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These are the user uploaded subtitles that are being translated: 1 1 00:00:01,180 --> 00:00:03,480 So in this lecture, we're gonna fix 2 2 00:00:03,480 --> 00:00:06,730 the request 404 error that we saw happening 3 3 00:00:06,730 --> 00:00:08,093 in the last lecture. 4 4 00:00:09,910 --> 00:00:13,090 And so as we saw in the last video, the problem here 5 5 00:00:13,090 --> 00:00:17,490 is that during the fetch, there was a 404 error, 6 6 00:00:17,490 --> 00:00:21,080 which is because our API couldn't find any country 7 7 00:00:21,080 --> 00:00:23,060 with this name. 8 8 00:00:23,060 --> 00:00:24,770 But still, even though there 9 9 00:00:24,770 --> 00:00:28,320 was obviously a big problem with this request, 10 10 00:00:28,320 --> 00:00:32,770 the fetch function still did not reject in this case, 11 11 00:00:32,770 --> 00:00:34,800 and by the way, many people, 12 12 00:00:34,800 --> 00:00:38,200 and that includes myself think that in this case, 13 13 00:00:38,200 --> 00:00:41,820 the promise should actually be rejected right away, 14 14 00:00:41,820 --> 00:00:43,880 but again, it just doesn't, 15 15 00:00:43,880 --> 00:00:45,823 and so we will have to do it manually. 16 16 00:00:47,660 --> 00:00:52,160 So to see what happens here, let's go back 17 17 00:00:52,160 --> 00:00:57,160 to this first then handler here, which gets access 18 18 00:00:57,480 --> 00:00:59,143 to the response immediately. 19 19 00:01:00,010 --> 00:01:01,543 So without even the data. 20 20 00:01:03,020 --> 00:01:05,460 So here, I'm adding back a block, 21 21 00:01:05,460 --> 00:01:07,903 and let's take a look at the response here. 22 22 00:01:08,870 --> 00:01:11,750 And also, since we added back the block, 23 23 00:01:11,750 --> 00:01:14,683 we now need to return this explicitly. 24 24 00:01:15,640 --> 00:01:18,320 So again, for now, we are just taking a look 25 25 00:01:18,320 --> 00:01:20,093 at the response object here. 26 26 00:01:21,280 --> 00:01:23,100 And so here it is, 27 27 00:01:23,100 --> 00:01:25,733 and so right here, you can see 28 28 00:01:25,733 --> 00:01:29,110 that the Ok property is set to false. 29 29 00:01:29,110 --> 00:01:30,840 And so the reason for that, 30 30 00:01:30,840 --> 00:01:32,703 is, of course, the status code 404. 31 31 00:01:34,270 --> 00:01:37,310 Now, when the request goes well, 32 32 00:01:37,310 --> 00:01:39,910 and so that's what's gonna happen when I click here. 33 33 00:01:41,940 --> 00:01:46,070 So then if we take a look at the response, Ok is true, 34 34 00:01:46,070 --> 00:01:48,960 and that's because status is 200. 35 35 00:01:48,960 --> 00:01:51,883 And so 200 literally stands for Ok. 36 36 00:01:55,410 --> 00:01:57,140 So we can now use the fact 37 37 00:01:57,140 --> 00:02:01,102 that this response has the Ok property set to false 38 38 00:02:01,102 --> 00:02:03,330 to basically reject the promise, 39 39 00:02:03,330 --> 00:02:07,510 or selves manually, now, right, 40 40 00:02:07,510 --> 00:02:11,050 so we can do that by creating a new error. 41 41 00:02:11,050 --> 00:02:16,050 So we can say, if there is no response, dot Ok, 42 42 00:02:17,310 --> 00:02:21,130 or in other words, if response dot Ok is false, 43 43 00:02:21,130 --> 00:02:26,130 then we right, throw new error, 44 44 00:02:26,252 --> 00:02:29,653 and then here we can define an error message. 45 45 00:02:31,250 --> 00:02:34,230 So let's write a very logical message here, 46 46 00:02:34,230 --> 00:02:37,880 which is country not found. 47 47 00:02:37,880 --> 00:02:41,370 So this is the real error message that we want to see here, 48 48 00:02:41,370 --> 00:02:43,730 and of course, not this one here 49 49 00:02:43,730 --> 00:02:47,661 with cannot read property of undefined. 50 50 00:02:47,661 --> 00:02:51,700 And here, we can then also take the status code, 51 51 00:02:51,700 --> 00:02:53,173 and display it as well. 52 52 00:02:55,530 --> 00:02:59,040 So this 404, in this case, let's just wrap it here 53 53 00:02:59,040 --> 00:03:03,640 into parenthesis, and Ok. 54 54 00:03:03,640 --> 00:03:06,860 So this is something that we never did before. 55 55 00:03:06,860 --> 00:03:08,863 So let's analyze what happens here. 56 56 00:03:09,820 --> 00:03:11,516 So we create the new error 57 57 00:03:11,516 --> 00:03:15,890 by using again, this constructor function, basically, 58 58 00:03:15,890 --> 00:03:18,540 and then we pass in a message, which is gonna be 59 59 00:03:18,540 --> 00:03:22,550 the error message, then we use the throw keyword here, 60 60 00:03:22,550 --> 00:03:25,990 which will immediately terminate the current function. 61 61 00:03:25,990 --> 00:03:28,850 So just like return does it. 62 62 00:03:28,850 --> 00:03:30,670 Now the effect of creating, 63 63 00:03:30,670 --> 00:03:34,290 and throwing an error in any of these then methods 64 64 00:03:34,290 --> 00:03:37,770 is that the promise will immediately reject. 65 65 00:03:37,770 --> 00:03:40,310 So basically, the promise returned 66 66 00:03:40,310 --> 00:03:44,097 by this then handler here will be a rejected promise. 67 67 00:03:44,097 --> 00:03:47,026 And that rejection will then propagate all the way down 68 68 00:03:47,026 --> 00:03:51,417 to the catch handler, which we already have set up here. 69 69 00:03:51,417 --> 00:03:55,200 And so now, if we will try to reload this, 70 70 00:03:55,200 --> 00:03:56,600 and actually, let's do that. 71 71 00:03:57,830 --> 00:04:01,230 So now this error message that we created here, 72 72 00:04:01,230 --> 00:04:04,300 so that's country not found 404. 73 73 00:04:04,300 --> 00:04:07,880 So exactly this, this is exactly the rejection 74 74 00:04:07,880 --> 00:04:11,530 that we created here by creating this new error. 75 75 00:04:11,530 --> 00:04:14,950 All right, so again, any error that happens 76 76 00:04:14,950 --> 00:04:16,985 in any of the callback functions here, 77 77 00:04:16,985 --> 00:04:20,550 so in any then handler, will immediately terminate 78 78 00:04:20,550 --> 00:04:23,170 that then handler and will propagate down 79 79 00:04:23,170 --> 00:04:26,630 to the catch method here. 80 80 00:04:26,630 --> 00:04:28,737 And then in there, we handle that error, 81 81 00:04:28,737 --> 00:04:31,167 and so therefore, that's why we then see 82 82 00:04:31,167 --> 00:04:32,653 the error here displayed. 83 83 00:04:33,820 --> 00:04:37,009 So that error dot message is exactly that message 84 84 00:04:37,009 --> 00:04:39,708 that we pass into the error here. 85 85 00:04:39,708 --> 00:04:43,604 And in fact, the same is true for any other error. 86 86 00:04:43,604 --> 00:04:46,250 So before we had this, 87 87 00:04:46,250 --> 00:04:49,830 the error that we saw was this one here, 88 88 00:04:49,830 --> 00:04:51,340 and that's because somewhere 89 89 00:04:51,340 --> 00:04:54,257 in the render country function, we are trying to read 90 90 00:04:54,257 --> 00:04:57,295 the flag from the data that we received, 91 91 00:04:57,295 --> 00:05:01,710 but the data that we received did not contain this flag. 92 92 00:05:01,710 --> 00:05:03,589 And so therefore it created this error, 93 93 00:05:03,589 --> 00:05:07,748 and then that error caused the rejection of the promise, 94 94 00:05:07,748 --> 00:05:12,748 which was then one more time handled down here, okay. 95 95 00:05:12,836 --> 00:05:17,836 So again, any error will cause any promise to reject, 96 96 00:05:18,040 --> 00:05:20,719 but here, we are simply creating our own error 97 97 00:05:20,719 --> 00:05:24,140 to basically reject the promise on purpose, 98 98 00:05:24,140 --> 00:05:25,540 so that we can then handle 99 99 00:05:25,540 --> 00:05:27,742 that error down here in the chain, 100 100 00:05:27,742 --> 00:05:31,240 so in this catch method, all right. 101 101 00:05:31,240 --> 00:05:32,874 But now, you might be wondering, 102 102 00:05:32,874 --> 00:05:36,650 why should we even bother handle all these errors? 103 103 00:05:36,650 --> 00:05:40,460 Isn't that just a bunch of work and a waste of time? 104 104 00:05:40,460 --> 00:05:44,240 Well, first, handling these errors is the only way 105 105 00:05:44,240 --> 00:05:47,902 in which we can actually display an error message like this 106 106 00:05:47,902 --> 00:05:50,380 on the screen for the user, 107 107 00:05:50,380 --> 00:05:54,050 but even more important, it's just a really bad practice 108 108 00:05:54,050 --> 00:05:57,140 to leave these rejected promises, 109 109 00:05:57,140 --> 00:05:59,790 hanging around without handling them. 110 110 00:05:59,790 --> 00:06:03,310 So don't do that, always use catch, 111 111 00:06:03,310 --> 00:06:07,013 and if necessary, you can also use finally, okay. 112 112 00:06:08,900 --> 00:06:13,780 And now, again, what if there 113 113 00:06:13,780 --> 00:06:16,363 was no error here in this promise? 114 114 00:06:17,770 --> 00:06:20,810 So if we got no problem in this one here, 115 115 00:06:20,810 --> 00:06:24,186 but then we get a problem in the second fetch. 116 116 00:06:24,186 --> 00:06:28,520 So let's say that, we don't do this, 117 117 00:06:28,520 --> 00:06:30,130 and so we have a reasonable country, 118 118 00:06:30,130 --> 00:06:32,766 which is Portugal when we click on the button, 119 119 00:06:32,766 --> 00:06:35,963 but then here, let's say that the neighbor is not this one, 120 120 00:06:37,030 --> 00:06:42,030 but this country code, which I'm sure does not exist. 121 121 00:06:44,320 --> 00:06:46,470 And so now there is gonna be a rejection 122 122 00:06:46,470 --> 00:06:49,483 in this promise, right. 123 123 00:06:51,970 --> 00:06:53,800 So we get another error here. 124 124 00:06:53,800 --> 00:06:58,600 In this case, it's a 400 error, which means something else, 125 125 00:06:58,600 --> 00:07:02,350 but in any case, the error is not handled. 126 126 00:07:02,350 --> 00:07:03,890 So we now need to go ahead, 127 127 00:07:03,890 --> 00:07:08,263 and copy this code here, also into this then handler. 128 128 00:07:09,140 --> 00:07:10,921 Let's actually do that, even though we know 129 129 00:07:10,921 --> 00:07:15,423 it's a bad practice, and we need to fix this later. 130 130 00:07:19,420 --> 00:07:24,420 Okay, and so now we get the error message that we want. 131 131 00:07:26,070 --> 00:07:28,240 So again, country not found, 132 132 00:07:28,240 --> 00:07:30,720 and this time with the code 400. 133 133 00:07:30,720 --> 00:07:33,883 But now, of course, we have all this duplicate code here. 134 134 00:07:34,880 --> 00:07:37,530 So we shouldn't have this here. 135 135 00:07:37,530 --> 00:07:41,510 And in fact, even all of this is kind of repetitive. 136 136 00:07:41,510 --> 00:07:44,000 It's really, if you think about it 137 137 00:07:44,000 --> 00:07:46,281 the same code as this one. 138 138 00:07:46,281 --> 00:07:49,010 And so now, I think that it's a good time 139 139 00:07:49,010 --> 00:07:53,070 to actually create ourselves a really nice helper function. 140 140 00:07:53,070 --> 00:07:55,270 And this helper function will wrap up 141 141 00:07:55,270 --> 00:07:57,110 the fetch the error handling, 142 142 00:07:57,110 --> 00:07:59,891 and also the conversion to JSON, 143 143 00:07:59,891 --> 00:08:03,560 because in my opinion, it's really a bit cumbersome 144 144 00:08:03,560 --> 00:08:06,343 to having to do all these steps all the time. 145 145 00:08:07,450 --> 00:08:10,730 So having the fetch, and then having to convert 146 146 00:08:10,730 --> 00:08:12,212 that response to JSON, 147 147 00:08:12,212 --> 00:08:16,870 and only then being able to handle it, right. 148 148 00:08:16,870 --> 00:08:20,460 So instead, we will basically encapsulate all of this here 149 149 00:08:20,460 --> 00:08:23,706 into one nice function, so let's do that, 150 150 00:08:23,706 --> 00:08:26,343 and I'm gonna call it getJSON. 151 151 00:08:29,360 --> 00:08:31,930 So basically, because it gets data, 152 152 00:08:31,930 --> 00:08:35,332 and it immediately converts to JSON. 153 153 00:08:35,332 --> 00:08:38,123 Now, what do we need here as parameters? 154 154 00:08:38,980 --> 00:08:41,670 Well, for sure, we need the URL, 155 155 00:08:41,670 --> 00:08:44,543 and so for now, that's okay, here. 156 156 00:08:46,090 --> 00:08:51,053 So let's get, or actually, we can just write it. 157 157 00:08:52,150 --> 00:08:54,370 So what we want here is fetch, 158 158 00:08:54,370 --> 00:08:57,820 and then the URL that we pass into the function, 159 159 00:08:57,820 --> 00:09:02,220 and then on that we want to call then, 160 160 00:09:02,220 --> 00:09:04,453 and then basically with all of this. 161 161 00:09:06,370 --> 00:09:08,523 So let's grab it here. 162 162 00:09:12,180 --> 00:09:16,520 And that, of course, we are missing 163 163 00:09:18,650 --> 00:09:20,353 the callback function itself here, 164 164 00:09:23,640 --> 00:09:26,733 so that should end somewhere here, 165 165 00:09:27,950 --> 00:09:31,263 and, well, apparently, it doesn't. 166 166 00:09:34,610 --> 00:09:39,480 Maybe like this, and yes, that's correct. 167 167 00:09:39,480 --> 00:09:41,170 All right, now in order 168 168 00:09:41,170 --> 00:09:44,650 to actually make this function her really generic, 169 169 00:09:44,650 --> 00:09:47,647 we do not want to hard code the error message, 170 170 00:09:47,647 --> 00:09:51,120 but instead, we want to pass the message in. 171 171 00:09:51,120 --> 00:09:53,900 So let's say error message, 172 172 00:09:53,900 --> 00:09:56,033 and we can set a nice default value. 173 173 00:09:57,470 --> 00:10:01,320 So let's say something went wrong, 174 174 00:10:01,320 --> 00:10:03,580 which again is very generic, 175 175 00:10:03,580 --> 00:10:07,030 and so here, inside of this, we then want 176 176 00:10:09,560 --> 00:10:12,060 the error message that we passed in, 177 177 00:10:12,060 --> 00:10:16,010 and along with response dot status. 178 178 00:10:16,010 --> 00:10:20,941 Okay, and now all we need to do is to return all of this. 179 179 00:10:20,941 --> 00:10:22,430 And so like this, 180 180 00:10:22,430 --> 00:10:26,720 this getJSON function will actually return a promise. 181 181 00:10:26,720 --> 00:10:29,585 And so this is then just like any other promise 182 182 00:10:29,585 --> 00:10:32,460 that we can call here in our chain, 183 183 00:10:32,460 --> 00:10:35,233 and basically replace all of this. 184 184 00:10:36,740 --> 00:10:41,563 So just so we can keep this code, I'm gonna copy it. 185 185 00:10:42,479 --> 00:10:46,680 And so you can then keep this code here, 186 186 00:10:46,680 --> 00:10:50,480 basically as a reference for when you need 187 187 00:10:50,480 --> 00:10:53,770 to study it again, okay. 188 188 00:10:53,770 --> 00:10:57,300 But again, I will now go ahead and take everything 189 189 00:10:57,300 --> 00:11:01,247 that we just nicely encapsulated into our getJSON function, 190 190 00:11:01,247 --> 00:11:03,313 and simply replace this. 191 191 00:11:05,510 --> 00:11:10,510 So getJSON, and then the URL is this one, 192 192 00:11:14,260 --> 00:11:18,533 and the error message is country not found, now, right. 193 193 00:11:22,840 --> 00:11:25,603 And so now, we can take away all of this. 194 194 00:11:27,590 --> 00:11:30,080 So this is already in our get JSON function, 195 195 00:11:30,080 --> 00:11:32,363 and so we don't need this. 196 196 00:11:34,440 --> 00:11:37,940 All right, and now the same down here. 197 197 00:11:37,940 --> 00:11:40,800 Instead of fetch, we can get JSON, 198 198 00:11:40,800 --> 00:11:45,440 and then we don't need this done here. 199 199 00:11:45,440 --> 00:11:49,723 And again, the error message is country not found. 200 200 00:11:53,750 --> 00:11:57,923 Okay, and so let's get rid of this, now, right. 201 201 00:12:02,430 --> 00:12:04,041 So let's check here, 202 202 00:12:04,041 --> 00:12:07,980 and indeed, we get, again, country not found 203 203 00:12:07,980 --> 00:12:10,823 with code 400 because of this one. 204 204 00:12:13,200 --> 00:12:15,010 So that's working great, 205 205 00:12:15,010 --> 00:12:16,460 but there's still one more thing 206 206 00:12:16,460 --> 00:12:19,650 that we need to do, which is to handle the fact 207 207 00:12:19,650 --> 00:12:22,550 that sometimes there might be no neighbor. 208 208 00:12:22,550 --> 00:12:24,570 So right now, what we're doing here when there 209 209 00:12:24,570 --> 00:12:27,544 is no neighbor, is to simply return, 210 210 00:12:27,544 --> 00:12:31,460 but as I mentioned before, that doesn't really do anything. 211 211 00:12:31,460 --> 00:12:36,460 So let me just try here, a country that has no neighbors. 212 212 00:12:36,694 --> 00:12:41,260 So let's say Australia, because that is an island, 213 213 00:12:41,260 --> 00:12:45,465 and so here again, we get cannot read property flag 214 214 00:12:45,465 --> 00:12:48,515 of undefined, and so that is happening, 215 215 00:12:48,515 --> 00:12:52,593 because it is trying to render a country that doesn't exist. 216 216 00:12:55,080 --> 00:12:58,951 Okay, so again, it's because of this neighbor, 217 217 00:12:58,951 --> 00:13:03,390 and just to make sure, to illustrate my point here, 218 218 00:13:03,390 --> 00:13:06,820 you see that the neighbor is indeed undefined. 219 219 00:13:06,820 --> 00:13:08,400 And so what we want to do here 220 220 00:13:08,400 --> 00:13:11,815 is to simply just like before, throw a new error 221 221 00:13:11,815 --> 00:13:14,103 that will then get caught down here 222 222 00:13:14,103 --> 00:13:17,593 in our catch handler, right. 223 223 00:13:20,390 --> 00:13:25,390 So throw new error, no neighbor, found, 224 224 00:13:29,180 --> 00:13:31,860 and that's a lot better. 225 225 00:13:31,860 --> 00:13:34,680 So now, this is a real error message 226 226 00:13:34,680 --> 00:13:37,570 that actually makes sense to the user. 227 227 00:13:37,570 --> 00:13:39,976 Of course, this could be all a bit better, 228 228 00:13:39,976 --> 00:13:43,080 but this is just a demonstration anyway, 229 229 00:13:43,080 --> 00:13:43,913 but I think that 230 230 00:13:43,913 --> 00:13:47,660 it nicely demonstrate how we can create a real error message 231 231 00:13:47,660 --> 00:13:49,860 that does actually make sense. 232 232 00:13:49,860 --> 00:13:51,860 And again, that is super important 233 233 00:13:51,860 --> 00:13:54,980 for any user interface that you're building, 234 234 00:13:54,980 --> 00:13:58,710 because in web applications like this very small one, 235 235 00:13:58,710 --> 00:14:01,770 errors will happen, that is just guaranteed, 236 236 00:14:01,770 --> 00:14:04,200 and so your application needs to be ready, 237 237 00:14:04,200 --> 00:14:06,825 and needs to be prepared for that. 238 238 00:14:06,825 --> 00:14:10,930 All right, so let's now quickly recap here. 239 239 00:14:10,930 --> 00:14:13,786 And I think that the big takeaway from this lecture 240 240 00:14:13,786 --> 00:14:17,124 is that whenever we want to create some error 241 241 00:14:17,124 --> 00:14:19,430 that we want to handle down here, 242 242 00:14:19,430 --> 00:14:22,833 in the catch handler, all we need to do is to throw, 243 243 00:14:22,833 --> 00:14:26,990 and create a new error, just like we did here. 244 244 00:14:26,990 --> 00:14:30,260 And of course, we can do that for multiple reasons. 245 245 00:14:30,260 --> 00:14:32,660 So in this case, here, we did it simply 246 246 00:14:32,660 --> 00:14:36,520 because in the situation, no neighbor can be found. 247 247 00:14:36,520 --> 00:14:39,750 And so that is a good reason to display an error message 248 248 00:14:39,750 --> 00:14:41,097 on the user interface, 249 249 00:14:41,097 --> 00:14:44,983 and since we do that down here in our error handler, 250 250 00:14:46,230 --> 00:14:50,080 the best way of doing that is to indeed throw an error. 251 251 00:14:50,080 --> 00:14:51,472 And remember that this works, 252 252 00:14:51,472 --> 00:14:55,760 because throwing an error inside of this callback function 253 253 00:14:55,760 --> 00:15:00,120 of this then method will immediately reject this promise. 254 254 00:15:00,120 --> 00:15:03,190 And so then that rejected promise will travel down 255 255 00:15:03,190 --> 00:15:07,880 the chain until it is eventually caught somewhere. 256 256 00:15:07,880 --> 00:15:10,608 So again, in this case, it is right here 257 257 00:15:10,608 --> 00:15:12,980 in this catch handler. 258 258 00:15:12,980 --> 00:15:15,700 So when working with real applications 259 259 00:15:15,700 --> 00:15:17,760 in the real world, really make sure 260 260 00:15:17,760 --> 00:15:19,670 to keep this technique in mind, 261 261 00:15:19,670 --> 00:15:21,483 because it's really important. 23099