1 00:00:03,650 --> 00:00:06,180 In the previous exercise, 2 00:00:06,180 --> 00:00:09,414 we learned about two new aspects of JavaScript, 3 00:00:09,414 --> 00:00:13,155 their support for first class functions and closures. 4 00:00:13,155 --> 00:00:20,250 We also saw how Node handles the asynchronous programming aspect and how Node 5 00:00:20,250 --> 00:00:24,015 supports asynchronous execution of code with 6 00:00:24,015 --> 00:00:28,365 the use of callbacks and also error handling. 7 00:00:28,365 --> 00:00:31,815 Let's look at an example of how we make use of this 8 00:00:31,815 --> 00:00:36,650 within our Node application in this exercise. 9 00:00:36,650 --> 00:00:42,315 Continuing with where we left off in the previous exercise, 10 00:00:42,315 --> 00:00:48,025 now I'm going to go and modify the rectangle Node module, 11 00:00:48,025 --> 00:00:51,670 so that it can take the help of callbacks. 12 00:00:51,670 --> 00:00:55,375 So I'm going to remove these exports here, 13 00:00:55,375 --> 00:00:59,800 and instead, they're going to modify this function. 14 00:00:59,800 --> 00:01:03,190 I'm going to be making use of these two functions a little bit later, 15 00:01:03,190 --> 00:01:05,925 so that's why I'm saving them at the bottom. 16 00:01:05,925 --> 00:01:07,515 So let me keep them aside. 17 00:01:07,515 --> 00:01:11,530 And then I will modify this rectangle module as follows. 18 00:01:11,530 --> 00:01:15,270 So we'll say module.exports. 19 00:01:15,270 --> 00:01:20,025 So we're are using the standard way the module is handled. 20 00:01:20,025 --> 00:01:26,185 And this is going to export this function which takes these three parameters, 21 00:01:26,185 --> 00:01:30,565 x and y and callback. 22 00:01:30,565 --> 00:01:37,300 The callback is a function that is going to be supplied in when this module is called. 23 00:01:37,300 --> 00:01:43,825 So this is the use of the first class functions that we have talked about earlier. 24 00:01:43,825 --> 00:01:46,775 So inside this callback, 25 00:01:46,775 --> 00:01:49,940 so you can see that you're getting two parameters, x and y. 26 00:01:49,940 --> 00:01:51,965 In this case, for the rectangle, 27 00:01:51,965 --> 00:01:55,520 the x and y correspond to the length and breadth of 28 00:01:55,520 --> 00:02:01,550 the rectangle that is being passed in as the two values. 29 00:02:01,550 --> 00:02:05,090 So inside here, we're going to check to see 30 00:02:05,090 --> 00:02:10,300 if x is less than or equal to zero or y is less than or equal to zero. 31 00:02:10,300 --> 00:02:13,255 We have written code like this here. 32 00:02:13,255 --> 00:02:17,630 So I'm just going to copy this code from here 33 00:02:17,630 --> 00:02:23,330 and then bring it over to here and then we'll edit. 34 00:02:23,330 --> 00:02:26,090 So in this case, this is if x is less 35 00:02:26,090 --> 00:02:30,135 than or equal to zero or y is less than or equal to zero. 36 00:02:30,135 --> 00:02:32,810 In this case, what do we do? 37 00:02:32,810 --> 00:02:35,435 In this case, we notice that 38 00:02:35,435 --> 00:02:39,750 the dimensions of the rectangle are less than or equal to zero. 39 00:02:39,750 --> 00:02:43,775 So we have to handle it differently, 40 00:02:43,775 --> 00:02:46,330 and we'll do the else part of it. 41 00:02:46,330 --> 00:02:52,790 The else part where we handle the situation where the rectangle is a valid rectangle. 42 00:02:52,790 --> 00:02:54,245 So in this case, 43 00:02:54,245 --> 00:03:04,580 what we will do is to then use a setTimeout. 44 00:03:04,580 --> 00:03:08,090 So here, I am simulating the fact that whatever 45 00:03:08,090 --> 00:03:11,860 is being done in this rectangle is going to take some time. 46 00:03:11,860 --> 00:03:14,375 So this is the asynchronous processing. 47 00:03:14,375 --> 00:03:18,710 Now, since I don't have a lot of work to do in the background, 48 00:03:18,710 --> 00:03:22,530 so I'm going to simulate that by simply using 49 00:03:22,530 --> 00:03:24,830 the setTimeout function of JavaScript and then 50 00:03:24,830 --> 00:03:28,825 delay before the callback function is called. 51 00:03:28,825 --> 00:03:32,210 So the way I arrange this is as follows. 52 00:03:32,210 --> 00:03:34,270 So inside the setTimeout, 53 00:03:34,270 --> 00:03:40,445 I'm going to call this function there 54 00:03:40,445 --> 00:03:44,855 and arrange a delay of 55 00:03:44,855 --> 00:03:50,570 2,000 milliseconds or two seconds after which this function is going to be called. 56 00:03:50,570 --> 00:03:52,003 So if you recall, 57 00:03:52,003 --> 00:03:54,890 the setTimeout that is supported in JavaScript, 58 00:03:54,890 --> 00:03:58,070 it takes as the first parameter a function and 59 00:03:58,070 --> 00:04:03,450 the second parameter would be the time period for which this is going to be delayed. 60 00:04:03,450 --> 00:04:07,120 So let me indent this in. 61 00:04:07,120 --> 00:04:09,255 Now, I need to fill in this function here. 62 00:04:09,255 --> 00:04:11,915 So I have started out with the arrow function here 63 00:04:11,915 --> 00:04:15,575 which takes no parameters and then when there is called, 64 00:04:15,575 --> 00:04:20,745 I'm going to issue a call to the callback. 65 00:04:20,745 --> 00:04:22,880 This callback is a callback function that is 66 00:04:22,880 --> 00:04:25,760 going to be passed in as the third parameter here. 67 00:04:25,760 --> 00:04:33,440 This callback function, as you noticed, takes two parameters. 68 00:04:33,440 --> 00:04:38,245 The first one is the error and the second one is the return value. 69 00:04:38,245 --> 00:04:40,625 So in this case, since we have an error, 70 00:04:40,625 --> 00:04:44,750 because x is less than or equal to zero and y is less than or equal to zero, 71 00:04:44,750 --> 00:04:50,820 so the first value I'm going to pass in as new error, 72 00:04:50,820 --> 00:05:00,603 and this error is exactly this string that I was using in the console.log then, 73 00:05:00,603 --> 00:05:03,915 and that is what I will pass in as the error here. 74 00:05:03,915 --> 00:05:06,160 Let me now delete that console.log, 75 00:05:06,160 --> 00:05:07,625 I no longer need it. 76 00:05:07,625 --> 00:05:08,890 So for the callback, 77 00:05:08,890 --> 00:05:13,210 I'm going to generate a new error object and then pass this 78 00:05:13,210 --> 00:05:18,850 in as the return value for the callback function, the first parameter. 79 00:05:18,850 --> 00:05:23,770 So I'll say rectangle dimensions should be greater than zero. 80 00:05:23,770 --> 00:05:28,240 So we'll say l, 81 00:05:28,240 --> 00:05:39,000 this is the x and the y value that we got as the input values there. 82 00:05:39,000 --> 00:05:40,650 So that is the callback. 83 00:05:40,650 --> 00:05:43,890 And the second part of this callback, 84 00:05:43,890 --> 00:05:51,290 I'm going to pass in as null because this is going to return an error. 85 00:05:51,290 --> 00:05:54,345 So when you return an error as the first parameter, 86 00:05:54,345 --> 00:05:59,250 the second parameter will be ignored when that callback was just received by 87 00:05:59,250 --> 00:06:06,510 the node module from where we are calling this particular function there. 88 00:06:06,510 --> 00:06:09,085 So, with this arrangement, 89 00:06:09,085 --> 00:06:10,700 so less than zero, 90 00:06:10,700 --> 00:06:18,590 so let me give some space here so that it's properly indented here. 91 00:06:18,590 --> 00:06:21,650 So, the way I am arranging this 92 00:06:21,650 --> 00:06:26,135 here is that if x is less than zero and y is less than zero, 93 00:06:26,135 --> 00:06:30,040 I'm going to callback the callback function that has been passed in, 94 00:06:30,040 --> 00:06:32,424 but the first parameter will pass in in 95 00:06:32,424 --> 00:06:36,307 error because here we notice that there is an error, 96 00:06:36,307 --> 00:06:38,930 and the second parameter will be null. 97 00:06:38,930 --> 00:06:40,940 If this is not the case, 98 00:06:40,940 --> 00:06:44,550 then I'm going to call the same callback, 99 00:06:44,550 --> 00:06:46,910 but with the first parameter. 100 00:06:46,910 --> 00:06:51,800 So let me copy that code in here. 101 00:06:51,800 --> 00:07:05,490 I'm still going to use the setTimeout here. 102 00:07:05,490 --> 00:07:10,727 But that first part is not going to be an error, 103 00:07:10,727 --> 00:07:13,005 instead, the first part, 104 00:07:13,005 --> 00:07:15,235 in this case, there is no error. 105 00:07:15,235 --> 00:07:20,455 So I'm just going to pass back that value as null. 106 00:07:20,455 --> 00:07:22,950 So that means that the error is set to null. 107 00:07:22,950 --> 00:07:26,335 So this is a valid rectangle. 108 00:07:26,335 --> 00:07:29,965 So we can compute the values for the rectangle. 109 00:07:29,965 --> 00:07:35,165 But instead, I'm going to simply pass in 110 00:07:35,165 --> 00:07:40,020 a JavaScript object containing the two functions 111 00:07:40,020 --> 00:07:45,330 as the two values inside this JavaScript object. 112 00:07:45,330 --> 00:07:50,355 So, here I am going to take this two, 113 00:07:50,355 --> 00:07:54,530 the perimeter and the area, 114 00:07:55,250 --> 00:08:05,639 and then these two will be passed in as the two values here, 115 00:08:05,639 --> 00:08:07,807 perimeter and the area. 116 00:08:07,807 --> 00:08:12,740 And these two, since they happen to be a JavaScript object, 117 00:08:12,740 --> 00:08:16,544 so perimeter and area will be 118 00:08:16,544 --> 00:08:22,690 the two properties that I will pass in inside those JavaScript object, 119 00:08:22,690 --> 00:08:25,295 and that is it. 120 00:08:25,295 --> 00:08:28,570 So here, in the second case, 121 00:08:28,570 --> 00:08:32,034 the error is set to null because there is no error, 122 00:08:32,034 --> 00:08:33,190 but the second part, 123 00:08:33,190 --> 00:08:37,715 notice that I am passing in a JavaScript object containing two properties, 124 00:08:37,715 --> 00:08:41,530 perimeter and area, which are two functions here. 125 00:08:41,530 --> 00:08:46,060 So the two functions are the perimeter function and area function. 126 00:08:46,060 --> 00:08:49,390 So this JavaScript object will be passed back as 127 00:08:49,390 --> 00:08:53,705 the return value for the callback here, the second value. 128 00:08:53,705 --> 00:08:56,035 And again, I am going to delay this by 129 00:08:56,035 --> 00:09:02,900 a two second interval before the value will be passed back by this function. 130 00:09:02,900 --> 00:09:07,075 The reason why I'm using the setTimeout is to simulate 131 00:09:07,075 --> 00:09:11,780 a delay before the callback comes in from the other side. 132 00:09:11,780 --> 00:09:15,280 So this sort of represents situations where, for example, 133 00:09:15,280 --> 00:09:18,565 you issue a database call to the database, 134 00:09:18,565 --> 00:09:23,300 and the database needs to be read before the value is passed back to you. 135 00:09:23,300 --> 00:09:25,855 So that is going to take a certain amount of time. 136 00:09:25,855 --> 00:09:32,560 So I'm basically simulating this at this moment by using the setTimeout function here. 137 00:09:32,560 --> 00:09:37,000 Later on, you will see that when we integrate the MongoDB with 138 00:09:37,000 --> 00:09:43,060 our Express in the later exercises, 139 00:09:43,060 --> 00:09:45,580 that delay would have to be simulated, 140 00:09:45,580 --> 00:09:48,670 it'll be automatically caused because of the fact that you need to do 141 00:09:48,670 --> 00:09:54,190 database operations behind the scenes before the data is obtained. 142 00:09:54,190 --> 00:10:00,325 So here, having completed this rectangle modules update, 143 00:10:00,325 --> 00:10:03,135 so here you see that the rectangle module takes in 144 00:10:03,135 --> 00:10:06,830 three parameters as it's a input call here, 145 00:10:06,830 --> 00:10:08,920 x, y and callback. 146 00:10:08,920 --> 00:10:12,445 And so, the callback is the callback function that is being supplied here. 147 00:10:12,445 --> 00:10:16,585 And this callback function will be called inside here, 148 00:10:16,585 --> 00:10:18,685 and when the callback function is called, 149 00:10:18,685 --> 00:10:22,495 either you pass back an error or you pass back 150 00:10:22,495 --> 00:10:28,045 a function that allows you to compute the perimeter and the area of the rectangle. 151 00:10:28,045 --> 00:10:33,835 Now, this pattern of calling in and passing in a callback function 152 00:10:33,835 --> 00:10:40,300 from one Node module and then the second Node module which when it completes, 153 00:10:40,300 --> 00:10:44,800 will pass back the result using the callback function is 154 00:10:44,800 --> 00:10:50,135 exactly the pattern that you will see often repeated in Node applications. 155 00:10:50,135 --> 00:10:52,360 So that's the reason why I'm illustrating it. 156 00:10:52,360 --> 00:10:58,735 Of course, this is a contrived example that I am illustrating here but it shows 157 00:10:58,735 --> 00:11:02,525 all the standard patterns that you will use with node 158 00:11:02,525 --> 00:11:07,065 and callback functions and also the error handling. 159 00:11:07,065 --> 00:11:10,955 Now, we have updated the rectangle module here. 160 00:11:10,955 --> 00:11:16,870 Let's go and fix that index.js file so that it can pass in the callback function and 161 00:11:16,870 --> 00:11:23,770 then handle the return value that is sent back from the rectangle module. 162 00:11:23,770 --> 00:11:26,940 Switching back to index.js, 163 00:11:26,940 --> 00:11:31,270 now we are going to update the index.js file as follows. 164 00:11:31,270 --> 00:11:37,955 I'm going to remove this from the index.js file instead. 165 00:11:37,955 --> 00:11:42,430 Here, what we will end up doing is call 166 00:11:42,430 --> 00:11:49,120 this rectangle module and 167 00:11:49,120 --> 00:11:53,880 then pass in the l and the b as the two parameters. 168 00:11:53,880 --> 00:12:00,300 And the third parameter that I'm going to pass in is that callback function. 169 00:12:00,300 --> 00:12:10,080 So the callback function has two parameters, err and rectangle. 170 00:12:10,080 --> 00:12:15,860 And this callback function is implemented as follows here. 171 00:12:15,860 --> 00:12:19,090 So here, you see that I'm calling 172 00:12:19,090 --> 00:12:23,785 this rectangle node module and I'm 173 00:12:23,785 --> 00:12:29,050 passing in the length and the breadth of the first two parameters. 174 00:12:29,050 --> 00:12:31,285 And the third parameter is, of course, 175 00:12:31,285 --> 00:12:34,355 a callback function which I'm implementing here. 176 00:12:34,355 --> 00:12:38,250 So, this is an arrow function that I'm implementing here. 177 00:12:38,250 --> 00:12:40,510 So inside this callback function, 178 00:12:40,510 --> 00:12:43,245 how do I handle the return value? 179 00:12:43,245 --> 00:12:46,860 So here I'll say if (err), 180 00:12:46,860 --> 00:12:51,335 so if the error value is returned to you, 181 00:12:51,335 --> 00:12:59,500 then I will simply do console.log and I'll say 182 00:12:59,500 --> 00:13:09,100 "ERROR" and then the second value is err.message. 183 00:13:09,100 --> 00:13:11,445 Recall that in the rectangle, 184 00:13:11,445 --> 00:13:14,820 we had created this new error object 185 00:13:14,820 --> 00:13:18,155 and then passed in this string inside this error object. 186 00:13:18,155 --> 00:13:20,100 This string will be attached to 187 00:13:20,100 --> 00:13:24,380 the error object to the message property of the error object here. 188 00:13:24,380 --> 00:13:26,830 So, in the index.js file, 189 00:13:26,830 --> 00:13:30,090 so I'm retrieving the message property of the error object 190 00:13:30,090 --> 00:13:33,755 and then printing it out onto my console here. 191 00:13:33,755 --> 00:13:35,310 So if an error occurs, 192 00:13:35,310 --> 00:13:39,215 I'm going to print out the error message using this. 193 00:13:39,215 --> 00:13:45,820 And so, this is how I would handle error that was returned by a callback function. 194 00:13:45,820 --> 00:13:51,360 Otherwise, of course in this case, 195 00:13:51,360 --> 00:13:57,450 the rectangle module has returned the rectangle function, 196 00:13:57,450 --> 00:14:01,620 the object that contains the perimeter and the area functions, 197 00:14:01,620 --> 00:14:08,108 so I can go ahead and print out that value. 198 00:14:08,108 --> 00:14:14,175 So here, I'll say the area of the rectangle 199 00:14:14,175 --> 00:14:23,030 of dimensions l equal to, 200 00:14:23,140 --> 00:14:28,270 so I will say, 201 00:14:29,170 --> 00:14:37,155 and b equal to. 202 00:14:37,155 --> 00:14:40,530 The reason why I am identifying it 203 00:14:40,530 --> 00:14:47,340 explicitly will become very clear when we run this example. 204 00:14:47,340 --> 00:15:01,480 I'll say is rectangle.area. 205 00:15:01,480 --> 00:15:04,188 Now, notice that for this area, 206 00:15:04,188 --> 00:15:09,555 I am not sending in any parameters here because goes to values, 207 00:15:09,555 --> 00:15:14,540 the length and the breath have been passed in already here in the l and b, 208 00:15:14,540 --> 00:15:19,065 and those would be already available to this 209 00:15:19,065 --> 00:15:24,245 and here because of the closure that JavaScript supports. 210 00:15:24,245 --> 00:15:27,470 Because these x and y have come in as the parameters, 211 00:15:27,470 --> 00:15:32,710 so those will be accessible right there in this callback function. 212 00:15:32,710 --> 00:15:37,315 So those x_y will be automatically available for us here. 213 00:15:37,315 --> 00:15:39,890 So, as a matter of fact, 214 00:15:39,890 --> 00:15:43,970 I don't need to supply this x and y parameters here at all because 215 00:15:43,970 --> 00:15:49,090 those x and y will be retrieved from this x and y right there. 216 00:15:49,090 --> 00:15:53,375 So I don't even need to pass those two values in here. 217 00:15:53,375 --> 00:15:55,385 So that's why, right there, 218 00:15:55,385 --> 00:15:57,905 I can call rectangle.area. 219 00:15:57,905 --> 00:16:03,170 And this area computation will automatically get the l and b that have been passed in 220 00:16:03,170 --> 00:16:09,505 by the rectangle call to direct node module earlier. 221 00:16:09,505 --> 00:16:14,980 So this is the closure of JavaScript operating here. 222 00:16:14,980 --> 00:16:17,945 So, that is the first one. 223 00:16:17,945 --> 00:16:23,305 I am printing out the area of the rectangle. 224 00:16:23,305 --> 00:16:32,990 The same thing, let me also print out the perimeters. 225 00:16:32,990 --> 00:16:34,608 So to print out the perimeters, 226 00:16:34,608 --> 00:16:36,720 so I'll say console.log. 227 00:16:36,720 --> 00:16:46,560 The perimeter of the rectangle will be rectangle perimeter. 228 00:16:46,560 --> 00:16:53,280 Now, I must tell you that this kind of approach for implementing takes a little bit 229 00:16:53,280 --> 00:17:02,335 of effort to understand and internalize the asynchronous function ability. 230 00:17:02,335 --> 00:17:06,360 Takes some time for you to completely understand how it really works. 231 00:17:06,360 --> 00:17:08,550 Now to illustrate the fact that 232 00:17:08,550 --> 00:17:14,642 this function call will result in a call to the Node module 233 00:17:14,642 --> 00:17:19,280 but this part will be executed only after 234 00:17:19,280 --> 00:17:24,830 a two-second delay because of the fact that we are using the set time mark. 235 00:17:24,830 --> 00:17:28,745 Now, as we studied in asynchronous operation, 236 00:17:28,745 --> 00:17:33,155 because we have moved off the processing to the back, 237 00:17:33,155 --> 00:17:41,420 let me also introduce one more console.log here to make a point. 238 00:17:41,420 --> 00:17:46,175 So we'll say this statement is 239 00:17:46,175 --> 00:17:53,869 after the call to rect. 240 00:17:53,869 --> 00:18:00,210 The reason why I am illustrating this point is that when you issue this call, 241 00:18:00,210 --> 00:18:06,130 this code will not be executed until after a two-second delay. 242 00:18:06,130 --> 00:18:08,070 So, in the meantime, 243 00:18:08,070 --> 00:18:10,360 your function, main function here, 244 00:18:10,360 --> 00:18:16,340 will continue on and then execute the next line of code that you see here. 245 00:18:16,340 --> 00:18:19,745 So this is the continuation that you will see here. 246 00:18:19,745 --> 00:18:21,805 So, with these changes, 247 00:18:21,805 --> 00:18:27,815 let's save the changes and then look at how this application executes now. 248 00:18:27,815 --> 00:18:29,410 So saving the changes. 249 00:18:29,410 --> 00:18:35,220 Let's go to the terminal and execute this node application. 250 00:18:35,220 --> 00:18:37,570 Now, going to the terminal, add the prompt. 251 00:18:37,570 --> 00:18:43,795 Let me type npm start and you will immediately notice 252 00:18:43,795 --> 00:18:51,790 that the solve and this statement were printed out earlier. 253 00:18:51,790 --> 00:18:54,505 And then after this certain delay, 254 00:18:54,505 --> 00:18:57,855 the area and the perimeter was printed out. 255 00:18:57,855 --> 00:19:02,130 So that is a two-second delay that we introduced using the set time out. 256 00:19:02,130 --> 00:19:05,755 So, you noticed that in the earlier version, 257 00:19:05,755 --> 00:19:10,290 this was printed and immediately the area and the perimeter was printed right below that. 258 00:19:10,290 --> 00:19:15,120 But now, notice that those values are printed a little bit later. 259 00:19:15,120 --> 00:19:20,460 So in between, their function calls with the different parameters that are already gone 260 00:19:20,460 --> 00:19:26,505 in and then the callbacks are called back after two second delay for each one of them. 261 00:19:26,505 --> 00:19:32,500 And because of the fact that there is the closure that the JavaScript supports, 262 00:19:32,500 --> 00:19:35,980 so the values that get passed in is preserved. 263 00:19:35,980 --> 00:19:38,440 And so when the callback function is called, 264 00:19:38,440 --> 00:19:40,615 the appropriate value is printed. 265 00:19:40,615 --> 00:19:42,730 So that's why you see that the area and 266 00:19:42,730 --> 00:19:46,330 the perimeter are printed correctly and these two here, 267 00:19:46,330 --> 00:19:48,070 these two statements here, 268 00:19:48,070 --> 00:19:51,340 correspond to this particular call to direct 269 00:19:51,340 --> 00:19:55,100 with l is equal to two and b is equal to four. 270 00:19:55,100 --> 00:19:56,605 And then these two, 271 00:19:56,605 --> 00:19:58,150 the next to two, of course, 272 00:19:58,150 --> 00:20:05,000 correspond to this particular one and the remaining ones as you see here. 273 00:20:05,000 --> 00:20:10,410 So, what I wanted to illustrate is the fact that when you do asynchronous computation, 274 00:20:10,410 --> 00:20:12,730 the asynchronous computation takes its own time 275 00:20:12,730 --> 00:20:16,270 to return the value while your main computation 276 00:20:16,270 --> 00:20:22,460 will proceed forward without waiting for the callback to be completed. 277 00:20:22,460 --> 00:20:25,245 So, when you need to do a certain amount of work, 278 00:20:25,245 --> 00:20:29,365 you passed in as a callback to that other module. 279 00:20:29,365 --> 00:20:31,375 And when that other module completes its work, 280 00:20:31,375 --> 00:20:35,010 it's going to callback and then that code will be executed. 281 00:20:35,010 --> 00:20:40,350 So that is the point that I have illustrated using this example. 282 00:20:40,350 --> 00:20:41,565 Again, as I said, 283 00:20:41,565 --> 00:20:45,210 this takes a little bit of imagination and understanding 284 00:20:45,210 --> 00:20:48,930 for you to internalize the way this works, 285 00:20:48,930 --> 00:20:55,185 but you will see that with Node and also Express and when we use MongoDB, 286 00:20:55,185 --> 00:21:01,785 you will see this kind of pattern repeating very often in the way we write our code. 287 00:21:01,785 --> 00:21:06,210 So with this simple illustration of callbacks and error handling, 288 00:21:06,210 --> 00:21:08,940 we complete this exercise. 289 00:21:08,940 --> 00:21:13,675 This is a good time for you to do a get comment with that message, 290 00:21:13,675 --> 00:21:17,700 node, callbacks, and error handling.