1
00:00:00,240 --> 00:00:05,250
In this video you're going to learn how to use the callback chaining pattern with your asynchronous

2
00:00:05,250 --> 00:00:07,080
note J S code.

3
00:00:07,080 --> 00:00:13,030
So right now in app that J S we do indeed have two asynchronous I O operations.

4
00:00:13,050 --> 00:00:17,370
The problem is that both are operating completely independently of the other.

5
00:00:17,460 --> 00:00:21,600
We fire both OP and they each get their response and just dump it to the console.

6
00:00:21,600 --> 00:00:25,870
That's actually not what we want with the callback chaining pattern.

7
00:00:25,950 --> 00:00:28,640
We're gonna be able to do one thing then the next.

8
00:00:28,650 --> 00:00:31,960
Which means we'll be able to geo code and address.

9
00:00:32,040 --> 00:00:38,620
Then we can take the coordinates and use that as the input for some other asynchronous I O operation.

10
00:00:38,640 --> 00:00:42,870
In this case we will pass the coordinates to forecast to get started.

11
00:00:42,870 --> 00:00:48,270
We need to take our call to forecast and move it to where we have the geo coding output.

12
00:00:48,270 --> 00:00:53,300
Where do we have that geo coding output in our callback function for geo code.

13
00:00:53,310 --> 00:00:59,220
So what I'm going to do is take this forecast call and cut it out copying it to the clipboard then up

14
00:00:59,220 --> 00:01:04,630
above I'm going to paste it right inside of the GEO code callback function.

15
00:01:04,650 --> 00:01:06,150
Now what do we have.

16
00:01:06,150 --> 00:01:08,470
Well we start with a call to geo code.

17
00:01:08,490 --> 00:01:13,740
We start this asynchronous operation and when it's done we know that the event loop is going to make

18
00:01:13,740 --> 00:01:16,860
sure our callback gets called from there.

19
00:01:16,860 --> 00:01:22,920
We're gonna kick off another asynchronous IO operation then we're gonna wait for that callback to finish

20
00:01:23,190 --> 00:01:27,240
and inside of here we'll have access to the final data.

21
00:01:27,270 --> 00:01:28,870
So this is callback chaining.

22
00:01:28,920 --> 00:01:34,630
We are chaining together multiple callbacks to do multiple things in a specific order.

23
00:01:34,650 --> 00:01:36,260
Now what do we have to do.

24
00:01:36,270 --> 00:01:43,170
Well we have to make sure that the data properties latitude and longitude get used as the input for

25
00:01:43,170 --> 00:01:44,550
the forecast function.

26
00:01:44,550 --> 00:01:50,070
Right here I'm gonna take our first argument where I have my static number and I'm going to remove that

27
00:01:50,310 --> 00:01:53,910
and instead I'm going to get the property off of data up above.

28
00:01:54,090 --> 00:01:57,240
So this is the success output from Geo code.

29
00:01:57,240 --> 00:02:03,630
Now we know that data has latitude long to dude and we also added that other property location all of

30
00:02:03,630 --> 00:02:11,480
which we could choose to use in this case we need the latitude property so data latitude as the first

31
00:02:11,570 --> 00:02:17,390
argument to forecast now as the second argument to forecast we want to use the longitude.

32
00:02:17,390 --> 00:02:20,100
So over here we're gonna go ahead and get that done.

33
00:02:20,240 --> 00:02:21,710
I'm gonna remove these static.

34
00:02:21,710 --> 00:02:28,440
No we have in place for that second argument and I'll replace it with data dot longitude.

35
00:02:28,570 --> 00:02:29,420
Perfect.

36
00:02:29,440 --> 00:02:36,040
Now that we have this in place the input for forecast comes from the output for geo code and the end

37
00:02:36,040 --> 00:02:41,620
result is that we're going to get the weather for Boston because that's what's passed into geo code

38
00:02:41,710 --> 00:02:42,770
up above.

39
00:02:42,790 --> 00:02:46,150
Let's go ahead and make sure this actually works as expected.

40
00:02:46,150 --> 00:02:49,710
Right here I'm going gonna save apt J S and down below.

41
00:02:49,720 --> 00:02:52,550
We're gonna run the file so node app dot J.

42
00:02:52,560 --> 00:02:53,820
S and what do we get.

43
00:02:53,830 --> 00:02:59,620
We're gonna get two pieces of output and you'll notice that the geo coding information printed first

44
00:02:59,850 --> 00:03:05,350
we got our data because things went well then down below we got our forecast because once again things

45
00:03:05,350 --> 00:03:06,420
went well.

46
00:03:06,430 --> 00:03:08,690
Now you'll notice something pretty suspicious.

47
00:03:08,710 --> 00:03:14,190
It currently says it is negative 18 degrees and that's in Fahrenheit in Boston.

48
00:03:14,290 --> 00:03:15,010
It's not.

49
00:03:15,010 --> 00:03:16,680
It is mid November.

50
00:03:16,720 --> 00:03:18,160
Clearly not that cold.

51
00:03:18,160 --> 00:03:24,280
The problem here is that I've actually reversed the order of latitude and longitude so the latitude

52
00:03:24,280 --> 00:03:30,220
comes second with the index of one and the longitude comes first with the index of zero.

53
00:03:30,220 --> 00:03:33,700
So that's going to actually bring our application to a working state.

54
00:03:33,700 --> 00:03:35,310
Sorry for the confusion there.

55
00:03:35,310 --> 00:03:38,780
And now we get a temperature that makes a bit more sense.

56
00:03:38,800 --> 00:03:40,200
I reran the program.

57
00:03:40,210 --> 00:03:45,580
We still get our data printing but we can see these properties are reversed to the correct ones and

58
00:03:45,580 --> 00:03:49,750
down below we get the forecast for Boston once again clear throughout the day.

59
00:03:49,930 --> 00:03:55,620
But now it's about 40 degrees so above freezing as opposed to minus 18.

60
00:03:55,660 --> 00:04:00,580
Now that we have the basics in place let's make sure we're still handling things like our errors.

61
00:04:00,580 --> 00:04:06,340
So right here if something goes wrong with GEO code we want to make sure to use the error message in

62
00:04:06,340 --> 00:04:07,480
some sort of way.

63
00:04:07,600 --> 00:04:11,470
And we want to make sure we don't run the forecast call down below.

64
00:04:11,470 --> 00:04:17,560
If something went wrong with GEO code we won't have the input necessary to actually run forecast.

65
00:04:17,560 --> 00:04:22,340
So right here we're going to use a condition if there is an error do something.

66
00:04:22,420 --> 00:04:23,870
What are we going to do for now.

67
00:04:23,890 --> 00:04:26,110
We'll continue to print it.

68
00:04:26,140 --> 00:04:28,060
Now the goal here is to print the error.

69
00:04:28,090 --> 00:04:32,170
But the goal is to also not run the other code down here.

70
00:04:32,200 --> 00:04:34,480
Now there are two ways we could get that done.

71
00:04:34,480 --> 00:04:37,050
One is to toss on and else clots.

72
00:04:37,060 --> 00:04:42,540
We put all of this code inside of else and we know that only one of the two will run.

73
00:04:42,610 --> 00:04:47,330
Another approach worth noting is that you could just use return inside of here.

74
00:04:47,440 --> 00:04:53,040
Return is going to stop the function execution after printing the error message to the console.

75
00:04:53,080 --> 00:04:55,070
And this is a very common pattern.

76
00:04:55,120 --> 00:04:58,930
So I'm going to use it here to give you a bit more exposure to it.

77
00:04:59,020 --> 00:05:05,800
Now if there is an error the function stops and the error prints otherwise down below we continue on.

78
00:05:05,800 --> 00:05:08,230
Now we don't need these console dot log calls.

79
00:05:08,230 --> 00:05:13,010
The next thing we want to do if we do have data is to call forecast.

80
00:05:13,030 --> 00:05:15,700
And that's exactly what we're currently doing.

81
00:05:15,700 --> 00:05:19,240
Now we know that things can go wrong with forecast as well.

82
00:05:19,240 --> 00:05:22,370
So let's go ahead and set up our if statement inside of there.

83
00:05:22,570 --> 00:05:28,270
If something goes wrong with forecast once again I'll use return to stop this callback function and

84
00:05:28,270 --> 00:05:32,510
I will just print the error message to the console so down below.

85
00:05:32,530 --> 00:05:37,630
This is where we put the code that runs if both requests worked.

86
00:05:37,630 --> 00:05:43,060
If we geo coded and we got the forecast we can put our final code right here.

87
00:05:43,060 --> 00:05:44,260
And what are we going to do here.

88
00:05:44,470 --> 00:05:46,920
Well the goal here is to print two things.

89
00:05:47,020 --> 00:05:53,100
I want to print the location property from Geo code so people know what they're seeing the weather for

90
00:05:53,440 --> 00:05:58,900
and I also want to obviously print the forecast which is the data that comes back from the forecast

91
00:05:58,900 --> 00:05:59,720
function.

92
00:05:59,830 --> 00:06:05,050
Both of those are going to print down below giving the user all of the information we were hoping to

93
00:06:05,050 --> 00:06:05,770
provide.

94
00:06:05,770 --> 00:06:09,610
Now the goal is to use the output from both geo code and forecast.

95
00:06:09,610 --> 00:06:13,420
The problem though is that the argument name is the same for each.

96
00:06:13,420 --> 00:06:20,420
So down below in our forecast callback function we're only ever gonna be able to access the data here.

97
00:06:20,440 --> 00:06:26,710
The data here is going to shadow over data up above all we have to do to address that is to come up

98
00:06:26,710 --> 00:06:29,320
with a unique name for one or both.

99
00:06:29,560 --> 00:06:33,730
So right here I could call this something like forecast data.

100
00:06:33,730 --> 00:06:38,500
Now I have data and forecast data there's unique names and I can access both.

101
00:06:38,500 --> 00:06:44,170
Down below I'm going to remove those example console dot log calls we had before and we're going to

102
00:06:44,170 --> 00:06:48,240
start by printing the location that is data dot location.

103
00:06:48,250 --> 00:06:52,850
The last property from the geo code output that we have not used yet.

104
00:06:52,930 --> 00:06:56,320
Then down below the location we're going to print the forecast.

105
00:06:56,320 --> 00:07:04,300
So console dot log forecast data now that we have this in place we have a pretty good setup in the terminal.

106
00:07:04,300 --> 00:07:07,670
We can run our application node space app dot and J.

107
00:07:07,670 --> 00:07:14,600
S and if I fire that command off what do I get I get my location information followed by my forecast.

108
00:07:14,620 --> 00:07:17,490
Now here I can see which Boston I'm looking for.

109
00:07:17,500 --> 00:07:22,570
So if I was looking for a Boston in a different state or a different country I would know that I'm not

110
00:07:22,570 --> 00:07:27,610
actually getting the weather for what I intended to get the weather for and I could refine my search

111
00:07:27,630 --> 00:07:30,170
result even more to get a different Boston.

112
00:07:30,520 --> 00:07:35,770
So after the location we have the forecast once again still clear throughout the day with a temperature

113
00:07:35,770 --> 00:07:40,930
of thirty nine point five five degrees and a zero percent chance of rain.

114
00:07:40,960 --> 00:07:46,930
So this is the final output we're going gonna have if things go as they were supposed to.

115
00:07:46,930 --> 00:07:53,590
The only thing left to do is to make sure that the user can actually provide the location without needing

116
00:07:53,590 --> 00:07:55,870
to modify the source code.

117
00:07:55,870 --> 00:08:02,140
So we want to get this value which is currently hardcoded in as a command line argument.

118
00:08:02,140 --> 00:08:07,090
Getting that done is going to be your challenge for the video and down below I have some challenged

119
00:08:07,090 --> 00:08:09,400
comments outlining what I'd like you to do.

120
00:08:09,520 --> 00:08:14,260
The big picture goal except a location via a command line argument.

121
00:08:14,260 --> 00:08:20,220
Now you're going to access this single value without using any module like yards.

122
00:08:20,320 --> 00:08:26,620
You can use the standard functionality we explored on process and that was the second video of Section

123
00:08:26,620 --> 00:08:27,350
3.

124
00:08:27,400 --> 00:08:29,460
So if you forget exactly how that worked.

125
00:08:29,470 --> 00:08:30,570
That's OK.

126
00:08:30,580 --> 00:08:34,400
Go back to the video to jog your memory or use the PDA guide.

127
00:08:34,480 --> 00:08:37,970
Then you can actually integrate it into the app here.

128
00:08:37,990 --> 00:08:43,720
Now once you have the argument you're going to use that string as the input for geo code.

129
00:08:43,930 --> 00:08:49,680
So instead of having these static String Boston you'll reference the value the user provided.

130
00:08:49,690 --> 00:08:53,740
Now you're obviously only going to do that if a location was provided.

131
00:08:53,830 --> 00:08:59,410
If you're not seeing any argument provided you can go ahead and print a message saying something like

132
00:08:59,410 --> 00:09:05,230
please provide an address and then you would not run geo code and you could just use and else and an

133
00:09:05,230 --> 00:09:07,370
if statement to get that done.

134
00:09:07,390 --> 00:09:10,840
Now the last thing is to test your work with a couple of locations.

135
00:09:10,930 --> 00:09:17,050
And this I will give you so down below it's going to look a bit like that snowed app dot J S there's

136
00:09:17,050 --> 00:09:21,260
not going to be anything fancy we're just going to provide the location right here.

137
00:09:21,310 --> 00:09:24,850
So if I wanted to search for Boston it would look like this.

138
00:09:24,940 --> 00:09:30,460
Now if I wanted to search for a search query that had two words I would have to put those in double

139
00:09:30,460 --> 00:09:33,130
quotes like new space York.

140
00:09:33,130 --> 00:09:38,290
So you want to make sure that both of these work and in the end of the day you're going to get this

141
00:09:38,680 --> 00:09:41,590
and this as the first argument value.

142
00:09:41,590 --> 00:09:47,230
So take some time to refer back to what we covered quite some time ago and use it in the application

143
00:09:47,230 --> 00:09:47,760
here.

144
00:09:47,890 --> 00:09:53,110
When you're done test with those two locations and make sure you get what at least look like accurate

145
00:09:53,170 --> 00:09:54,210
temperatures.

146
00:09:54,340 --> 00:10:02,530
All right take some time to knock that out and when you're done come back and click play.

147
00:10:02,540 --> 00:10:04,190
How would you do to get this done.

148
00:10:04,190 --> 00:10:09,090
You needed to use the ARG the property on process which we explored earlier.

149
00:10:09,110 --> 00:10:14,840
So if I just dump that to the console we can see what we get when we run commands like these process

150
00:10:14,900 --> 00:10:16,010
dot arg.

151
00:10:16,490 --> 00:10:22,400
I'm going to save the program down below I'm going to rerun one of those commands and we can see what's

152
00:10:22,460 --> 00:10:23,540
on that ARG.

153
00:10:23,540 --> 00:10:31,100
The variable right here as we talked about earlier the third value is the first thing we provided the

154
00:10:31,100 --> 00:10:34,370
first thing is the path to the node executable.

155
00:10:34,370 --> 00:10:37,730
The second thing is the path to the script we ran.

156
00:10:37,730 --> 00:10:42,610
And the third is in this case New York or in the case of the other command Boston.

157
00:10:42,650 --> 00:10:44,180
That is what we want.

158
00:10:44,180 --> 00:10:51,350
So right here you could have created a variable like address setting it equal to a process dot ARG v

159
00:10:51,560 --> 00:10:54,950
accessing the third item with the index of 2.

160
00:10:54,950 --> 00:11:00,680
So now we have access to the command line argument and we want to use it as the input for geo code down

161
00:11:00,680 --> 00:11:01,110
below.

162
00:11:01,520 --> 00:11:08,660
So instead of the static String Boston I'm going to grab whatever value is stored on the address variable.

163
00:11:08,660 --> 00:11:13,880
Now this doesn't account for if no location was provided but for the moment we can at least test things

164
00:11:13,910 --> 00:11:15,480
out down below.

165
00:11:15,560 --> 00:11:20,390
I'm going to rerun the exact same command using New York as my search term.

166
00:11:20,570 --> 00:11:22,190
If I run it what do I get.

167
00:11:22,310 --> 00:11:26,900
I get New York New York in the United States and I get the temperature for that.

168
00:11:26,900 --> 00:11:33,650
Now I could run a different location node app J S Philadelphia I don't have to put it in quotes because

169
00:11:33,650 --> 00:11:35,510
it's just a single word.

170
00:11:35,660 --> 00:11:40,520
And if I fire that off I get the weather for where I'm currently in filming this video.

171
00:11:40,520 --> 00:11:46,550
So now that we have that in place we want to make sure we only geo code if an address is provided.

172
00:11:46,550 --> 00:11:52,410
Otherwise we'll print some sort of little string that's going to be what needs to happen for step three.

173
00:11:52,520 --> 00:11:54,560
And we could get that done with an if statement.

174
00:11:54,560 --> 00:12:01,690
So right here if there is no address what are we going to do.

175
00:12:01,750 --> 00:12:11,070
Console log please provide an address otherwise inside of an else clause.

176
00:12:11,080 --> 00:12:12,920
We're gonna go ahead and run the code.

177
00:12:13,000 --> 00:12:17,990
We had down below so I'll take my geo code call from beginning all the way to end.

178
00:12:18,040 --> 00:12:23,820
I'm going to cut it out of its current location and I'm going to paste it right inside of else.

179
00:12:23,950 --> 00:12:27,760
So that takes care of steps one two and three.

180
00:12:27,760 --> 00:12:34,240
Now we're just going to test our work and make sure we do get the error message when no address is provided.

181
00:12:34,240 --> 00:12:38,770
I'm going to remove the challenge comments as well as a couple of those extra lines and I'm going to

182
00:12:38,770 --> 00:12:41,210
test things out so down below.

183
00:12:41,230 --> 00:12:46,110
Let's run it node space app dot J s when I do that I would expect to see.

184
00:12:46,120 --> 00:12:53,420
Please provide an address and that is exactly what I get now node app dot J S Dublin.

185
00:12:53,550 --> 00:12:57,890
I'm gonna fire that off and this time we do indeed get a valid location.

186
00:12:57,900 --> 00:13:01,720
Dublin Ireland and I do indeed get the temperature for that.

187
00:13:01,830 --> 00:13:08,430
Now if I was looking for a different Dublin I could be more specific inside of quotes like Dublin P.A..

188
00:13:08,580 --> 00:13:15,040
That's where I grew up and if I fire that off now I'm getting Dublin in Pennsylvania in the United States.

189
00:13:15,060 --> 00:13:17,660
With that in place the challenge is all done.

190
00:13:17,670 --> 00:13:20,420
Now there are plenty of ways you could have solved this.

191
00:13:20,430 --> 00:13:26,430
You might not have used the exact same setup and that is a ok as long as you're getting similar results

192
00:13:26,430 --> 00:13:27,830
for those commands.

193
00:13:27,840 --> 00:13:29,620
That's where we're going to stop for this one.

194
00:13:29,640 --> 00:13:34,530
I'm excited to continue on in the next video so let's go ahead and jump right in.