1
00:00:01,070 --> 00:00:01,950
‫All right!

2
00:00:01,950 --> 00:00:04,040
‫And now to finish this section,

3
00:00:04,040 --> 00:00:07,043
‫let's learn how to catch uncaught exceptions.

4
00:00:08,660 --> 00:00:11,930
‫But what exactly are uncaught exceptions?

5
00:00:11,930 --> 00:00:15,290
‫Well, all errors, or let's also call them bugs,

6
00:00:15,290 --> 00:00:17,280
‫that occur in our synchronous code

7
00:00:17,280 --> 00:00:21,230
‫but are not handled anywhere are called uncaught exceptions.

8
00:00:21,230 --> 00:00:25,340
‫And like before, so just like with the unhandled rejections,

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00:00:25,340 --> 00:00:28,730
‫we also have a way of handling uncaught exceptions.

10
00:00:28,730 --> 00:00:30,720
‫So let me quickly show you

11
00:00:30,720 --> 00:00:32,823
‫an example of an uncaught exception.

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00:00:34,370 --> 00:00:37,400
‫So let's simply pretend that we're trying

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00:00:37,400 --> 00:00:40,423
‫to console.log something that doesn't exist.

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00:00:42,110 --> 00:00:46,600
‫And so right away, we get this error here, x is not defined,

15
00:00:46,600 --> 00:00:50,080
‫and then by default, we get this entire stack trace here

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00:00:50,080 --> 00:00:51,960
‫printed to the console.

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00:00:51,960 --> 00:00:52,793
‫All right.

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00:00:52,793 --> 00:00:57,660
‫And now, to fix this, this is very similar to doing

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00:00:57,660 --> 00:00:59,380
‫the unhandled rejections.

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00:00:59,380 --> 00:01:02,040
‫So again, we're listening to an event,

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00:01:02,040 --> 00:01:04,040
‫this time it's called uncaughtException.

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00:01:08,662 --> 00:01:11,912
‫And of course it needs to be in quotes.

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00:01:12,840 --> 00:01:17,390
‫And then just like before, we pass in our callback function.

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00:01:17,390 --> 00:01:18,223
‫Okay.

25
00:01:18,223 --> 00:01:20,960
‫And now actually what we're doing here is very similar

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00:01:20,960 --> 00:01:22,350
‫to what we did here.

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00:01:22,350 --> 00:01:24,970
‫So we wanna lock the error to the console

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00:01:24,970 --> 00:01:29,690
‫so that it then shows up in the logs in our server, okay?

29
00:01:29,690 --> 00:01:32,430
‫So, giving us a way of then fixing the problem.

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00:01:32,430 --> 00:01:35,223
‫And then we want to gracefully shut down the server.

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00:01:36,750 --> 00:01:39,010
‫So let's, well actually here I want

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00:01:39,010 --> 00:01:41,590
‫to print the entire error to the console,

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00:01:41,590 --> 00:01:43,940
‫and not just the message, okay?

34
00:01:43,940 --> 00:01:45,993
‫And let's also change the order here,

35
00:01:47,300 --> 00:01:48,660
‫but that's it.

36
00:01:48,660 --> 00:01:50,140
‫So the rest is the same.

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00:01:50,140 --> 00:01:51,290
‫And here it's of course

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00:01:53,310 --> 00:01:58,310
‫UNCAUGHT EXCEPTION!

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00:01:58,320 --> 00:02:01,810
‫Now I could refactor this here into a nice function,

40
00:02:01,810 --> 00:02:05,020
‫but let's just keep it like this, all right?

41
00:02:05,020 --> 00:02:07,123
‫Okay, so let's now rerun this.

42
00:02:08,330 --> 00:02:11,090
‫And so let's take a look at what we have here.

43
00:02:11,090 --> 00:02:13,700
‫And indeed, we now see UNCAUGHT EXCEPTION!

44
00:02:13,700 --> 00:02:14,780
‫Shutting down.

45
00:02:14,780 --> 00:02:15,750
‫All right?

46
00:02:15,750 --> 00:02:18,770
‫And actually this again prints the entire stack

47
00:02:18,770 --> 00:02:21,400
‫to the console, but I don't really want that.

48
00:02:21,400 --> 00:02:22,660
‫I don't like that.

49
00:02:22,660 --> 00:02:25,860
‫So let's just actually go back to what we had here.

50
00:02:25,860 --> 00:02:28,683
‫So err.name and then err.message.

51
00:02:30,890 --> 00:02:32,610
‫Okay, and so now we have the ReferenceError

52
00:02:32,610 --> 00:02:36,290
‫that x is not defined, okay?

53
00:02:36,290 --> 00:02:38,520
‫And let's just put the same here.

54
00:02:38,520 --> 00:02:39,983
‫So just like it was before.

55
00:02:40,950 --> 00:02:41,783
‫All right.

56
00:02:41,783 --> 00:02:43,070
‫And that's actually all there is

57
00:02:43,070 --> 00:02:45,370
‫to catching uncaught exceptions.

58
00:02:45,370 --> 00:02:48,180
‫Now while here in the unhandled rejection,

59
00:02:48,180 --> 00:02:51,230
‫crashing the application like we did here is optional,

60
00:02:51,230 --> 00:02:53,630
‫when there is an uncaught exception,

61
00:02:53,630 --> 00:02:57,150
‫we really, really need to crash our application

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00:02:57,150 --> 00:02:59,970
‫because after there was an uncaught exception,

63
00:02:59,970 --> 00:03:01,610
‫the entire node process is

64
00:03:01,610 --> 00:03:04,370
‫in a so-called unclean state, all right?

65
00:03:04,370 --> 00:03:07,770
‫And so to fix that, the process need to terminate

66
00:03:07,770 --> 00:03:09,940
‫and then to be restarted, all right?

67
00:03:09,940 --> 00:03:11,330
‫And again, in production,

68
00:03:11,330 --> 00:03:13,460
‫we should then have a tool in place

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00:03:13,460 --> 00:03:16,640
‫which will restart the application after crashing.

70
00:03:16,640 --> 00:03:19,270
‫And many hosting services already do that

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00:03:19,270 --> 00:03:20,860
‫out of the box, all right?

72
00:03:20,860 --> 00:03:23,170
‫So, completely automatically without

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00:03:23,170 --> 00:03:24,980
‫us having to do anything.

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00:03:24,980 --> 00:03:27,820
‫Now, in Node.js, it's not really a good practice

75
00:03:27,820 --> 00:03:31,580
‫to just blindly rely on these two error handlers

76
00:03:31,580 --> 00:03:34,170
‫that we just implemented here, okay?

77
00:03:34,170 --> 00:03:36,470
‫So ideally errors should really be handled

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00:03:36,470 --> 00:03:38,120
‫right where they occur.

79
00:03:38,120 --> 00:03:42,150
‫So for example, in the problem connecting to the database,

80
00:03:42,150 --> 00:03:44,660
‫we should of course add a catch handler there

81
00:03:44,660 --> 00:03:46,480
‫and not just simply rely on the

82
00:03:46,480 --> 00:03:50,160
‫unhandled rejection callback that we have here, okay?

83
00:03:50,160 --> 00:03:53,600
‫And some people even say that we shouldn't use these at all,

84
00:03:53,600 --> 00:03:54,770
‫but I disagree with that.

85
00:03:54,770 --> 00:03:58,090
‫I think that as a safety net, let's say,

86
00:03:58,090 --> 00:04:02,470
‫they can be very useful and play a part in our application.

87
00:04:02,470 --> 00:04:04,020
‫Now actually this handler here

88
00:04:04,020 --> 00:04:07,300
‫should be at the very top of our code, okay?

89
00:04:07,300 --> 00:04:11,280
‫Or at least before any other code is really executed.

90
00:04:11,280 --> 00:04:14,980
‫Because watch what happens if I move this line of code here

91
00:04:14,980 --> 00:04:16,513
‫before this handler.

92
00:04:17,350 --> 00:04:19,593
‫So if I put it here, for example,

93
00:04:21,140 --> 00:04:23,070
‫and then give it a save,

94
00:04:23,070 --> 00:04:24,680
‫then you actually see that our handler

95
00:04:24,680 --> 00:04:29,410
‫does not catch this exception, so this error here, right?

96
00:04:29,410 --> 00:04:31,830
‫And so that's because only here at the end

97
00:04:31,830 --> 00:04:35,010
‫we actually start listening for an uncaught exception.

98
00:04:35,010 --> 00:04:37,580
‫But in this case here, the uncaught exception

99
00:04:37,580 --> 00:04:40,380
‫happens before we even listen to that event.

100
00:04:40,380 --> 00:04:44,630
‫And so therefore, we have no way of catching it, right?

101
00:04:44,630 --> 00:04:48,920
‫And so we should ideally put it here, right at the top,

102
00:04:48,920 --> 00:04:51,030
‫again before any other code executes.

103
00:04:51,030 --> 00:04:53,423
‫Especially the one in our application.

104
00:04:54,580 --> 00:04:57,760
‫So let's put it actually right here.

105
00:04:57,760 --> 00:04:59,550
‫Now the problem here will be

106
00:04:59,550 --> 00:05:02,820
‫that the server is not defined at this point.

107
00:05:02,820 --> 00:05:04,160
‫But that's not a problem,

108
00:05:04,160 --> 00:05:06,180
‫because actually we don't need the server here

109
00:05:06,180 --> 00:05:08,060
‫at all, all right?

110
00:05:08,060 --> 00:05:09,480
‫And that's because these errors,

111
00:05:09,480 --> 00:05:11,270
‫so these uncaught exceptions,

112
00:05:11,270 --> 00:05:13,670
‫they are not gonna happen asynchronously.

113
00:05:13,670 --> 00:05:15,630
‫So they are not gonna have anything to do

114
00:05:15,630 --> 00:05:17,660
‫with the server actually.

115
00:05:17,660 --> 00:05:19,360
‫So we don't need this here at all.

116
00:05:20,470 --> 00:05:22,250
‫We can just save it here,

117
00:05:22,250 --> 00:05:25,480
‫and so now we have it before we actually require

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00:05:25,480 --> 00:05:26,890
‫our main application.

119
00:05:26,890 --> 00:05:28,663
‫And so if we now have an error,

120
00:05:29,940 --> 00:05:31,850
‫let's try it first here,

121
00:05:31,850 --> 00:05:33,500
‫so you see that now we're back

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00:05:33,500 --> 00:05:36,710
‫to actually catching it in our error handler.

123
00:05:36,710 --> 00:05:41,333
‫But if we now had this code, for example inside of app.js,

124
00:05:42,350 --> 00:05:45,810
‫let's say, it doesn't really matter,

125
00:05:45,810 --> 00:05:47,410
‫let's just say it's here,

126
00:05:47,410 --> 00:05:50,260
‫and if we now run this, well then you see

127
00:05:50,260 --> 00:05:52,530
‫that we're still catching that exception

128
00:05:52,530 --> 00:05:56,290
‫in our error handler, which before would not be the case.

129
00:05:56,290 --> 00:05:57,180
‫All right?

130
00:05:57,180 --> 00:05:59,590
‫And now just for an experiment,

131
00:05:59,590 --> 00:06:03,120
‫what do you think if we put this code

132
00:06:03,120 --> 00:06:06,100
‫right here in this middleware function?

133
00:06:06,100 --> 00:06:07,690
‫What do you think is gonna happen

134
00:06:07,690 --> 00:06:09,510
‫when we save this file now?

135
00:06:09,510 --> 00:06:12,200
‫So x is still not defined anywhere,

136
00:06:12,200 --> 00:06:13,840
‫but let's take a look at what happens

137
00:06:13,840 --> 00:06:15,373
‫when we now run this code.

138
00:06:16,980 --> 00:06:19,510
‫And so now we actually have no error.

139
00:06:19,510 --> 00:06:21,300
‫Or actually we do, but that's only

140
00:06:21,300 --> 00:06:24,130
‫because of our unhandled rejection,

141
00:06:24,130 --> 00:06:26,020
‫which is due to the fact

142
00:06:26,020 --> 00:06:27,920
‫that our password here is still wrong.

143
00:06:29,290 --> 00:06:32,170
‫So if we save that and now save it again,

144
00:06:32,170 --> 00:06:34,560
‫you should see no error at all.

145
00:06:34,560 --> 00:06:35,730
‫And indeed, we don't.

146
00:06:35,730 --> 00:06:36,953
‫And why is that?

147
00:06:37,830 --> 00:06:40,940
‫Well, it's because this middleware function here of course

148
00:06:40,940 --> 00:06:42,450
‫is only called as soon

149
00:06:42,450 --> 00:06:45,220
‫as there actually is a request, right?

150
00:06:45,220 --> 00:06:48,113
‫And so let's see what happens when we do a request.

151
00:06:50,760 --> 00:06:54,180
‫And so let's do a Get All Tours request.

152
00:06:54,180 --> 00:06:55,720
‫And let's wait for it.

153
00:06:55,720 --> 00:06:58,980
‫And, here we get a "Something went very wrong!"

154
00:06:58,980 --> 00:07:01,420
‫error message with the 500,

155
00:07:01,420 --> 00:07:03,560
‫and that's also because we are still

156
00:07:03,560 --> 00:07:05,820
‫in production mode, okay?

157
00:07:05,820 --> 00:07:08,990
‫But anyway, getting this message here, remember,

158
00:07:08,990 --> 00:07:12,120
‫means that we have a nonoperational error here.

159
00:07:12,120 --> 00:07:15,630
‫So basically an error that we did not create ourselves.

160
00:07:15,630 --> 00:07:19,580
‫And so, right this, we now enter here

161
00:07:19,580 --> 00:07:24,380
‫in our errorController, we go right here, right?

162
00:07:24,380 --> 00:07:28,780
‫So into this else branch here in the sendErrorProd function.

163
00:07:28,780 --> 00:07:30,380
‫And actually you see the error

164
00:07:30,380 --> 00:07:32,970
‫down here in the console as well, okay?

165
00:07:32,970 --> 00:07:35,600
‫And so that's what happens when there is an error

166
00:07:35,600 --> 00:07:39,430
‫inside of any Express middleware, right?

167
00:07:39,430 --> 00:07:41,370
‫So Express, when there is an error,

168
00:07:41,370 --> 00:07:44,480
‫will automatically go to the error-handling middleware

169
00:07:44,480 --> 00:07:46,330
‫with that error, okay?

170
00:07:46,330 --> 00:07:48,100
‫And so that's why when we get an error

171
00:07:48,100 --> 00:07:50,410
‫here in any of our middleware function,

172
00:07:50,410 --> 00:07:55,240
‫it will immediately go here into this handler, okay?

173
00:07:55,240 --> 00:07:56,520
‫And since we're in production,

174
00:07:56,520 --> 00:07:58,730
‫it will enter this block here,

175
00:07:58,730 --> 00:08:00,994
‫but since it's not a CastError,

176
00:08:00,994 --> 00:08:03,700
‫and not this error, and not a ValidationError,

177
00:08:03,700 --> 00:08:06,830
‫then as soon as the error is actually sent,

178
00:08:06,830 --> 00:08:09,640
‫right here, we then enter this block.

179
00:08:09,640 --> 00:08:14,640
‫And again, that's why we send this generic error, all right?

180
00:08:15,235 --> 00:08:17,500
‫Let's now actually exit this mode here

181
00:08:17,500 --> 00:08:20,053
‫and go back to our normal npm start.

182
00:08:22,310 --> 00:08:26,130
‫And so right now, our error will be sent like this, right?

183
00:08:26,130 --> 00:08:29,440
‫So send development error will give us all the details

184
00:08:29,440 --> 00:08:31,773
‫about the error that is happening.

185
00:08:33,610 --> 00:08:38,490
‫And so now indeed we get ReferenceError: x is not defined.

186
00:08:38,490 --> 00:08:39,323
‫Okay?

187
00:08:39,323 --> 00:08:41,533
‫So in app.js, line 21.

188
00:08:42,480 --> 00:08:46,300
‫And so, line 21 is where it is,

189
00:08:46,300 --> 00:08:47,520
‫but let's get rid of it.

190
00:08:47,520 --> 00:08:49,760
‫I just wanted to show it to you.

191
00:08:49,760 --> 00:08:51,350
‫Okay?

192
00:08:51,350 --> 00:08:53,770
‫Anyway, with this, we actually close

193
00:08:53,770 --> 00:08:55,620
‫our error handling section.

194
00:08:55,620 --> 00:08:58,830
‫And there could be an entire course on error handling

195
00:08:58,830 --> 00:09:01,940
‫with Node and Express, because it really is

196
00:09:01,940 --> 00:09:03,830
‫an insanely complex topic.

197
00:09:03,830 --> 00:09:06,470
‫But, in this section, you learn the essentials

198
00:09:06,470 --> 00:09:08,830
‫that you need to know in order to really get started

199
00:09:08,830 --> 00:09:10,943
‫building amazing applications.