1 00:00:01,330 --> 00:00:02,870 Welcome back. 2 00:00:02,870 --> 00:00:06,140 Let's now talk about our first data structure. 3 00:00:06,140 --> 00:00:07,703 And that's gonna be Arrays. 4 00:00:09,740 --> 00:00:12,690 And let's say, I want it to store my friends' names 5 00:00:12,690 --> 00:00:17,120 in variables so that I could use them later in my program. 6 00:00:17,120 --> 00:00:19,660 And so with the knowledge that we have so far 7 00:00:19,660 --> 00:00:21,710 this is how we would do it. 8 00:00:21,710 --> 00:00:22,840 So const 9 00:00:25,170 --> 00:00:27,120 friend one 10 00:00:27,120 --> 00:00:30,668 let's say Michael, 11 00:00:30,668 --> 00:00:33,170 and let's spell this correct. 12 00:00:33,170 --> 00:00:34,580 Then friend two 13 00:00:39,530 --> 00:00:40,730 Steven and then 14 00:00:43,240 --> 00:00:46,570 friend three 15 00:00:46,570 --> 00:00:48,350 let's say Peter. 16 00:00:48,350 --> 00:00:51,910 Now this isn't really fun to do, right? 17 00:00:51,910 --> 00:00:53,580 Because imagine that 18 00:00:53,580 --> 00:00:56,620 we actually wanted to represent 10 friends 19 00:00:56,620 --> 00:00:59,870 and then we would have to create 10 variables. 20 00:00:59,870 --> 00:01:02,530 So that doesn't sound like fun. 21 00:01:02,530 --> 00:01:04,440 Instead, wouldn't it be great 22 00:01:04,440 --> 00:01:08,180 to essentially bundle all of these values together 23 00:01:08,180 --> 00:01:11,270 into some bigger container? 24 00:01:11,270 --> 00:01:15,550 Well, that's why we have data structures in JavaScript. 25 00:01:15,550 --> 00:01:18,870 And Arrays are such a data structure. 26 00:01:18,870 --> 00:01:21,770 So an Array is like a big container 27 00:01:21,770 --> 00:01:24,090 into which we can throw variables 28 00:01:24,090 --> 00:01:26,510 and then later reference them. 29 00:01:26,510 --> 00:01:28,050 And that's super important. 30 00:01:28,050 --> 00:01:30,860 Because programming is most of the time, 31 00:01:30,860 --> 00:01:32,620 all about data. 32 00:01:32,620 --> 00:01:37,050 So we get data from somewhere we store and process data 33 00:01:37,050 --> 00:01:39,390 and then we give some data back. 34 00:01:39,390 --> 00:01:42,060 And that data, it has to go somewhere. 35 00:01:42,060 --> 00:01:44,800 So it has to be stored in some place. 36 00:01:44,800 --> 00:01:48,700 And for that, we use data structures, just like Arrays. 37 00:01:48,700 --> 00:01:52,410 So in case we have more, than just a single value. 38 00:01:52,410 --> 00:01:54,790 The two most important data structures 39 00:01:54,790 --> 00:01:58,970 at least in JavaScript, are Arrays and Objects. 40 00:01:58,970 --> 00:02:02,003 And so now let's learn all about Arrays. 41 00:02:02,880 --> 00:02:04,940 So instead of doing this 42 00:02:04,940 --> 00:02:06,140 let's create a variable, 43 00:02:07,680 --> 00:02:09,290 which I'm gonna call a friends 44 00:02:10,550 --> 00:02:13,740 and then we use the brackets, to create a new Array. 45 00:02:13,740 --> 00:02:17,260 And then in there, we can put different values, 46 00:02:17,260 --> 00:02:19,270 separated by commas. 47 00:02:19,270 --> 00:02:21,913 And so let's now use the exact same friends. 48 00:02:23,220 --> 00:02:25,723 So Michael comma, 49 00:02:26,800 --> 00:02:30,250 then Steven comma 50 00:02:30,250 --> 00:02:33,460 and Peter. 51 00:02:33,460 --> 00:02:35,860 And make sure that these commas here 52 00:02:35,860 --> 00:02:37,730 are outside of the string. 53 00:02:37,730 --> 00:02:40,330 So that's an error I see people do. 54 00:02:40,330 --> 00:02:42,860 And so this needs to be one string. 55 00:02:42,860 --> 00:02:44,860 So you need to start and finish it, 56 00:02:44,860 --> 00:02:48,330 then the comma and then another value then another comma 57 00:02:48,330 --> 00:02:50,183 and then another value. 58 00:02:52,840 --> 00:02:53,673 Okay. 59 00:02:53,673 --> 00:02:57,773 And now let's take a look at it in the console. 60 00:02:59,800 --> 00:03:00,930 Okay. 61 00:03:00,930 --> 00:03:04,980 And so this is what the Array looks like, in the console. 62 00:03:04,980 --> 00:03:09,240 So basically just as we wrote it in our code. 63 00:03:09,240 --> 00:03:12,980 And that is the absolute basics about Arrays. 64 00:03:12,980 --> 00:03:14,720 So that's how we create one. 65 00:03:14,720 --> 00:03:17,440 At least that's the way that we use the most 66 00:03:17,440 --> 00:03:19,860 but there's actually another way. 67 00:03:19,860 --> 00:03:21,743 So let me show that to you as well. 68 00:03:22,870 --> 00:03:26,093 So let's say we wanted an Array of years. 69 00:03:27,070 --> 00:03:29,000 And so instead of the brackets, 70 00:03:29,000 --> 00:03:30,410 we could also write 71 00:03:30,410 --> 00:03:34,460 new, Array and then parenthesis. 72 00:03:34,460 --> 00:03:37,730 And in there we would then specify all our values. 73 00:03:37,730 --> 00:03:40,100 Let's say 1991, 74 00:03:40,100 --> 00:03:41,950 1984, 75 00:03:41,950 --> 00:03:42,783 2008, 76 00:03:43,620 --> 00:03:46,520 and let's say 2020. 77 00:03:46,520 --> 00:03:50,130 So on the rate can hold as many values as we want. 78 00:03:50,130 --> 00:03:53,870 And also values of any type that we'd like. 79 00:03:53,870 --> 00:03:56,090 So it doesn't have to be strings like here, 80 00:03:56,090 --> 00:03:58,403 of course, numbers work just the same. 81 00:03:59,340 --> 00:04:02,670 And so here we used a different way of creating the Array, 82 00:04:02,670 --> 00:04:05,060 which was using this Array function. 83 00:04:05,060 --> 00:04:08,350 So you see that it's a function because we called it here 84 00:04:08,350 --> 00:04:10,050 basically using these parenthesis. 85 00:04:11,150 --> 00:04:14,120 And we also needed to use the new keyword 86 00:04:14,120 --> 00:04:16,710 because otherwise it's not gonna work. 87 00:04:16,710 --> 00:04:20,240 But anyway, it's way more usual to just use the brackets 88 00:04:20,240 --> 00:04:24,173 like I did up here, which is called the a literal syntax. 89 00:04:26,210 --> 00:04:27,043 Great. 90 00:04:27,043 --> 00:04:29,270 So now we know how to create Arrays. 91 00:04:29,270 --> 00:04:32,660 So basically how to put elements into an Array, 92 00:04:32,660 --> 00:04:36,290 but of course we need to have a way to get them out. 93 00:04:36,290 --> 00:04:39,393 And for that, we use the square bracket syntax again. 94 00:04:40,390 --> 00:04:43,140 So let's say that we want to log to the console, 95 00:04:43,140 --> 00:04:46,250 the first element of this friend's Array. 96 00:04:46,250 --> 00:04:47,583 So that would be Michael. 97 00:04:48,520 --> 00:04:51,230 So console.log, 98 00:04:51,230 --> 00:04:53,913 and now we write friends. 99 00:04:54,870 --> 00:04:56,537 So that's the name of the Array. 100 00:04:56,537 --> 00:04:58,510 And then we use the square brackets 101 00:04:58,510 --> 00:05:01,090 and then the number zero. 102 00:05:01,090 --> 00:05:03,900 That's because Arrays are zero-based, 103 00:05:03,900 --> 00:05:06,380 which means that this first element 104 00:05:06,380 --> 00:05:08,210 is the element number zero. 105 00:05:08,210 --> 00:05:10,570 Then this one is element number one 106 00:05:10,570 --> 00:05:13,750 number two, and so on and so forth. 107 00:05:13,750 --> 00:05:17,230 And so if we want the element at position zero, 108 00:05:17,230 --> 00:05:18,940 so this first one, 109 00:05:18,940 --> 00:05:22,623 then we write friends square bracket zero. 110 00:05:23,510 --> 00:05:24,343 And that's it. 111 00:05:25,200 --> 00:05:27,980 And now let's say we wanted Peter here as well. 112 00:05:27,980 --> 00:05:30,963 So that would be element zero, one, two. 113 00:05:32,240 --> 00:05:35,113 And so, let's just write two. 114 00:05:36,010 --> 00:05:37,950 And check it out. 115 00:05:37,950 --> 00:05:40,950 And indeed we get Michael and Peter. 116 00:05:40,950 --> 00:05:41,783 Cool. 117 00:05:42,660 --> 00:05:46,170 Now we can also get the actual number of elements 118 00:05:46,170 --> 00:05:47,477 that is in the Array. 119 00:05:48,980 --> 00:05:52,490 So let's also log that one to the console. 120 00:05:52,490 --> 00:05:54,460 And this one works by saying, 121 00:05:54,460 --> 00:05:58,350 friends.length. 122 00:05:58,350 --> 00:05:59,350 Okay. 123 00:05:59,350 --> 00:06:03,310 And this dot length is something called a property 124 00:06:03,310 --> 00:06:05,320 which we're gonna talk about a little bit later 125 00:06:05,320 --> 00:06:08,190 in the section when we talk about objects. 126 00:06:08,190 --> 00:06:11,470 Anyway what matters, is that this is gonna be the exact 127 00:06:11,470 --> 00:06:14,270 amount of elements that is in the Array. 128 00:06:14,270 --> 00:06:15,980 And it's not zero based. 129 00:06:15,980 --> 00:06:17,390 So it will not be two, 130 00:06:17,390 --> 00:06:21,130 even though the last one here is element number two. 131 00:06:21,130 --> 00:06:23,050 Instead it really gives us the number 132 00:06:23,050 --> 00:06:24,890 of elements in the Array. 133 00:06:24,890 --> 00:06:27,550 So we should expect this to log three 134 00:06:28,560 --> 00:06:30,820 and yeah, here we go. 135 00:06:30,820 --> 00:06:32,540 Now we can use this to 136 00:06:32,540 --> 00:06:36,110 automatically get the last element of any Array. 137 00:06:36,110 --> 00:06:38,860 And that is useful so that we don't have to count 138 00:06:38,860 --> 00:06:41,820 how many elements are in the Array. 139 00:06:41,820 --> 00:06:44,533 So let me show you how we could do that. 140 00:06:46,250 --> 00:06:47,630 Friends, 141 00:06:47,630 --> 00:06:51,150 and then remember to retrieve an element from the Array, 142 00:06:51,150 --> 00:06:53,250 we need to square brackets. 143 00:06:53,250 --> 00:06:54,890 So just like this. 144 00:06:54,890 --> 00:06:58,020 And then to get the index of the last element in the Array 145 00:06:58,020 --> 00:07:01,343 all we need to do, is friends.length minus one. 146 00:07:02,640 --> 00:07:05,400 So friends.length 147 00:07:06,300 --> 00:07:08,300 minus one. 148 00:07:08,300 --> 00:07:10,250 And that's because again, 149 00:07:10,250 --> 00:07:12,860 friends.length is not zero based. 150 00:07:12,860 --> 00:07:14,850 So this is three I remember, 151 00:07:14,850 --> 00:07:16,940 but this last element is two. 152 00:07:16,940 --> 00:07:21,593 And so we always need to subtract one from this number here. 153 00:07:22,770 --> 00:07:23,790 Okay. 154 00:07:23,790 --> 00:07:25,240 So what this also means, 155 00:07:25,240 --> 00:07:29,270 is that inside of these brackets, we can put any expression. 156 00:07:29,270 --> 00:07:31,790 It doesn't just have to be a number 157 00:07:31,790 --> 00:07:34,173 literally like we did here and here. 158 00:07:35,330 --> 00:07:37,700 So here we have this expression 159 00:07:37,700 --> 00:07:39,190 and remember that an expression 160 00:07:39,190 --> 00:07:41,970 is something that produces a value. 161 00:07:41,970 --> 00:07:45,420 And so this one here is gonna be calculated first. 162 00:07:45,420 --> 00:07:48,080 So it will compute friends.length 163 00:07:48,080 --> 00:07:50,550 which is three minus one is two, 164 00:07:50,550 --> 00:07:53,223 and then it will get friends at position two. 165 00:07:54,530 --> 00:07:57,000 And that's also one of the reasons 166 00:07:57,000 --> 00:07:58,540 why it's important that you know 167 00:07:58,540 --> 00:08:02,290 what is an expression and what is a statement. 168 00:08:02,290 --> 00:08:06,200 Because once again, we could not put a statement in here. 169 00:08:06,200 --> 00:08:08,160 So inside of these square brackets, 170 00:08:08,160 --> 00:08:11,713 JavaScript expects an expression, not a statement. 171 00:08:12,800 --> 00:08:16,090 Anyway, we got to the correct element from the Array, 172 00:08:16,090 --> 00:08:17,607 which is the element number two. 173 00:08:17,607 --> 00:08:21,270 And so that means that this automatic retrieval 174 00:08:21,270 --> 00:08:23,893 of the last element works just fine. 175 00:08:24,840 --> 00:08:26,210 All right. 176 00:08:26,210 --> 00:08:29,850 And moving on, there is more stuff that we can do. 177 00:08:29,850 --> 00:08:33,720 So this square bracket syntax, that we used here 178 00:08:33,720 --> 00:08:37,350 is not only for retrieving elements from the Array 179 00:08:37,350 --> 00:08:41,610 but we can also change it to add elements to the Array. 180 00:08:41,610 --> 00:08:44,040 So let's say that for some reason, 181 00:08:44,040 --> 00:08:46,330 I'm no longer friends with Peter 182 00:08:46,330 --> 00:08:49,930 and that I want to replace him with some other friends. 183 00:08:49,930 --> 00:08:53,740 And so now we can change or mutate the Array 184 00:08:53,740 --> 00:08:54,783 in the same way. 185 00:08:55,770 --> 00:08:57,210 So we can say 186 00:08:57,210 --> 00:09:00,920 friends at position number two 187 00:09:00,920 --> 00:09:03,380 and I'm doing it manually here again. 188 00:09:03,380 --> 00:09:06,890 So we can say that this should be equal to, something else. 189 00:09:06,890 --> 00:09:08,800 And let's say Jay. 190 00:09:08,800 --> 00:09:12,860 And if we now log this friends Array to the console, 191 00:09:12,860 --> 00:09:15,353 then we should already see the new Array. 192 00:09:17,110 --> 00:09:17,943 So, 193 00:09:19,160 --> 00:09:22,593 friends and let's see, 194 00:09:23,490 --> 00:09:26,360 and indeed the element number two. 195 00:09:26,360 --> 00:09:30,103 So this one here, is now replaced with Jay, 196 00:09:32,480 --> 00:09:33,360 okay. 197 00:09:33,360 --> 00:09:35,930 So in the beginning, when we first logged 198 00:09:35,930 --> 00:09:38,440 the friends Array to the console, 199 00:09:38,440 --> 00:09:39,990 it was still in this state. 200 00:09:39,990 --> 00:09:41,540 So it still had Peter. 201 00:09:41,540 --> 00:09:42,960 And that's why here in the beginning, 202 00:09:42,960 --> 00:09:45,720 we still see that original state. 203 00:09:45,720 --> 00:09:48,730 But then as we move on through the code, 204 00:09:48,730 --> 00:09:51,980 at some point here, we changed that Array. 205 00:09:51,980 --> 00:09:54,920 And so when we then log the Array again 206 00:09:54,920 --> 00:09:58,993 that change is of course reflected here in the output. 207 00:09:59,870 --> 00:10:00,920 But wait! 208 00:10:00,920 --> 00:10:03,170 Didn't I say in the last section, 209 00:10:03,170 --> 00:10:07,620 that variables declared with const, cannot be changed. 210 00:10:07,620 --> 00:10:11,140 And we did in fact declare the friends variable 211 00:10:11,140 --> 00:10:13,540 here with const, right? 212 00:10:13,540 --> 00:10:16,500 But I was still able to change one element 213 00:10:16,500 --> 00:10:19,910 of the Array here from Peter to Jay, right? 214 00:10:19,910 --> 00:10:22,380 So isn't that a contradiction? 215 00:10:22,380 --> 00:10:24,750 Well what I didn't tell you at the time, 216 00:10:24,750 --> 00:10:28,330 is that only primitive values, are immutable. 217 00:10:28,330 --> 00:10:31,350 But an Array is not a primitive value. 218 00:10:31,350 --> 00:10:35,780 And so we can actually always change it so we can mutate it. 219 00:10:35,780 --> 00:10:36,920 And it works this way 220 00:10:36,920 --> 00:10:41,830 because of the way that JavaScript stores things in memory. 221 00:10:41,830 --> 00:10:44,640 And once more we will have a whole lecture 222 00:10:44,640 --> 00:10:46,250 just on that topic alone, 223 00:10:46,250 --> 00:10:49,720 in a special section about how JavaScript works 224 00:10:49,720 --> 00:10:51,370 behind the scenes. 225 00:10:51,370 --> 00:10:53,010 So what you need to know for now 226 00:10:53,010 --> 00:10:56,158 is that we can actually mutate Arrays 227 00:10:56,158 --> 00:10:59,210 even though they were declared with const. 228 00:10:59,210 --> 00:11:00,610 Now what we can not do 229 00:11:00,610 --> 00:11:03,780 is to actually replace the entire Array. 230 00:11:03,780 --> 00:11:06,270 So we cannot do this. 231 00:11:06,270 --> 00:11:10,400 We cannot say that the entire friends' Array should now be 232 00:11:11,630 --> 00:11:15,570 Bob and Ellis. 233 00:11:15,570 --> 00:11:17,940 That would be illegal. 234 00:11:17,940 --> 00:11:19,470 So let's see. 235 00:11:19,470 --> 00:11:22,800 And now we get assignment to constant variable. 236 00:11:22,800 --> 00:11:24,760 And so that's why 237 00:11:24,760 --> 00:11:25,940 that we can not do. 238 00:11:25,940 --> 00:11:27,790 But let's just keep it here anyway. 239 00:11:27,790 --> 00:11:30,503 So you get this as a reference. 240 00:11:31,480 --> 00:11:34,430 Next up, I want to show you that an Array 241 00:11:34,430 --> 00:11:37,410 can actually hold values with different types 242 00:11:37,410 --> 00:11:39,330 all at the same time. 243 00:11:39,330 --> 00:11:42,780 So let's experiment with that. 244 00:11:42,780 --> 00:11:45,230 So let's create an Array which holds 245 00:11:45,230 --> 00:11:47,483 all kinds of information about me. 246 00:11:48,750 --> 00:11:52,490 So let's say Jonas, so that would be the first name 247 00:11:54,050 --> 00:11:57,260 then the last name, then the age. 248 00:11:57,260 --> 00:11:59,520 And here we can actually calculate the age 249 00:11:59,520 --> 00:12:01,320 from other values. 250 00:12:01,320 --> 00:12:03,550 And that works because in each position, 251 00:12:03,550 --> 00:12:06,470 JavaScript simply expects an expression. 252 00:12:06,470 --> 00:12:07,883 And so if we do this, 253 00:12:09,040 --> 00:12:12,570 then this is perfectly fine. 254 00:12:12,570 --> 00:12:13,500 Okay. 255 00:12:13,500 --> 00:12:15,950 So this will here produce a value. 256 00:12:15,950 --> 00:12:18,680 And so that's the value that will then be stored 257 00:12:18,680 --> 00:12:21,780 at position number two of the new Array. 258 00:12:21,780 --> 00:12:24,380 And that also works for variables. 259 00:12:24,380 --> 00:12:25,830 So we could, for example do 260 00:12:26,840 --> 00:12:31,400 first name equal Jonas. 261 00:12:31,400 --> 00:12:32,420 And then 262 00:12:32,420 --> 00:12:35,170 here instead of literally writing that, 263 00:12:35,170 --> 00:12:37,810 we could use this variable name. 264 00:12:37,810 --> 00:12:41,003 And so this would then be replaced, with the actual string. 265 00:12:43,450 --> 00:12:45,300 So let's put some more data 266 00:12:45,300 --> 00:12:47,960 and actually we could even put other Arrays 267 00:12:47,960 --> 00:12:49,220 inside of an Array. 268 00:12:49,220 --> 00:12:51,200 So that's really cool. 269 00:12:51,200 --> 00:12:54,730 So let's put the friends Array here, 270 00:12:54,730 --> 00:12:56,810 in the Jonas Array. 271 00:12:56,810 --> 00:12:57,990 And then with this, 272 00:12:57,990 --> 00:13:01,640 we have all the relevant data about Jonas 273 00:13:01,640 --> 00:13:04,150 in one handy data structure. 274 00:13:04,150 --> 00:13:06,230 And that's really convenient because 275 00:13:06,230 --> 00:13:08,550 then we don't have to create one variable 276 00:13:08,550 --> 00:13:10,163 for each of the data points. 277 00:13:11,520 --> 00:13:13,080 So let's, 278 00:13:13,080 --> 00:13:15,430 of course, inspect is now in the console 279 00:13:15,430 --> 00:13:16,780 so that you can see 280 00:13:16,780 --> 00:13:19,790 that all of these expressions here basically. 281 00:13:19,790 --> 00:13:21,150 So this 282 00:13:21,150 --> 00:13:22,700 and this here 283 00:13:23,600 --> 00:13:24,950 and this, 284 00:13:24,950 --> 00:13:26,950 so all of them are gonna be replaced 285 00:13:26,950 --> 00:13:29,103 with the values that they produce. 286 00:13:30,110 --> 00:13:30,943 And so 287 00:13:31,990 --> 00:13:33,660 indeed, we now get Jonas. 288 00:13:33,660 --> 00:13:35,600 We have to calculate at age, 289 00:13:35,600 --> 00:13:38,130 and here have the Array at three. 290 00:13:38,130 --> 00:13:39,210 And we can expand that 291 00:13:39,210 --> 00:13:41,790 by clicking on this small triangle here 292 00:13:41,790 --> 00:13:45,670 and then we can see the actual Array in here. 293 00:13:45,670 --> 00:13:47,200 We also have the length. 294 00:13:47,200 --> 00:13:48,890 So length is five. 295 00:13:48,890 --> 00:13:50,720 And so that's where JavaScript 296 00:13:50,720 --> 00:13:54,223 gets this friends.length from. 297 00:13:57,490 --> 00:13:58,960 Jonas.length, 298 00:13:58,960 --> 00:14:00,390 would of course also work. 299 00:14:00,390 --> 00:14:02,583 And it would be this five that we see here. 300 00:14:04,920 --> 00:14:05,753 Okay. 301 00:14:05,753 --> 00:14:06,853 So that's the one. 302 00:14:07,700 --> 00:14:08,533 Great. 303 00:14:08,533 --> 00:14:12,870 So these are the very fundamentals of Arrays in JavaScript. 304 00:14:12,870 --> 00:14:14,660 And now just to finish this video, 305 00:14:14,660 --> 00:14:18,030 let's work on a very small Array exercise. 306 00:14:18,030 --> 00:14:22,620 So that we can see why Arrays are actually so useful. 307 00:14:22,620 --> 00:14:26,783 And to do that, let's bring back our old calcAge function. 308 00:14:28,550 --> 00:14:29,700 So 309 00:14:30,960 --> 00:14:33,010 it's gotta be somewhere here. 310 00:14:33,010 --> 00:14:34,320 Yes. 311 00:14:34,320 --> 00:14:36,010 Let's use this one. 312 00:14:36,010 --> 00:14:37,700 We've coded this so many times 313 00:14:37,700 --> 00:14:39,493 that I don't want to repeat it. 314 00:14:41,990 --> 00:14:43,333 So exercise. 315 00:14:46,360 --> 00:14:48,210 So calcAge. 316 00:14:48,210 --> 00:14:52,200 And now let's say that we have an Array of birth years. 317 00:14:52,200 --> 00:14:56,440 And then we want to calculate the ages for some of them. 318 00:14:56,440 --> 00:14:57,560 So, 319 00:14:57,560 --> 00:14:59,390 a new Array 320 00:15:00,610 --> 00:15:02,990 and let's say 1990, 321 00:15:02,990 --> 00:15:04,763 1967, 322 00:15:05,600 --> 00:15:06,643 2002, 323 00:15:08,282 --> 00:15:12,250 2010 and 2018. 324 00:15:12,250 --> 00:15:15,300 And now we will be able to use the calcAge function 325 00:15:15,300 --> 00:15:19,060 and then store the results of calculating the ages 326 00:15:19,060 --> 00:15:22,880 for some of these years into a new Array. 327 00:15:22,880 --> 00:15:24,490 So let's see how 328 00:15:24,490 --> 00:15:27,880 and let's see first how we could not do it. 329 00:15:27,880 --> 00:15:28,920 Okay. 330 00:15:28,920 --> 00:15:32,680 So for example, if we wanted to use the calcAge function 331 00:15:32,680 --> 00:15:35,110 for all of the elements in this Array, 332 00:15:35,110 --> 00:15:37,033 we could not do this. 333 00:15:39,310 --> 00:15:40,910 This would be 334 00:15:40,910 --> 00:15:42,680 not illegal, I would say 335 00:15:42,680 --> 00:15:44,670 but it's not gonna work 336 00:15:44,670 --> 00:15:47,570 because this years is an Array. 337 00:15:47,570 --> 00:15:51,090 And if we pass this years' Array as an argument 338 00:15:51,090 --> 00:15:52,800 to the calcAge function, 339 00:15:52,800 --> 00:15:54,660 it doesn't know what to do with it. 340 00:15:54,660 --> 00:15:59,420 Because this operation here expects a single value 341 00:15:59,420 --> 00:16:01,810 and we cannot do a number 342 00:16:01,810 --> 00:16:03,183 minus and array. 343 00:16:04,720 --> 00:16:06,610 So let me actually show you what's gonna happen 344 00:16:06,610 --> 00:16:08,240 if we run this, 345 00:16:08,240 --> 00:16:13,240 and well that's not the error I wanted to show you. 346 00:16:13,270 --> 00:16:14,103 So what we get is, 347 00:16:14,103 --> 00:16:17,390 "identifier 'years' has already been declared." 348 00:16:17,390 --> 00:16:20,240 So this means that somewhere in the code, we already have 349 00:16:21,470 --> 00:16:25,140 an Array called years and yeah, here it is. 350 00:16:25,140 --> 00:16:28,403 So that's changed the name of this one to just Y. 351 00:16:29,780 --> 00:16:32,750 So that one doesn't really matter anyway 352 00:16:32,750 --> 00:16:35,210 but now let's see what happens 353 00:16:35,210 --> 00:16:37,500 and well 354 00:16:37,500 --> 00:16:39,870 not much did happen. 355 00:16:39,870 --> 00:16:44,870 So we actually need to inspect the result, of this. 356 00:16:46,430 --> 00:16:48,490 So console.log, 357 00:16:48,490 --> 00:16:50,203 and then the result of this. 358 00:16:51,320 --> 00:16:56,090 So let's see, and now we get the result, "not a number." 359 00:16:56,090 --> 00:16:57,240 And so that's the result 360 00:16:57,240 --> 00:17:00,660 of attempting to subtract a whole Array 361 00:17:00,660 --> 00:17:03,603 which would be this year's Array, from a number. 362 00:17:04,620 --> 00:17:05,810 Okay. 363 00:17:05,810 --> 00:17:08,820 So let's actually do that here as well. 364 00:17:08,820 --> 00:17:11,080 Remember that we have access to all the variables 365 00:17:11,080 --> 00:17:12,860 that we declare in the script, 366 00:17:12,860 --> 00:17:14,640 also in the console. 367 00:17:14,640 --> 00:17:18,240 So that's the years Array, that we just declared. 368 00:17:18,240 --> 00:17:21,770 But years, for example, plus 10 369 00:17:21,770 --> 00:17:23,110 does not work. 370 00:17:23,110 --> 00:17:26,220 At least not in a way that we expected it. 371 00:17:26,220 --> 00:17:28,330 So the plus operator what it does here, 372 00:17:28,330 --> 00:17:30,750 is to convert everything to a string, 373 00:17:30,750 --> 00:17:32,440 just like I showed you before. 374 00:17:32,440 --> 00:17:33,970 And so the string here, 375 00:17:33,970 --> 00:17:36,780 is then basically this Array as a string, 376 00:17:36,780 --> 00:17:39,160 and then it adds the 10 here. 377 00:17:39,160 --> 00:17:41,290 So that's not meaningful at all. 378 00:17:41,290 --> 00:17:45,030 And if we use it with minus 10, then it's even weirder. 379 00:17:45,030 --> 00:17:47,040 So we get not a number. 380 00:17:47,040 --> 00:17:48,530 So all of this is to say 381 00:17:48,530 --> 00:17:52,150 that we cannot do operations with Arrays. 382 00:17:52,150 --> 00:17:55,020 But we can still use the calcAge function 383 00:17:55,020 --> 00:17:58,463 on individual elements of the Array of course. 384 00:17:59,780 --> 00:18:01,870 So let's do that. 385 00:18:01,870 --> 00:18:04,990 And we will assume that I want to calculate the ages 386 00:18:04,990 --> 00:18:08,830 for the first, the second and the last Array element. 387 00:18:08,830 --> 00:18:09,996 So this one, this one 388 00:18:09,996 --> 00:18:11,423 and this one. 389 00:18:12,620 --> 00:18:15,180 So let's 390 00:18:16,450 --> 00:18:17,930 calculate this 391 00:18:17,930 --> 00:18:21,320 and then store the result into a variable called age one 392 00:18:23,200 --> 00:18:26,150 and we need to get rid of this parenthesis. 393 00:18:26,150 --> 00:18:27,350 And so, as I said 394 00:18:27,350 --> 00:18:31,680 I want to calculate the age for this first element. 395 00:18:31,680 --> 00:18:33,320 And so what we do, 396 00:18:33,320 --> 00:18:36,590 is to take years at position zero. 397 00:18:36,590 --> 00:18:40,600 And so this year will then basically be replaced with 1990. 398 00:18:40,600 --> 00:18:43,810 And then calcAge is called with 1990, 399 00:18:43,810 --> 00:18:45,963 and to resolve the stores to age one. 400 00:18:47,050 --> 00:18:49,650 Now let's do this some more times 401 00:18:53,680 --> 00:18:57,090 and say, age two age three. 402 00:18:57,090 --> 00:18:59,700 And here I'm using one. 403 00:18:59,700 --> 00:19:03,580 So remember I want this, this and the last one. 404 00:19:03,580 --> 00:19:05,710 But I don't want to count 405 00:19:05,710 --> 00:19:07,850 the number of elements in the Array. 406 00:19:07,850 --> 00:19:10,000 And so I used that trick, that I just showed you 407 00:19:10,000 --> 00:19:11,250 in the beginning 408 00:19:11,250 --> 00:19:14,930 which is to first compute the length of the Array. 409 00:19:14,930 --> 00:19:17,770 So years.length, 410 00:19:17,770 --> 00:19:20,323 and then minus one to account for the fact 411 00:19:20,323 --> 00:19:22,803 that the index of the Array is zero based. 412 00:19:23,810 --> 00:19:28,810 And now we can quickly check out if this actually worked. 413 00:19:28,970 --> 00:19:31,520 So age one, age two, 414 00:19:31,520 --> 00:19:32,683 age three. 415 00:19:37,370 --> 00:19:38,203 And yeah. 416 00:19:38,203 --> 00:19:40,170 So we got three values here, 417 00:19:40,170 --> 00:19:42,330 which are these three ages. 418 00:19:42,330 --> 00:19:46,160 But we can take this even a little bit further. 419 00:19:46,160 --> 00:19:48,500 So since we started with an Array, 420 00:19:48,500 --> 00:19:53,070 it's usually a good idea, to also end up with a new Array. 421 00:19:53,070 --> 00:19:55,319 So let's create an Array 422 00:19:55,319 --> 00:19:57,923 const ages, 423 00:19:59,340 --> 00:20:01,890 and now what should I put in here? 424 00:20:01,890 --> 00:20:04,940 Well, remember how I said up here 425 00:20:04,940 --> 00:20:07,400 that any position of the Array 426 00:20:07,400 --> 00:20:09,220 simply needs to be an expression. 427 00:20:09,220 --> 00:20:11,690 So something that produces a value. 428 00:20:11,690 --> 00:20:13,250 And as we already know, 429 00:20:13,250 --> 00:20:16,280 this here produces a value as well, right? 430 00:20:16,280 --> 00:20:17,830 It is an expression. 431 00:20:17,830 --> 00:20:19,540 And so I can take this 432 00:20:19,540 --> 00:20:22,663 and simply put it as a position in this Array. 433 00:20:23,600 --> 00:20:25,670 And so JavaScript will then go ahead 434 00:20:25,670 --> 00:20:29,313 compute this value and place it in the Array. 435 00:20:30,260 --> 00:20:33,440 So basically we can place function calls 436 00:20:33,440 --> 00:20:35,440 into an Array just fine, 437 00:20:35,440 --> 00:20:37,140 because they will produce a value. 438 00:20:38,040 --> 00:20:39,420 And now here. 439 00:20:39,420 --> 00:20:40,660 We need to again, 440 00:20:40,660 --> 00:20:45,660 do years.length minus one. 441 00:20:49,581 --> 00:20:54,180 And now finally logging it to the console 442 00:20:54,180 --> 00:20:55,343 and then we're done. 443 00:20:57,200 --> 00:20:59,980 And indeed we get the three same values, 444 00:20:59,980 --> 00:21:02,140 that we had up here separately. 445 00:21:02,140 --> 00:21:05,610 Now, all nicely placed into this Array. 446 00:21:05,610 --> 00:21:06,910 And this of course works 447 00:21:06,910 --> 00:21:09,770 because JavaScript will start by 448 00:21:09,770 --> 00:21:11,260 running these function calls, 449 00:21:11,260 --> 00:21:13,570 these three function calls here 450 00:21:13,570 --> 00:21:16,963 and then place all the results in an Array afterwards. 451 00:21:17,830 --> 00:21:18,760 Nice. 452 00:21:18,760 --> 00:21:22,000 And I think that should be enough for one video. 453 00:21:22,000 --> 00:21:24,920 I hope that you see how fun Arrays can be, 454 00:21:24,920 --> 00:21:27,060 and how useful they can be. 455 00:21:27,060 --> 00:21:28,100 And in the next lecture, 456 00:21:28,100 --> 00:21:30,170 we will make them even more useful 457 00:21:30,170 --> 00:21:32,333 by doing some operations with them.