1 00:00:01,300 --> 00:00:02,620 So until now, 2 00:00:02,620 --> 00:00:05,399 we have always assumed that everything went well 3 00:00:05,399 --> 00:00:10,399 with our AJAX calls, so we never handled any errors. 4 00:00:10,590 --> 00:00:14,000 However, an important part of web development 5 00:00:14,000 --> 00:00:15,689 is to actually handle the errors 6 00:00:15,689 --> 00:00:19,080 because it's very common that errors happen 7 00:00:19,080 --> 00:00:21,020 in web applications. 8 00:00:21,020 --> 00:00:22,600 And so in this lecture, 9 00:00:22,600 --> 00:00:24,930 let's talk about how to handle errors 10 00:00:24,930 --> 00:00:26,073 in promises. 11 00:00:28,120 --> 00:00:30,950 And to start, remember that a promise 12 00:00:30,950 --> 00:00:35,000 in which an error happens is a rejected promise. 13 00:00:35,000 --> 00:00:36,210 And so in this video, 14 00:00:36,210 --> 00:00:39,910 we're gonna learn how to handle promise rejections. 15 00:00:39,910 --> 00:00:41,920 Now, actually the only way 16 00:00:41,920 --> 00:00:44,049 in which the fetch promise rejects 17 00:00:44,049 --> 00:00:48,000 is when the user loses his internet connection. 18 00:00:48,000 --> 00:00:50,903 And so for now, that's gonna be the only error 19 00:00:50,903 --> 00:00:53,160 that we will handle here. 20 00:00:53,160 --> 00:00:56,910 Now to simulate losing the internet connection 21 00:00:56,910 --> 00:00:59,010 we can go here to Network 22 00:00:59,010 --> 00:01:03,880 and then we can basically change the speed here to Offline. 23 00:01:03,880 --> 00:01:06,675 However, when we then reload the page 24 00:01:06,675 --> 00:01:09,202 then basically everything will disappear, 25 00:01:09,202 --> 00:01:12,970 and so that's not really what we want. 26 00:01:12,970 --> 00:01:17,680 We want to simulate that the page was first still loaded 27 00:01:17,680 --> 00:01:22,080 but then as the user does the request without internet 28 00:01:22,080 --> 00:01:25,850 then we want to see the error happening. 29 00:01:25,850 --> 00:01:28,780 And so let's set it back to Online 30 00:01:28,780 --> 00:01:30,950 and so now what we want to do 31 00:01:30,950 --> 00:01:33,840 is to basically only call this function here 32 00:01:33,840 --> 00:01:36,610 whenever the user clicks on a button. 33 00:01:36,610 --> 00:01:38,977 And so that will then make it easier for us 34 00:01:38,977 --> 00:01:42,683 to simulate losing the internet connection. 35 00:01:43,890 --> 00:01:47,197 So here in the HTML we already have a button, 36 00:01:47,197 --> 00:01:50,240 so let's comment this one out 37 00:01:51,570 --> 00:01:55,413 and then in the script I believe, here, 38 00:01:55,413 --> 00:01:59,963 so we already have it here selected as this btn. 39 00:02:02,846 --> 00:02:06,330 And so all we will do now is btn.addEventListener 40 00:02:08,590 --> 00:02:11,970 click, and then here function 41 00:02:15,220 --> 00:02:16,423 and just like this. 42 00:02:18,460 --> 00:02:21,690 Now wait, so just to see what happens here 43 00:02:22,800 --> 00:02:25,830 so indeed that works just fine 44 00:02:25,830 --> 00:02:29,593 but now watch what happens when we set ourselves Offline. 45 00:02:30,780 --> 00:02:34,283 Let's go back to the console and now when we do the request, 46 00:02:35,560 --> 00:02:38,290 then we get these errors here. 47 00:02:38,290 --> 00:02:41,599 So first this one here, but the one that's most important 48 00:02:41,599 --> 00:02:45,820 is that we now have an Uncaught promise, 49 00:02:45,820 --> 00:02:49,220 and so because we have failed to fetch. 50 00:02:49,220 --> 00:02:51,760 And so at this point for the first time 51 00:02:51,760 --> 00:02:54,740 the promise that's returned from the fetch function 52 00:02:54,740 --> 00:02:57,030 was actually rejected. 53 00:02:57,030 --> 00:03:00,190 And so let's no handle that rejection. 54 00:03:00,190 --> 00:03:03,330 Now there are two ways of handling rejections 55 00:03:03,330 --> 00:03:06,671 and the first one is to pass a second callback function 56 00:03:06,671 --> 00:03:08,483 into the then method. 57 00:03:10,120 --> 00:03:11,980 So the first callback function here 58 00:03:11,980 --> 00:03:15,450 is always gonna be called for the fulfilled promise. 59 00:03:15,450 --> 00:03:17,630 so for a successful one. 60 00:03:17,630 --> 00:03:20,290 But we can also pass in a second callback 61 00:03:20,290 --> 00:03:24,310 which will be called when the promise was rejected. 62 00:03:24,310 --> 00:03:25,453 So let's do that. 63 00:03:26,597 --> 00:03:29,530 And this callback function will be called with an argument 64 00:03:29,530 --> 00:03:32,053 which is basically the error itself. 65 00:03:33,700 --> 00:03:36,803 And so let's simply alert the error. 66 00:03:38,680 --> 00:03:41,433 Alright, so let's try that. 67 00:03:42,345 --> 00:03:46,200 And now we are back to having No Internet 68 00:03:46,200 --> 00:03:50,110 so we first need to do everything normally, 69 00:03:50,110 --> 00:03:51,960 so load the page. 70 00:03:51,960 --> 00:03:56,790 Now we lose the connection and now we do the request. 71 00:03:56,790 --> 00:03:59,463 And so now actually we handled the error 72 00:03:59,463 --> 00:04:02,800 by displaying this alert window. 73 00:04:02,800 --> 00:04:07,290 And the error that we saw previously down here is now gone. 74 00:04:07,290 --> 00:04:09,230 So now, in fact, we no longer 75 00:04:09,230 --> 00:04:11,920 have this Uncaught error down here 76 00:04:11,920 --> 00:04:15,453 because we did actually catch the error right here. 77 00:04:16,490 --> 00:04:20,290 So handling the error is also called catching to error, 78 00:04:20,290 --> 00:04:22,990 and so again, that's the reason why 79 00:04:22,990 --> 00:04:26,030 this error that we had here before disappeared. 80 00:04:26,030 --> 00:04:28,501 So it said Uncaught error, but again 81 00:04:28,501 --> 00:04:31,890 now we are actually catching it here. 82 00:04:31,890 --> 00:04:34,890 And so with this we are now handling the error 83 00:04:34,890 --> 00:04:39,840 that might occur in this promise here, okay. 84 00:04:39,840 --> 00:04:42,661 Now, in this case there are then no more errors 85 00:04:42,661 --> 00:04:45,900 because basically the chain stops here 86 00:04:45,900 --> 00:04:49,600 when this error happens and when it's handled right here. 87 00:04:49,600 --> 00:04:52,485 But now what if there was actually no error 88 00:04:52,485 --> 00:04:55,333 in this fetch promise here? 89 00:04:56,240 --> 00:04:58,570 So basically what if this fetch promise 90 00:04:58,570 --> 00:05:02,010 was actually fulfilled but then the second one here 91 00:05:02,010 --> 00:05:03,123 was rejected. 92 00:05:04,160 --> 00:05:08,400 Well then we would also have to catch an error here. 93 00:05:08,400 --> 00:05:10,200 So we would have to come here 94 00:05:11,520 --> 00:05:14,030 get this function and paste it here 95 00:05:14,030 --> 00:05:17,370 and also handle the error right here. 96 00:05:17,370 --> 00:05:20,130 However, that is a little bit annoying 97 00:05:20,130 --> 00:05:22,820 and so in fact there is a better way 98 00:05:22,820 --> 00:05:25,498 of basically handling all these errors globally 99 00:05:25,498 --> 00:05:28,220 just in one central place. 100 00:05:28,220 --> 00:05:32,370 So instead of all of these callback functions here 101 00:05:32,370 --> 00:05:34,203 let's just delete them. 102 00:05:36,160 --> 00:05:39,160 okay. So this is a lot nicer 103 00:05:39,160 --> 00:05:41,960 just having one callback in the then 104 00:05:41,960 --> 00:05:44,890 and then instead we can handle all the errors 105 00:05:44,890 --> 00:05:46,986 no matter where they appear in the chain 106 00:05:46,986 --> 00:05:51,986 right at the end of the chain by adding a catch method. 107 00:05:53,220 --> 00:05:54,870 And then here we can actually use 108 00:05:54,870 --> 00:05:56,430 the same call back function 109 00:05:57,560 --> 00:05:59,380 because the callback function here 110 00:05:59,380 --> 00:06:02,823 will also be called with the error object that occurred 111 00:06:02,823 --> 00:06:06,200 and so then we can handle it in some way. 112 00:06:06,200 --> 00:06:09,690 All right, so again this catch method here 113 00:06:09,690 --> 00:06:12,934 at the end of the chain will basically catch any errors 114 00:06:12,934 --> 00:06:16,860 that occur in any place in this whole promise chain 115 00:06:16,860 --> 00:06:19,160 and no matter where that is. 116 00:06:19,160 --> 00:06:22,230 So errors basically propagate down the chain 117 00:06:22,230 --> 00:06:24,100 until they are caught, 118 00:06:24,100 --> 00:06:26,460 and only if they're not caught anywhere 119 00:06:26,460 --> 00:06:28,840 then we get that Uncaught error 120 00:06:28,840 --> 00:06:30,590 that we saw right in the beginning. 121 00:06:32,090 --> 00:06:34,630 All right, so let's now try this again 122 00:06:34,630 --> 00:06:39,630 and we will have to put back the internet, reload the page, 123 00:06:40,070 --> 00:06:41,233 get back Offline. 124 00:06:42,750 --> 00:06:45,923 And indeed we get the same error now here, 125 00:06:45,923 --> 00:06:49,543 so the same alert window that we had before. 126 00:06:53,123 --> 00:06:57,042 Now instead of having that annoying alert window 127 00:06:57,042 --> 00:07:00,020 let's just lock the error to the console 128 00:07:00,020 --> 00:07:02,523 and create some string here. 129 00:07:03,990 --> 00:07:07,300 So error, and then let's just add some emojis 130 00:07:07,300 --> 00:07:09,480 so that we know exactly which 131 00:07:11,160 --> 00:07:13,173 errors were printed by ourselves. 132 00:07:14,760 --> 00:07:17,010 Well, lets add a couple of them here. 133 00:07:17,010 --> 00:07:19,950 And in fact, we can actually also style this one 134 00:07:19,950 --> 00:07:21,940 with console.error. 135 00:07:21,940 --> 00:07:24,170 Remember, I think I showed you this one 136 00:07:25,080 --> 00:07:27,960 at some point in the beginning of the course. 137 00:07:27,960 --> 00:07:32,180 But anyway, usually simply logging the error to the console 138 00:07:32,180 --> 00:07:34,740 is not enough in a real application 139 00:07:34,740 --> 00:07:37,110 with a real user interface. 140 00:07:37,110 --> 00:07:40,610 And so instead of just logging something to the console 141 00:07:40,610 --> 00:07:44,113 let's also display an error message for the user to see. 142 00:07:45,640 --> 00:07:49,823 And so that's then a more real use case of this catch block. 143 00:07:51,030 --> 00:07:54,700 All right, so I'm adding a new block here 144 00:07:54,700 --> 00:07:58,880 so we still want to of course lock the error to the console 145 00:07:58,880 --> 00:08:01,973 but besides that let's actually now create a function 146 00:08:01,973 --> 00:08:06,973 that will also render some kind of error, okay. 147 00:08:07,010 --> 00:08:08,680 So let's do that outside here 148 00:08:10,150 --> 00:08:12,103 and I'm calling it renderError. 149 00:08:16,660 --> 00:08:20,100 So this is a function and it can take in a message 150 00:08:21,180 --> 00:08:23,130 just call it like this. 151 00:08:23,130 --> 00:08:24,750 And just like the country elements 152 00:08:24,750 --> 00:08:28,123 we want to attach this to countriesContainer 153 00:08:29,666 --> 00:08:31,910 and then insert.adjacent. 154 00:08:31,910 --> 00:08:36,440 And this time not HTML but insertAdjacent text. 155 00:08:36,440 --> 00:08:40,630 So this is a new one but basically it does the same thing 156 00:08:41,490 --> 00:08:45,803 as insertAdjacent HTML but simply with text, 157 00:08:46,930 --> 00:08:49,473 so it doesn't create any new HTML elements. 158 00:08:51,050 --> 00:08:53,700 Okay. And then besides that 159 00:08:53,700 --> 00:08:57,893 remember we always have set the opacity back to one. 160 00:08:59,110 --> 00:09:03,730 So just as we did before in the render countries function, 161 00:09:03,730 --> 00:09:05,900 because otherwise this container 162 00:09:05,900 --> 00:09:08,139 is not even gonna be visible. 163 00:09:08,139 --> 00:09:09,634 And actually let's put this function 164 00:09:09,634 --> 00:09:11,830 right at the very top of the file 165 00:09:13,350 --> 00:09:16,323 together with the render country function. 166 00:09:20,100 --> 00:09:22,923 So just like this. 167 00:09:25,400 --> 00:09:27,910 Okay, so again in both cases 168 00:09:27,910 --> 00:09:30,070 in order to actually see the container 169 00:09:30,070 --> 00:09:31,963 we set the opacity to one. 170 00:09:33,760 --> 00:09:37,030 Okay. And so now here let's use that 171 00:09:39,220 --> 00:09:42,513 render, so not country of course Error, 172 00:09:44,798 --> 00:09:49,610 let's say something went wrong. 173 00:09:49,610 --> 00:09:52,523 Then here we can use our emoji again, why not? 174 00:09:54,790 --> 00:09:57,172 And then here let's again print the error 175 00:09:57,172 --> 00:10:00,660 and actually this error that is generated here 176 00:10:00,660 --> 00:10:03,220 is a real JavaScript object. 177 00:10:03,220 --> 00:10:07,210 So we can create errors in JavaScript with a constructor, 178 00:10:07,210 --> 00:10:10,113 for example, just like a map or a set. 179 00:10:11,260 --> 00:10:15,010 And any error in JavaScript that was created like this 180 00:10:15,010 --> 00:10:17,580 contains the message property. 181 00:10:17,580 --> 00:10:19,230 So we can use that here 182 00:10:20,070 --> 00:10:23,230 to basically only print the message of that error 183 00:10:23,230 --> 00:10:26,430 and not the whole object itself. 184 00:10:26,430 --> 00:10:28,550 And we will actually learn more about 185 00:10:28,550 --> 00:10:32,083 the built in error object of JavaScript in the next video. 186 00:10:33,250 --> 00:10:36,810 Now to try this we need to again, get ourselves Online 187 00:10:38,960 --> 00:10:42,473 then Offline, check the console. 188 00:10:43,380 --> 00:10:47,890 And indeed we get our custom error down here 189 00:10:47,890 --> 00:10:49,510 visible by the emojis 190 00:10:49,510 --> 00:10:53,060 and then our own message here for the user. 191 00:10:53,060 --> 00:10:56,480 Now it's not super pretty but that's not the point here 192 00:10:57,550 --> 00:10:59,470 and as you see indeed this here 193 00:10:59,470 --> 00:11:01,197 is simply the error.msg. 194 00:11:02,180 --> 00:11:05,000 While down here we have the entire error. 195 00:11:05,000 --> 00:11:08,050 So that includes the TypeError 196 00:11:08,050 --> 00:11:13,050 and even it includes this so-called stack trace 197 00:11:13,320 --> 00:11:15,823 so it shows us exactly where the error comes from. 198 00:11:17,200 --> 00:11:19,870 All right, and so that's how we handle errors 199 00:11:19,870 --> 00:11:24,220 happening in promises in any then handler. 200 00:11:24,220 --> 00:11:27,065 So basically handling any promise rejection 201 00:11:27,065 --> 00:11:30,390 no matter where it happens in the chain. 202 00:11:30,390 --> 00:11:33,300 Now just to finish there is one more quick method 203 00:11:33,300 --> 00:11:34,710 that I want to show you 204 00:11:34,710 --> 00:11:37,970 and that is also available on all promises. 205 00:11:37,970 --> 00:11:42,970 So besides then and catch there is also the finally method. 206 00:11:43,550 --> 00:11:48,200 So let's add a finally here, finally. 207 00:11:48,200 --> 00:11:51,570 And then the callback function that we defined here 208 00:11:51,570 --> 00:11:55,630 will always be called whatever happens with the promise. 209 00:11:55,630 --> 00:11:59,860 So no matter if the promise is fulfilled or rejected 210 00:11:59,860 --> 00:12:01,902 this callback function that we define here 211 00:12:01,902 --> 00:12:04,970 is gonna be called always. 212 00:12:04,970 --> 00:12:07,740 So that's the difference between the other two 213 00:12:07,740 --> 00:12:10,150 so the then method is only called 214 00:12:10,150 --> 00:12:13,610 when the promise is fulfilled while this one is only called 215 00:12:13,610 --> 00:12:16,550 while the promise is rejected. 216 00:12:16,550 --> 00:12:20,000 Now the finally method is not always useful, 217 00:12:20,000 --> 00:12:22,170 but sometimes it actually is. 218 00:12:22,170 --> 00:12:23,700 So we use this method 219 00:12:23,700 --> 00:12:26,210 for something that always needs to happen 220 00:12:26,210 --> 00:12:29,120 no matter the result of the promise. 221 00:12:29,120 --> 00:12:33,080 And one good example of that is to hide a loading spinner 222 00:12:33,080 --> 00:12:36,200 like these rotating circles that you see everywhere 223 00:12:36,200 --> 00:12:39,570 in web applications when you load some data. 224 00:12:39,570 --> 00:12:42,070 So these applications show a spinner 225 00:12:42,070 --> 00:12:44,750 when an asynchronous operation starts 226 00:12:44,750 --> 00:12:47,780 and then hide it once the operation completes. 227 00:12:47,780 --> 00:12:49,520 And that happens no matter 228 00:12:49,520 --> 00:12:52,690 if the operation was successfully or not. 229 00:12:52,690 --> 00:12:56,520 And so for that to finally method is perfect. 230 00:12:56,520 --> 00:12:59,180 And in our case, what we always need to do 231 00:12:59,180 --> 00:13:01,570 is to fade-in the container. 232 00:13:01,570 --> 00:13:05,420 So, basically this part here 233 00:13:05,420 --> 00:13:08,810 always happens no matter what, right? 234 00:13:08,810 --> 00:13:10,860 So no matter if we render the country 235 00:13:10,860 --> 00:13:12,160 in the case of success 236 00:13:12,160 --> 00:13:16,110 or if we render the error in case of an error, 237 00:13:16,110 --> 00:13:19,080 no matter what we always need to do this. 238 00:13:19,080 --> 00:13:21,349 And so let's take this out of here, 239 00:13:21,349 --> 00:13:23,020 so I'm copying it here 240 00:13:24,590 --> 00:13:28,450 and deactivating this in both cases 241 00:13:28,450 --> 00:13:33,450 and then let's put it here into the finally handler, okay. 242 00:13:33,810 --> 00:13:36,600 And now we no longer need the Offline 243 00:13:37,730 --> 00:13:41,610 so let's just reload, put it here 244 00:13:41,610 --> 00:13:44,053 and yeah that works beautifully. 245 00:13:45,186 --> 00:13:50,020 Just notice that this actually works because catch itself 246 00:13:50,020 --> 00:13:52,050 also returns a promise. 247 00:13:52,050 --> 00:13:55,360 So that's the only way why this here can work 248 00:13:55,360 --> 00:13:58,700 so of course this only works on promises. 249 00:13:58,700 --> 00:14:01,770 And so, yeah, again, this can only work 250 00:14:01,770 --> 00:14:05,193 if catch itself also returns a promise. 251 00:14:07,030 --> 00:14:11,070 All right, now let's try to simulate another error 252 00:14:11,070 --> 00:14:13,923 so that I can show you what we need to do next. 253 00:14:14,810 --> 00:14:18,050 So let's say that we're trying to search for a country 254 00:14:18,050 --> 00:14:20,050 that simply doesn't exist. 255 00:14:20,050 --> 00:14:24,420 And so, or API is not gonna find any result for that. 256 00:14:24,420 --> 00:14:28,459 Let's say just this, so let's see. 257 00:14:28,459 --> 00:14:31,363 And well we get to this weird error 258 00:14:31,363 --> 00:14:36,363 that we Cannot read property flag of undefined in line 152. 259 00:14:38,320 --> 00:14:41,731 Now, so basically it's coming from this place 260 00:14:41,731 --> 00:14:44,570 but of course then in stack trace 261 00:14:44,570 --> 00:14:47,670 we can see where exactly it's coming from, 262 00:14:47,670 --> 00:14:50,043 so it's coming from getCountryData. 263 00:14:51,600 --> 00:14:53,930 Well, in this case it's not that helpful 264 00:14:53,930 --> 00:14:57,050 but sometimes inspecting this stack trace 265 00:14:57,050 --> 00:14:59,443 can be helpful indeed. 266 00:15:00,520 --> 00:15:03,310 So anyway, this error here is weird 267 00:15:03,310 --> 00:15:05,900 and it doesn't really reflect the true error 268 00:15:05,900 --> 00:15:07,940 which is simply that our API 269 00:15:07,940 --> 00:15:11,200 cannot find any country with this name. 270 00:15:11,200 --> 00:15:13,493 So the true error is of course not 271 00:15:13,493 --> 00:15:16,267 that we Cannot read flag of undefined 272 00:15:16,267 --> 00:15:21,267 but in fact it is that our API cannot find any country. 273 00:15:21,400 --> 00:15:26,100 And so that's reflected here with the status code 404. 274 00:15:26,100 --> 00:15:28,120 However, as I said in the beginning 275 00:15:28,120 --> 00:15:30,190 the fetch promise only rejects 276 00:15:30,190 --> 00:15:32,800 when there is no internet connection, 277 00:15:32,800 --> 00:15:37,272 but with a 404 error like this which is not a real error 278 00:15:37,272 --> 00:15:39,960 but well it kind of is. 279 00:15:39,960 --> 00:15:43,560 But anyway with this 404 the fetch promise 280 00:15:43,560 --> 00:15:45,710 will still get fulfilled. 281 00:15:45,710 --> 00:15:48,926 So there is no rejection and so our catch handler 282 00:15:48,926 --> 00:15:52,030 cannot pick up on this error. 283 00:15:52,030 --> 00:15:56,530 It does pick up on this other error so in this one here, 284 00:15:56,530 --> 00:15:59,580 but that's not the one that we actually want to handle. 285 00:15:59,580 --> 00:16:01,860 In this case we really want to tell the user 286 00:16:01,860 --> 00:16:05,370 that no country was found with this name. 287 00:16:05,370 --> 00:16:08,290 And so that is what we will do in the next lecture, 288 00:16:08,290 --> 00:16:10,963 so that this one doesn't become all too long.