:00:48,430 --> 00:00:51,760 ‫Instead, they depend on the user, or the system, 20 00:00:51,760 --> 00:00:53,050 ‫or the network. 21 00:00:53,050 --> 00:00:57,320 ‫So, things like a user accessing an invalid route, 22 00:00:57,320 --> 00:01:01,430 ‫inputting invalid data, or an application failing to connect 23 00:01:01,430 --> 00:01:02,870 ‫to the database. 24 00:01:02,870 --> 00:01:05,290 ‫All these are operational errors 25 00:01:05,290 --> 00:01:06,920 ‫that we will need to handle in order 26 00:01:06,920 --> 00:01:10,440 ‫to prepare our application for these cases. 27 00:01:10,440 --> 00:01:12,590 ‫You will also see the term, "exception" 28 00:01:12,590 --> 00:01:14,680 ‫being used instead of error 29 00:01:14,680 --> 00:01:17,200 ‫and while they are conceptually different, 30 00:01:17,200 --> 00:01:19,290 ‫many people use the terms error 31 00:01:19,290 --> 00:01:21,670 ‫and exception interchangeably. 32 00:01:21,670 --> 00:01:23,370 ‫And I'm just gonna call them, "errors," 33 00:01:23,370 --> 00:01:26,860 ‫in this course as well in order to avoid confusion. 34 00:01:26,860 --> 00:01:30,460 ‫Anyway, on the other hand, we have programming errors. 35 00:01:30,460 --> 00:01:32,800 ‫Which are simply bugs that we developers 36 00:01:32,800 --> 00:01:34,810 ‫introduce into our code. 37 00:01:34,810 --> 00:01:37,010 ‫Like, for example, trying to read properties 38 00:01:37,010 --> 00:01:41,220 ‫from an undefined variable, using await without async, 39 00:01:41,220 --> 00:01:46,070 ‫accidentally using request.query instead of request.body, 40 00:01:46,070 --> 00:01:49,020 ‫or many other errors, really, that we might make. 41 00:01:49,020 --> 00:01:51,530 ‫So, you know how it works, right? 42 00:01:51,530 --> 00:01:53,540 ‫And they are really inevitable 43 00:01:53,540 --> 00:01:56,810 ‫but also more difficult to find and to handle. 44 00:01:56,810 --> 00:01:59,720 ‫Okay, so, it's important that you understand 45 00:01:59,720 --> 00:02:02,480 ‫this crucial difference between operational errors 46 00:02:02,480 --> 00:02:04,230 ‫and programming errors. 47 00:02:04,230 --> 00:02:07,330 ‫So, when we're talking about error handling with Express, 48 00:02:07,330 --> 00:02:09,930 ‫we mainly just mean operational errors. 49 00:02:09,930 --> 00:02:11,890 ‫Because these are the ones that are easy 50 00:02:11,890 --> 00:02:15,650 ‫to catch and to handle with our Express application. 51 00:02:15,650 --> 00:02:18,070 ‫And Express actually comes with error handling 52 00:02:18,070 --> 00:02:19,390 ‫out of the box. 53 00:02:19,390 --> 00:02:21,160 ‫So, all we have to do is to write 54 00:02:21,160 --> 00:02:24,010 ‫a global express error handling middleware 55 00:02:24,010 --> 00:02:26,090 ‫which will then catch errors coming 56 00:02:26,090 --> 00:02:28,050 ‫from all over the application. 57 00:02:28,050 --> 00:02:31,260 ‫So, no matter if it's an error coming from a route handler, 58 00:02:31,260 --> 00:02:34,950 ‫or a model validator or really, someplace else, 59 00:02:34,950 --> 00:02:37,240 ‫the goal is that all these errors end up 60 00:02:37,240 --> 00:02:39,920 ‫in one central error handling middleware. 61 00:02:39,920 --> 00:02:43,610 ‫So that we can send a nice response back to the client 62 00:02:43,610 --> 00:02:45,510 ‫letting them know what happened. 63 00:02:45,510 --> 00:02:48,340 ‫And so, really, handling in this case just means 64 00:02:48,340 --> 00:02:52,040 ‫sending a response letting the user know what happened. 65 00:02:52,040 --> 00:02:55,050 ‫But handling can also mean, in other cases, 66 00:02:55,050 --> 00:02:58,220 ‫retrying the operation or crashing the server, 67 00:02:58,220 --> 00:03:00,960 ‫or just ignoring the error altogether. 68 00:03:00,960 --> 00:03:03,980 ‫Sometimes, that's just the best option. 69 00:03:03,980 --> 00:03:07,470 ‫Now the beauty of having a global error handling middleware 70 00:03:07,470 --> 00:03:10,830 ‫is that it allows for a nice separation of concerns. 71 00:03:10,830 --> 00:03:13,170 ‫So, we don't have to worry about error handling 72 00:03:13,170 --> 00:03:16,220 ‫right in our business logic or our controllers, 73 00:03:16,220 --> 00:03:18,750 ‫or really anywhere in our application. 74 00:03:18,750 --> 00:03:22,090 ‫We can simply send the errors down to the error handler 75 00:03:22,090 --> 00:03:25,090 ‫which will then decide what to do with them next. 76 00:03:25,090 --> 00:03:26,340 ‫All right? 77 00:03:26,340 --> 00:03:28,170 ‫So, with all that being said, 78 00:03:28,170 --> 00:03:30,463 ‫let's now actually start implementing this.