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These are the user uploaded subtitles that are being translated: 1 1 00:00:01,580 --> 00:00:03,030 So in the last lecture, 2 2 00:00:03,030 --> 00:00:06,170 we talked about values and variables. 3 3 00:00:06,170 --> 00:00:08,830 Now in every programming language values 4 4 00:00:08,830 --> 00:00:10,700 can have different types. 5 5 00:00:10,700 --> 00:00:14,810 Depending on the type of data that we want them to hold. 6 6 00:00:14,810 --> 00:00:17,580 And we already saw strings and numbers 7 7 00:00:17,580 --> 00:00:19,990 but there are actually more data types. 8 8 00:00:19,990 --> 00:00:22,580 So let's now take a look at all the types 9 9 00:00:22,580 --> 00:00:24,533 that we have in JavaScript. 10 10 00:00:25,950 --> 00:00:28,500 And first of all, in JavaScript, 11 11 00:00:28,500 --> 00:00:31,470 every value is either an object 12 12 00:00:31,470 --> 00:00:35,830 which looks something like this or a primitive value. 13 13 00:00:35,830 --> 00:00:40,500 So a value is only a primitive when it's not an object 14 14 00:00:40,500 --> 00:00:42,950 and we will learn all about objects later 15 15 00:00:42,950 --> 00:00:46,943 but for now let's really focus on primitive data types. 16 16 00:00:48,500 --> 00:00:51,290 So there are seven primitive data types, 17 17 00:00:51,290 --> 00:00:55,390 number, string, bullion, undefined, null, 18 18 00:00:55,390 --> 00:00:57,810 symbol and big int. 19 19 00:00:57,810 --> 00:01:00,390 So let's look at them one by one. 20 20 00:01:00,390 --> 00:01:03,320 First, we have the number of data type 21 21 00:01:03,320 --> 00:01:06,760 and numbers are always so-called floating point numbers 22 22 00:01:06,760 --> 00:01:09,630 which means that they always have decimals 23 23 00:01:09,630 --> 00:01:12,950 even if we don't see them or don't define them. 24 24 00:01:12,950 --> 00:01:16,470 For example, the 23 value debt we have here 25 25 00:01:16,470 --> 00:01:20,100 is exactly like having 23.0. 26 26 00:01:20,100 --> 00:01:23,410 But they are both simply the number data type. 27 27 00:01:23,410 --> 00:01:25,410 In many other programming languages, 28 28 00:01:25,410 --> 00:01:28,160 you will find different data types for integers 29 29 00:01:28,160 --> 00:01:31,060 and for decimals but not in JavaScript. 30 30 00:01:31,060 --> 00:01:34,250 All numbers are simply of the type number. 31 31 00:01:34,250 --> 00:01:37,650 Okay, next up we have strengths 32 32 00:01:37,650 --> 00:01:40,770 which are simply a sequence of characters. 33 33 00:01:40,770 --> 00:01:45,210 And so they're just used for text as we already learned 34 34 00:01:45,210 --> 00:01:46,990 and always put them in quotes, 35 35 00:01:46,990 --> 00:01:49,990 no matter if double quotes or single quotes 36 36 00:01:49,990 --> 00:01:52,660 otherwise JavaScript will actually confuse them 37 37 00:01:52,660 --> 00:01:54,203 with variable names. 38 38 00:01:55,110 --> 00:01:59,330 The Boolean data type is essentially a logical type 39 39 00:01:59,330 --> 00:02:03,870 that can only take one of the logical values true or false. 40 40 00:02:03,870 --> 00:02:08,700 In other words, a Boolean is always either true or false. 41 41 00:02:08,700 --> 00:02:12,300 All right, we use Boolean values to take decisions 42 42 00:02:12,300 --> 00:02:15,250 with code as we will see later. 43 43 00:02:15,250 --> 00:02:18,080 And these are the three most important data types 44 44 00:02:18,080 --> 00:02:20,230 that we will deal with the most 45 45 00:02:20,230 --> 00:02:22,330 but there are still four more 46 46 00:02:22,330 --> 00:02:25,010 which might be a bit more confusing. 47 47 00:02:25,010 --> 00:02:29,830 So first, undefined is the value taken by a variable 48 48 00:02:29,830 --> 00:02:31,980 that is not yet defined. 49 49 00:02:31,980 --> 00:02:34,630 And the variable thats not yet defined 50 50 00:02:34,630 --> 00:02:37,300 is simply just a variable that we declared 51 51 00:02:37,300 --> 00:02:39,790 but without assigning a value 52 52 00:02:39,790 --> 00:02:43,200 for example like this children variable here. 53 53 00:02:43,200 --> 00:02:47,920 So basically we can say that undefined means empty value 54 54 00:02:47,920 --> 00:02:51,800 then there's also null which is actually pretty similar 55 55 00:02:51,800 --> 00:02:54,550 because it also means empty value 56 56 00:02:54,550 --> 00:02:57,210 but it's used in different circumstances. 57 57 00:02:57,210 --> 00:02:59,950 But for now, don't worry about the details. 58 58 00:02:59,950 --> 00:03:04,910 I just want you to know that null also exists for now, okay. 59 59 00:03:04,910 --> 00:03:09,833 Anyway, next we have symbol which was introduced in ES2015 60 60 00:03:11,090 --> 00:03:13,960 and this data type is not really useful for us, 61 61 00:03:13,960 --> 00:03:16,840 it's simply defines a value that is unique 62 62 00:03:16,840 --> 00:03:18,620 and cannot be changed. 63 63 00:03:18,620 --> 00:03:20,500 And we will talk about this briefly 64 64 00:03:20,500 --> 00:03:22,890 by the end of the course. 65 65 00:03:22,890 --> 00:03:27,850 Finally, starting in ES2020 there is also BigInt 66 66 00:03:27,850 --> 00:03:30,730 which is for integers that are too large 67 67 00:03:30,730 --> 00:03:33,460 to be represented by the number type. 68 68 00:03:33,460 --> 00:03:37,020 So basically it's another tire before numbers 69 69 00:03:37,020 --> 00:03:40,420 and we will talk about this one in another section. 70 70 00:03:40,420 --> 00:03:45,050 So these are the seven primitive data types in JavaScript. 71 71 00:03:45,050 --> 00:03:49,470 But there is another fundamental thing to know about types 72 72 00:03:49,470 --> 00:03:51,640 which is the fact that JavaScript 73 73 00:03:51,640 --> 00:03:54,790 has a feature called dynamic typing. 74 74 00:03:54,790 --> 00:03:57,810 This means that when you create a new variable, 75 75 00:03:57,810 --> 00:04:01,260 you do not have to manually define the data type 76 76 00:04:01,260 --> 00:04:03,520 of the value that it contains. 77 77 00:04:03,520 --> 00:04:05,205 In many other programming languages, 78 78 00:04:05,205 --> 00:04:09,010 you actually have to do that but not in JavaScript. 79 79 00:04:09,010 --> 00:04:12,020 Instead, Java script automatically determines 80 80 00:04:12,020 --> 00:04:16,660 the data type of a value when it's stored into a variable. 81 81 00:04:16,660 --> 00:04:19,930 And this distinction between value and variable 82 82 00:04:19,930 --> 00:04:22,880 is pretty important because in JavaScript, 83 83 00:04:22,880 --> 00:04:27,130 it's the value that has a type, not the variable. 84 84 00:04:27,130 --> 00:04:31,020 So variables simply store values that have a type. 85 85 00:04:31,020 --> 00:04:35,170 Many people don't know about this detail or just don't care 86 86 00:04:35,170 --> 00:04:37,660 but this is how it actually works. 87 87 00:04:37,660 --> 00:04:41,041 Now another important application of dynamic typing 88 88 00:04:41,041 --> 00:04:44,930 is that later in our code, we can assign a new value 89 89 00:04:44,930 --> 00:04:47,930 with a different data type to the same variable 90 90 00:04:47,930 --> 00:04:49,300 without a problem. 91 91 00:04:49,300 --> 00:04:53,090 For example, variable x can initially be a number 92 92 00:04:53,090 --> 00:04:56,720 and then later a string, that's not a problem at all. 93 93 00:04:56,720 --> 00:04:59,170 And this can of course be very useful 94 94 00:04:59,170 --> 00:05:02,310 but it can also be the source of some difficult 95 95 00:05:02,310 --> 00:05:06,630 to find books which means errors in our code. 96 96 00:05:06,630 --> 00:05:10,643 All right, so let's not actually see these things in action. 97 97 00:05:12,120 --> 00:05:14,550 But before we talk about data types, 98 98 00:05:14,550 --> 00:05:17,670 let's quickly talk about code commenting. 99 99 00:05:17,670 --> 00:05:21,390 So in programming, we use comments to literally come 100 100 00:05:21,390 --> 00:05:25,410 and code or to deactivate code without deleting it. 101 101 00:05:25,410 --> 00:05:29,270 For example, here, I could write a single line comment 102 102 00:05:29,270 --> 00:05:34,270 using // and then I can describe what this code is doing. 103 103 00:05:34,400 --> 00:05:39,400 For example here, I can say, variable name conventions 104 104 00:05:41,340 --> 00:05:44,199 and JavaScript will completely ignore anything 105 105 00:05:44,199 --> 00:05:45,750 that is a comment. 106 106 00:05:45,750 --> 00:05:50,750 So anything that comes after // like this, okay. 107 107 00:05:51,097 --> 00:05:54,260 And this can be really useful to kind of divide 108 108 00:05:54,260 --> 00:05:56,610 or code into different sections 109 109 00:05:56,610 --> 00:05:59,450 and to explain what different pieces of code 110 110 00:05:59,450 --> 00:06:01,590 are actually doing in our code. 111 111 00:06:01,590 --> 00:06:04,460 So that's a very useful application of comments 112 112 00:06:04,460 --> 00:06:07,880 but another one is to simply comment out code 113 113 00:06:07,880 --> 00:06:11,400 that we don't want to be executed for some reason. 114 114 00:06:11,400 --> 00:06:14,170 For example, let's say we don't want this code here 115 115 00:06:14,170 --> 00:06:17,960 to be executed but we also don't want to delete it. 116 116 00:06:17,960 --> 00:06:21,450 So we can simply comment it out like this. 117 117 00:06:21,450 --> 00:06:23,910 And so if I save it and then 118 118 00:06:23,910 --> 00:06:26,513 when I reload this 61 should be gone. 119 119 00:06:27,900 --> 00:06:30,830 So you see, that's because JavaScript 120 120 00:06:30,830 --> 00:06:34,230 is now ignoring this code, it's just a comment. 121 121 00:06:34,230 --> 00:06:35,650 But since we didn't delete it, 122 122 00:06:35,650 --> 00:06:37,840 we can of course bring it back. 123 123 00:06:37,840 --> 00:06:42,020 So we can just delete it and then it's back to normal. 124 124 00:06:42,020 --> 00:06:45,300 Now in VS code we can also automatically create comments 125 125 00:06:45,300 --> 00:06:47,823 using the command slash shortcut. 126 126 00:06:49,010 --> 00:06:52,050 So that's command or control slash to turn on 127 127 00:06:52,050 --> 00:06:54,510 and then again to turn off. 128 128 00:06:54,510 --> 00:06:56,460 So that single line comments 129 129 00:06:56,460 --> 00:06:58,170 but we can go even further 130 130 00:06:58,170 --> 00:07:01,090 and also create multi line comments. 131 131 00:07:01,090 --> 00:07:04,780 And to do that, we do it like this. 132 132 00:07:04,780 --> 00:07:09,560 We start a multi line comment using slash and an asterisk 133 133 00:07:09,560 --> 00:07:13,810 and then we ended with the asterisk or star 134 134 00:07:13,810 --> 00:07:16,710 and then the slash and whatever comes here 135 135 00:07:16,710 --> 00:07:18,910 is then a normal code. 136 136 00:07:18,910 --> 00:07:21,720 Okay, and with this, I effectively got rid 137 137 00:07:21,720 --> 00:07:25,560 of this first code that we typed in the first lecture 138 138 00:07:25,560 --> 00:07:27,380 but without deleting it. 139 139 00:07:27,380 --> 00:07:31,460 And so I can keep it here basically as a reference. 140 140 00:07:31,460 --> 00:07:33,260 All right, and so that's what I'm gonna do 141 141 00:07:33,260 --> 00:07:34,610 in every lecture now. 142 142 00:07:34,610 --> 00:07:38,360 We'll always kind of get rid of the previous code 143 143 00:07:38,360 --> 00:07:41,163 by using a multi line comment like this. 144 144 00:07:42,070 --> 00:07:45,120 But now let's get back to the topic of this lecture 145 145 00:07:45,120 --> 00:07:47,180 which is data types. 146 146 00:07:47,180 --> 00:07:48,660 So in a previous lecture, 147 147 00:07:48,660 --> 00:07:52,640 we already saw numbers and strengths, right. 148 148 00:07:52,640 --> 00:07:56,100 So here we have a value which has a number 149 149 00:07:56,100 --> 00:07:59,540 and here we have a value which is a string. 150 150 00:07:59,540 --> 00:08:02,170 So nothing new, right. 151 151 00:08:02,170 --> 00:08:05,130 That's will be learned at the beginning of the video 152 152 00:08:05,130 --> 00:08:08,230 but we also talked about booleans, right? 153 153 00:08:08,230 --> 00:08:12,760 And so Boolean is a value that can only be true or false. 154 154 00:08:12,760 --> 00:08:14,473 And so if I write true, 155 155 00:08:15,870 --> 00:08:19,180 that is then automatically a Boolean value 156 156 00:08:19,180 --> 00:08:21,133 which is in this case true. 157 157 00:08:22,030 --> 00:08:25,290 Okay, and actually I can also lock that to the console 158 158 00:08:30,153 --> 00:08:31,890 and then here you get true 159 159 00:08:31,890 --> 00:08:33,770 and you can see that it's not a string 160 160 00:08:33,770 --> 00:08:36,250 because strings are printed in white 161 161 00:08:36,250 --> 00:08:39,570 and this value is printed in this pink. 162 162 00:08:39,570 --> 00:08:43,030 And so it' actually really a Boolean value. 163 163 00:08:43,030 --> 00:08:47,780 And of course we can also store Booleans in variables. 164 164 00:08:47,780 --> 00:08:52,780 So let's say Java script is fun using 165 165 00:08:54,440 --> 00:08:57,400 the camel case notation that we learned about before 166 166 00:08:58,370 --> 00:09:00,803 and done, set it to true. 167 167 00:09:03,040 --> 00:09:07,140 And now we can log JavaScriptIsFun instead 168 168 00:09:08,240 --> 00:09:10,700 but of course the variable declaration here needs 169 169 00:09:10,700 --> 00:09:12,860 to happen before the lock. 170 170 00:09:12,860 --> 00:09:15,810 And that's because JavaScript programs are executed 171 171 00:09:15,810 --> 00:09:19,260 from top to bottom like this 172 172 00:09:19,260 --> 00:09:22,030 and more about that a bit later, okay. 173 173 00:09:22,030 --> 00:09:24,630 What matters is that this need to be at the end. 174 174 00:09:24,630 --> 00:09:27,600 So let's cut it and put it here 175 175 00:09:28,620 --> 00:09:31,703 and also get rid of this one, save it. 176 176 00:09:33,400 --> 00:09:35,410 And here we still get true. 177 177 00:09:35,410 --> 00:09:37,590 And so here we created a nice 178 178 00:09:37,590 --> 00:09:42,590 and descriptive variable name holding a Boolean value. 179 179 00:09:43,260 --> 00:09:45,460 And remember that it's actually the value 180 180 00:09:45,460 --> 00:09:49,490 that holds the data type and not the variable 181 181 00:09:49,490 --> 00:09:52,200 because I could now actually go ahead and change this 182 182 00:09:52,200 --> 00:09:54,600 to a value with another type. 183 183 00:09:54,600 --> 00:09:57,000 And actually I will do that in a second 184 184 00:09:57,000 --> 00:09:58,700 but for now let's take a look 185 185 00:09:58,700 --> 00:10:01,920 at a special operator called type of. 186 186 00:10:01,920 --> 00:10:04,989 So type of it's an operator just like the plus 187 187 00:10:04,989 --> 00:10:06,989 or the minus operator 188 188 00:10:06,989 --> 00:10:11,050 that we can use to show the type of a value. 189 189 00:10:11,050 --> 00:10:12,433 So let me show you how. 190 190 00:10:16,680 --> 00:10:20,160 So again, we use console.log because we actually want to see 191 191 00:10:20,160 --> 00:10:23,728 the result of this operator in the console. 192 192 00:10:23,728 --> 00:10:28,728 So type of and then let's say just true, okay. 193 193 00:10:30,050 --> 00:10:32,730 And more about how operators actually work 194 194 00:10:32,730 --> 00:10:35,760 and what they are a bit later in the section. 195 195 00:10:35,760 --> 00:10:39,600 For now, let's just see what happens when we reload this 196 196 00:10:40,640 --> 00:10:43,060 and we see that it's a Boolean. 197 197 00:10:43,060 --> 00:10:47,100 So just as I said, the result of using this operator here 198 198 00:10:47,100 --> 00:10:49,780 on a value will be a new value 199 199 00:10:49,780 --> 00:10:54,780 which is a string with the type of this value here, okay. 200 200 00:10:55,410 --> 00:10:57,143 So let's do that again here. 201 201 00:10:57,143 --> 00:10:59,290 I will copy it just a couple of times 202 202 00:11:02,460 --> 00:11:05,330 just like this to make it a bit faster. 203 203 00:11:05,330 --> 00:11:08,337 And so here let's use JavaScriptIsFun. 204 204 00:11:09,990 --> 00:11:12,270 And so this should also be a Boolean 205 205 00:11:12,270 --> 00:11:15,710 but just to make sure. 206 206 00:11:15,710 --> 00:11:20,710 Here let's now use a number and finally a string. 207 207 00:11:23,030 --> 00:11:24,530 So just to prove you 208 208 00:11:24,530 --> 00:11:28,000 that these are actually numbers and strings. 209 209 00:11:28,000 --> 00:11:32,160 All right, and indeed we see that JavaScript 210 210 00:11:32,160 --> 00:11:37,160 is fun is a Boolean, 23 is a number and Jonas is a string. 211 211 00:11:38,910 --> 00:11:40,890 And speaking about strings, 212 212 00:11:40,890 --> 00:11:44,380 I just want to quickly emphasize that we really always need 213 213 00:11:44,380 --> 00:11:48,961 to use quotes in order to create a string, okay. 214 214 00:11:48,961 --> 00:11:51,660 Let's actually show that here in the console. 215 215 00:11:51,660 --> 00:11:54,540 So we can use double quotes or single quotes 216 216 00:11:54,540 --> 00:11:58,530 but we really do need quotes in order to create a string. 217 217 00:11:58,530 --> 00:12:03,530 So Jonah's, for example is a string, okay, 218 218 00:12:03,640 --> 00:12:06,090 and so is Jonah's with single quotes 219 219 00:12:07,230 --> 00:12:11,280 but if we do it like this then it's not gonna work. 220 220 00:12:11,280 --> 00:12:15,320 So it needs to be the same at the beginning and the end. 221 221 00:12:15,320 --> 00:12:17,340 but without any quotes at all, 222 222 00:12:17,340 --> 00:12:21,230 JavaScript will see this as a variable. 223 223 00:12:21,230 --> 00:12:23,690 And so then we get this reference error 224 224 00:12:23,690 --> 00:12:27,010 which is saying that Jonah's is not defined. 225 225 00:12:27,010 --> 00:12:29,930 And that's because it's looking for a variable 226 226 00:12:29,930 --> 00:12:31,340 that does not exist. 227 227 00:12:31,340 --> 00:12:35,090 We never declared a variable called Jonas, right. 228 228 00:12:35,090 --> 00:12:38,390 And so that's why we get this error here. 229 229 00:12:38,390 --> 00:12:41,600 So when you need a string, don't forget the quotes. 230 230 00:12:41,600 --> 00:12:45,440 This is a mistake that I see beginners make all the time. 231 231 00:12:45,440 --> 00:12:48,890 And so really be careful about that. 232 232 00:12:48,890 --> 00:12:53,890 Now moving on, let's actually see dynamic typing in action. 233 233 00:12:53,940 --> 00:12:56,473 And remember that dynamic typing simply means 234 234 00:12:56,473 --> 00:13:00,003 that we can easily change the type of a value 235 235 00:13:00,003 --> 00:13:02,360 that is hold by a variable. 236 236 00:13:02,360 --> 00:13:05,190 So that sounds a bit fancy 237 237 00:13:05,190 --> 00:13:07,513 but it's really as simple as this. 238 238 00:13:08,760 --> 00:13:13,760 So we already have a variable called JavaScriptIsFun 239 239 00:13:14,270 --> 00:13:17,300 and so let's now reassign that variable. 240 240 00:13:17,300 --> 00:13:20,140 So we do that like this. 241 241 00:13:20,140 --> 00:13:24,452 So we write the name of the variable then equal 242 242 00:13:24,452 --> 00:13:27,270 and then a new value. 243 243 00:13:27,270 --> 00:13:29,830 And what matters is that we do not write 244 244 00:13:29,830 --> 00:13:31,890 the let here again. 245 245 00:13:31,890 --> 00:13:33,570 So this let keyword. 246 246 00:13:33,570 --> 00:13:36,230 So the first time that we declare a new variable 247 247 00:13:36,230 --> 00:13:38,610 we need to do it using let 248 248 00:13:38,610 --> 00:13:42,330 but then when we want to change the value of the variable, 249 249 00:13:42,330 --> 00:13:46,830 we simply write it again like this but without the let. 250 250 00:13:46,830 --> 00:13:49,530 So we simply assign a new value here 251 251 00:13:49,530 --> 00:13:52,050 to this already existing variable. 252 252 00:13:52,050 --> 00:13:55,000 And this is actually the first time that we did that 253 253 00:13:55,000 --> 00:13:58,020 and so it's important to understand what happened here. 254 254 00:13:58,020 --> 00:14:03,020 Okay, so again, we first declared a new variable 255 255 00:14:03,360 --> 00:14:07,240 with this name and assigned it this true value. 256 256 00:14:07,240 --> 00:14:10,690 And then down here, we changed the value data's 257 257 00:14:10,690 --> 00:14:11,883 in the variable. 258 258 00:14:12,720 --> 00:14:15,610 So using the box analogy from before, 259 259 00:14:15,610 --> 00:14:18,980 it's like initially putting a book in a box 260 260 00:14:18,980 --> 00:14:22,100 and then later taking the book out of the box 261 261 00:14:22,100 --> 00:14:25,160 and instead putting a phone in a box. 262 262 00:14:25,160 --> 00:14:29,170 And so just like here, we changed the content of the box. 263 263 00:14:29,170 --> 00:14:32,497 In our example here, we changed from true 264 264 00:14:32,497 --> 00:14:35,663 to the string of yes. 265 265 00:14:36,760 --> 00:14:41,360 Okay, and now let's just grab this piece of code here 266 266 00:14:41,360 --> 00:14:45,560 so that we can see that dynamic typing actually works. 267 267 00:14:45,560 --> 00:14:47,293 So if I reload now, 268 268 00:14:48,520 --> 00:14:50,440 then let's see. 269 269 00:14:50,440 --> 00:14:54,550 Well, we get here from line 34. 270 270 00:14:54,550 --> 00:14:56,823 Let's actually simplify this a little bit. 271 271 00:14:58,170 --> 00:15:01,860 So commenting out this code and I'm using command 272 272 00:15:01,860 --> 00:15:06,860 or control slash, that's a reload again. 273 273 00:15:08,170 --> 00:15:12,040 And so you'll see that initially the type of JavaScript 274 274 00:15:12,040 --> 00:15:15,010 is fun is a bullion, right. 275 275 00:15:15,010 --> 00:15:18,460 But then we change it to a string down here. 276 276 00:15:18,460 --> 00:15:20,920 And so indeed when we then test the type 277 277 00:15:20,920 --> 00:15:24,480 of the variable again in line 39, 278 278 00:15:24,480 --> 00:15:26,810 then we get the result of a string. 279 279 00:15:26,810 --> 00:15:31,020 All right, and that basically is dynamic typing. 280 280 00:15:31,020 --> 00:15:35,000 Great, and now let's see an example of undefined. 281 281 00:15:35,000 --> 00:15:38,560 So remember that a number string and bullion 282 282 00:15:38,560 --> 00:15:40,720 are the most used data types 283 283 00:15:40,720 --> 00:15:43,980 but there are more like undefined and null 284 284 00:15:43,980 --> 00:15:47,250 and also symbol in BigInt but in this lecture, 285 285 00:15:47,250 --> 00:15:51,370 I'm only gonna talk about the three most important ones 286 286 00:15:51,370 --> 00:15:54,290 and then also undefined and null. 287 287 00:15:54,290 --> 00:15:57,410 So remember that undefined is the value taken 288 288 00:15:57,410 --> 00:16:00,600 by a variable that is not yet defined. 289 289 00:16:00,600 --> 00:16:04,430 So basically undefined means an empty value. 290 290 00:16:04,430 --> 00:16:08,490 So it's perfectly legal in JavaScript to do this. 291 291 00:16:08,490 --> 00:16:11,713 So to define a variable without value. 292 292 00:16:12,930 --> 00:16:17,410 And if we now take a look at this variable, 293 293 00:16:18,750 --> 00:16:21,320 then we should see undefined. 294 294 00:16:21,320 --> 00:16:23,230 And actually let's also check the type 295 295 00:16:24,250 --> 00:16:27,623 so that I can show you something, here. 296 296 00:16:33,540 --> 00:16:37,690 Okay, and so you get both undefined. 297 297 00:16:37,690 --> 00:16:40,680 What this means is that whenever we declare 298 298 00:16:40,680 --> 00:16:43,960 an empty variable, the value of the variable 299 299 00:16:43,960 --> 00:16:47,250 will be undefined, enter type of the variable 300 300 00:16:47,250 --> 00:16:49,400 is also undefined. 301 301 00:16:49,400 --> 00:16:53,460 Now that maybe sounds a bit confusing but it's really not. 302 302 00:16:53,460 --> 00:16:57,260 So simply put undefined is both the value 303 303 00:16:57,260 --> 00:17:00,040 and the type of the value and undefined 304 304 00:17:00,040 --> 00:17:03,210 is different than other types in this way. 305 305 00:17:03,210 --> 00:17:05,460 But this detail is not really important. 306 306 00:17:05,460 --> 00:17:07,860 What matters is that when you declare 307 307 00:17:07,860 --> 00:17:10,190 an empty variable like this 308 308 00:17:10,190 --> 00:17:13,190 it will automatically hold the value of undefined. 309 309 00:17:13,190 --> 00:17:16,000 And that's why we get undefined here. 310 310 00:17:16,000 --> 00:17:20,343 Then just like before, we can also reassign 311 311 00:17:20,343 --> 00:17:21,993 this variable basically. 312 312 00:17:23,740 --> 00:17:28,330 So we can now go ahead and say year 1991. 313 313 00:17:28,330 --> 00:17:30,830 And once more without the let 314 314 00:17:30,830 --> 00:17:33,730 because we are not creating a new variable, 315 315 00:17:33,730 --> 00:17:37,350 we are simply assigning a new value to you. 316 316 00:17:37,350 --> 00:17:41,630 So it used to be undefined, right, as we just discussed 317 317 00:17:41,630 --> 00:17:44,820 and now I'm changing it to 1991. 318 318 00:17:44,820 --> 00:17:49,200 And so if I test for the type now again, 319 319 00:17:49,200 --> 00:17:52,470 it should now be a number, right. 320 320 00:17:52,470 --> 00:17:54,050 So let's check that. 321 321 00:17:54,050 --> 00:17:59,050 And indeed we can again see the effect of dynamic typing. 322 322 00:17:59,130 --> 00:18:03,270 Great, and now finally just to quickly finish, 323 323 00:18:03,270 --> 00:18:05,800 I want to show you an error that exists 324 324 00:18:05,800 --> 00:18:08,060 in the type of operator. 325 325 00:18:08,060 --> 00:18:11,010 So all I'm going to do here is to quickly say, 326 326 00:18:11,010 --> 00:18:15,640 console.log type of null. 327 327 00:18:17,120 --> 00:18:21,090 So remember that null is just another data type 328 328 00:18:21,090 --> 00:18:23,010 and it's similar to undefined. 329 329 00:18:23,010 --> 00:18:26,210 And it's also similar in the fact that both the value 330 330 00:18:26,210 --> 00:18:30,330 and the type of the value are null, okay. 331 331 00:18:30,330 --> 00:18:35,330 Now about this bug, Java script says that a type of null 332 332 00:18:35,820 --> 00:18:40,050 is an object and this doesn't make any sense at all. 333 333 00:18:40,050 --> 00:18:45,050 And so this is regarded as a buck or an error in JavaScript. 334 334 00:18:45,140 --> 00:18:49,800 However, this bug is never corrected for legacy reasons 335 335 00:18:49,800 --> 00:18:52,620 but now it's of course not an object. 336 336 00:18:52,620 --> 00:18:54,360 This should return null 337 337 00:18:54,360 --> 00:18:58,230 just as type of undefined returns undefined. 338 338 00:18:58,230 --> 00:19:01,670 So just keep that in mind when working with type of 339 339 00:19:01,670 --> 00:19:04,510 but we will also come back to this later in the course 340 340 00:19:04,510 --> 00:19:08,270 to make sure that you don't create any accidental bugs 341 341 00:19:08,270 --> 00:19:11,130 because of this weird behavior. 342 342 00:19:11,130 --> 00:19:12,730 Okay, and that's all I had 343 343 00:19:12,730 --> 00:19:16,653 to tell you about data types in JavaScript. 30656