WEBVTT 1 00:00:01.580 --> 00:00:03.030 So in the last lecture, 2 00:00:03.030 --> 00:00:06.170 we talked about values and variables. 3 00:00:06.170 --> 00:00:08.830 Now in every programming language values 4 00:00:08.830 --> 00:00:10.700 can have different types. 5 00:00:10.700 --> 00:00:14.810 Depending on the type of data that we want them to hold. 6 00:00:14.810 --> 00:00:17.580 And we already saw strings and numbers 7 00:00:17.580 --> 00:00:19.990 but there are actually more data types. 8 00:00:19.990 --> 00:00:22.580 So let's now take a look at all the types 9 00:00:22.580 --> 00:00:24.533 that we have in JavaScript. 10 00:00:25.950 --> 00:00:28.500 And first of all, in JavaScript, 11 00:00:28.500 --> 00:00:31.470 every value is either an object 12 00:00:31.470 --> 00:00:35.830 which looks something like this or a primitive value. 13 00:00:35.830 --> 00:00:40.500 So a value is only a primitive when it's not an object 14 00:00:40.500 --> 00:00:42.950 and we will learn all about objects later 15 00:00:42.950 --> 00:00:46.943 but for now let's really focus on primitive data types. 16 00:00:48.500 --> 00:00:51.290 So there are seven primitive data types, 17 00:00:51.290 --> 00:00:55.390 number, string, bullion, undefined, null, 18 00:00:55.390 --> 00:00:57.810 symbol and big int. 19 00:00:57.810 --> 00:01:00.390 So let's look at them one by one. 20 00:01:00.390 --> 00:01:03.320 First, we have the number of data type 21 00:01:03.320 --> 00:01:06.760 and numbers are always so-called floating point numbers 22 00:01:06.760 --> 00:01:09.630 which means that they always have decimals 23 00:01:09.630 --> 00:01:12.950 even if we don't see them or don't define them. 24 00:01:12.950 --> 00:01:16.470 For example, the 23 value debt we have here 25 00:01:16.470 --> 00:01:20.100 is exactly like having 23.0. 26 00:01:20.100 --> 00:01:23.410 But they are both simply the number data type. 27 00:01:23.410 --> 00:01:25.410 In many other programming languages, 28 00:01:25.410 --> 00:01:28.160 you will find different data types for integers 29 00:01:28.160 --> 00:01:31.060 and for decimals but not in JavaScript. 30 00:01:31.060 --> 00:01:34.250 All numbers are simply of the type number. 31 00:01:34.250 --> 00:01:37.650 Okay, next up we have strengths 32 00:01:37.650 --> 00:01:40.770 which are simply a sequence of characters. 33 00:01:40.770 --> 00:01:45.210 And so they're just used for text as we already learned 34 00:01:45.210 --> 00:01:46.990 and always put them in quotes, 35 00:01:46.990 --> 00:01:49.990 no matter if double quotes or single quotes 36 00:01:49.990 --> 00:01:52.660 otherwise JavaScript will actually confuse them 37 00:01:52.660 --> 00:01:54.203 with variable names. 38 00:01:55.110 --> 00:01:59.330 The Boolean data type is essentially a logical type 39 00:01:59.330 --> 00:02:03.870 that can only take one of the logical values true or false. 40 00:02:03.870 --> 00:02:08.700 In other words, a Boolean is always either true or false. 41 00:02:08.700 --> 00:02:12.300 All right, we use Boolean values to take decisions 42 00:02:12.300 --> 00:02:15.250 with code as we will see later. 43 00:02:15.250 --> 00:02:18.080 And these are the three most important data types 44 00:02:18.080 --> 00:02:20.230 that we will deal with the most 45 00:02:20.230 --> 00:02:22.330 but there are still four more 46 00:02:22.330 --> 00:02:25.010 which might be a bit more confusing. 47 00:02:25.010 --> 00:02:29.830 So first, undefined is the value taken by a variable 48 00:02:29.830 --> 00:02:31.980 that is not yet defined. 49 00:02:31.980 --> 00:02:34.630 And the variable thats not yet defined 50 00:02:34.630 --> 00:02:37.300 is simply just a variable that we declared 51 00:02:37.300 --> 00:02:39.790 but without assigning a value 52 00:02:39.790 --> 00:02:43.200 for example like this children variable here. 53 00:02:43.200 --> 00:02:47.920 So basically we can say that undefined means empty value 54 00:02:47.920 --> 00:02:51.800 then there's also null which is actually pretty similar 55 00:02:51.800 --> 00:02:54.550 because it also means empty value 56 00:02:54.550 --> 00:02:57.210 but it's used in different circumstances. 57 00:02:57.210 --> 00:02:59.950 But for now, don't worry about the details. 58 00:02:59.950 --> 00:03:04.910 I just want you to know that null also exists for now, okay. 59 00:03:04.910 --> 00:03:09.833 Anyway, next we have symbol which was introduced in ES2015 60 00:03:11.090 --> 00:03:13.960 and this data type is not really useful for us, 61 00:03:13.960 --> 00:03:16.840 it's simply defines a value that is unique 62 00:03:16.840 --> 00:03:18.620 and cannot be changed. 63 00:03:18.620 --> 00:03:20.500 And we will talk about this briefly 64 00:03:20.500 --> 00:03:22.890 by the end of the course. 65 00:03:22.890 --> 00:03:27.850 Finally, starting in ES2020 there is also BigInt 66 00:03:27.850 --> 00:03:30.730 which is for integers that are too large 67 00:03:30.730 --> 00:03:33.460 to be represented by the number type. 68 00:03:33.460 --> 00:03:37.020 So basically it's another tire before numbers 69 00:03:37.020 --> 00:03:40.420 and we will talk about this one in another section. 70 00:03:40.420 --> 00:03:45.050 So these are the seven primitive data types in JavaScript. 71 00:03:45.050 --> 00:03:49.470 But there is another fundamental thing to know about types 72 00:03:49.470 --> 00:03:51.640 which is the fact that JavaScript 73 00:03:51.640 --> 00:03:54.790 has a feature called dynamic typing. 74 00:03:54.790 --> 00:03:57.810 This means that when you create a new variable, 75 00:03:57.810 --> 00:04:01.260 you do not have to manually define the data type 76 00:04:01.260 --> 00:04:03.520 of the value that it contains. 77 00:04:03.520 --> 00:04:05.205 In many other programming languages, 78 00:04:05.205 --> 00:04:09.010 you actually have to do that but not in JavaScript. 79 00:04:09.010 --> 00:04:12.020 Instead, Java script automatically determines 80 00:04:12.020 --> 00:04:16.660 the data type of a value when it's stored into a variable. 81 00:04:16.660 --> 00:04:19.930 And this distinction between value and variable 82 00:04:19.930 --> 00:04:22.880 is pretty important because in JavaScript, 83 00:04:22.880 --> 00:04:27.130 it's the value that has a type, not the variable. 84 00:04:27.130 --> 00:04:31.020 So variables simply store values that have a type. 85 00:04:31.020 --> 00:04:35.170 Many people don't know about this detail or just don't care 86 00:04:35.170 --> 00:04:37.660 but this is how it actually works. 87 00:04:37.660 --> 00:04:41.041 Now another important application of dynamic typing 88 00:04:41.041 --> 00:04:44.930 is that later in our code, we can assign a new value 89 00:04:44.930 --> 00:04:47.930 with a different data type to the same variable 90 00:04:47.930 --> 00:04:49.300 without a problem. 91 00:04:49.300 --> 00:04:53.090 For example, variable x can initially be a number 92 00:04:53.090 --> 00:04:56.720 and then later a string, that's not a problem at all. 93 00:04:56.720 --> 00:04:59.170 And this can of course be very useful 94 00:04:59.170 --> 00:05:02.310 but it can also be the source of some difficult 95 00:05:02.310 --> 00:05:06.630 to find books which means errors in our code. 96 00:05:06.630 --> 00:05:10.643 All right, so let's not actually see these things in action. 97 00:05:12.120 --> 00:05:14.550 But before we talk about data types, 98 00:05:14.550 --> 00:05:17.670 let's quickly talk about code commenting. 99 00:05:17.670 --> 00:05:21.390 So in programming, we use comments to literally come 100 00:05:21.390 --> 00:05:25.410 and code or to deactivate code without deleting it. 101 00:05:25.410 --> 00:05:29.270 For example, here, I could write a single line comment 102 00:05:29.270 --> 00:05:34.270 using // and then I can describe what this code is doing. 103 00:05:34.400 --> 00:05:39.400 For example here, I can say, variable name conventions 104 00:05:41.340 --> 00:05:44.199 and JavaScript will completely ignore anything 105 00:05:44.199 --> 00:05:45.750 that is a comment. 106 00:05:45.750 --> 00:05:50.750 So anything that comes after // like this, okay. 107 00:05:51.097 --> 00:05:54.260 And this can be really useful to kind of divide 108 00:05:54.260 --> 00:05:56.610 or code into different sections 109 00:05:56.610 --> 00:05:59.450 and to explain what different pieces of code 110 00:05:59.450 --> 00:06:01.590 are actually doing in our code. 111 00:06:01.590 --> 00:06:04.460 So that's a very useful application of comments 112 00:06:04.460 --> 00:06:07.880 but another one is to simply comment out code 113 00:06:07.880 --> 00:06:11.400 that we don't want to be executed for some reason. 114 00:06:11.400 --> 00:06:14.170 For example, let's say we don't want this code here 115 00:06:14.170 --> 00:06:17.960 to be executed but we also don't want to delete it. 116 00:06:17.960 --> 00:06:21.450 So we can simply comment it out like this. 117 00:06:21.450 --> 00:06:23.910 And so if I save it and then 118 00:06:23.910 --> 00:06:26.513 when I reload this 61 should be gone. 119 00:06:27.900 --> 00:06:30.830 So you see, that's because JavaScript 120 00:06:30.830 --> 00:06:34.230 is now ignoring this code, it's just a comment. 121 00:06:34.230 --> 00:06:35.650 But since we didn't delete it, 122 00:06:35.650 --> 00:06:37.840 we can of course bring it back. 123 00:06:37.840 --> 00:06:42.020 So we can just delete it and then it's back to normal. 124 00:06:42.020 --> 00:06:45.300 Now in VS code we can also automatically create comments 125 00:06:45.300 --> 00:06:47.823 using the command slash shortcut. 126 00:06:49.010 --> 00:06:52.050 So that's command or control slash to turn on 127 00:06:52.050 --> 00:06:54.510 and then again to turn off. 128 00:06:54.510 --> 00:06:56.460 So that single line comments 129 00:06:56.460 --> 00:06:58.170 but we can go even further 130 00:06:58.170 --> 00:07:01.090 and also create multi line comments. 131 00:07:01.090 --> 00:07:04.780 And to do that, we do it like this. 132 00:07:04.780 --> 00:07:09.560 We start a multi line comment using slash and an asterisk 133 00:07:09.560 --> 00:07:13.810 and then we ended with the asterisk or star 134 00:07:13.810 --> 00:07:16.710 and then the slash and whatever comes here 135 00:07:16.710 --> 00:07:18.910 is then a normal code. 136 00:07:18.910 --> 00:07:21.720 Okay, and with this, I effectively got rid 137 00:07:21.720 --> 00:07:25.560 of this first code that we typed in the first lecture 138 00:07:25.560 --> 00:07:27.380 but without deleting it. 139 00:07:27.380 --> 00:07:31.460 And so I can keep it here basically as a reference. 140 00:07:31.460 --> 00:07:33.260 All right, and so that's what I'm gonna do 141 00:07:33.260 --> 00:07:34.610 in every lecture now. 142 00:07:34.610 --> 00:07:38.360 We'll always kind of get rid of the previous code 143 00:07:38.360 --> 00:07:41.163 by using a multi line comment like this. 144 00:07:42.070 --> 00:07:45.120 But now let's get back to the topic of this lecture 145 00:07:45.120 --> 00:07:47.180 which is data types. 146 00:07:47.180 --> 00:07:48.660 So in a previous lecture, 147 00:07:48.660 --> 00:07:52.640 we already saw numbers and strengths, right. 148 00:07:52.640 --> 00:07:56.100 So here we have a value which has a number 149 00:07:56.100 --> 00:07:59.540 and here we have a value which is a string. 150 00:07:59.540 --> 00:08:02.170 So nothing new, right. 151 00:08:02.170 --> 00:08:05.130 That's will be learned at the beginning of the video 152 00:08:05.130 --> 00:08:08.230 but we also talked about booleans, right? 153 00:08:08.230 --> 00:08:12.760 And so Boolean is a value that can only be true or false. 154 00:08:12.760 --> 00:08:14.473 And so if I write true, 155 00:08:15.870 --> 00:08:19.180 that is then automatically a Boolean value 156 00:08:19.180 --> 00:08:21.133 which is in this case true. 157 00:08:22.030 --> 00:08:25.290 Okay, and actually I can also lock that to the console 158 00:08:30.153 --> 00:08:31.890 and then here you get true 159 00:08:31.890 --> 00:08:33.770 and you can see that it's not a string 160 00:08:33.770 --> 00:08:36.250 because strings are printed in white 161 00:08:36.250 --> 00:08:39.570 and this value is printed in this pink. 162 00:08:39.570 --> 00:08:43.030 And so it' actually really a Boolean value. 163 00:08:43.030 --> 00:08:47.780 And of course we can also store Booleans in variables. 164 00:08:47.780 --> 00:08:52.780 So let's say Java script is fun using 165 00:08:54.440 --> 00:08:57.400 the camel case notation that we learned about before 166 00:08:58.370 --> 00:09:00.803 and done, set it to true. 167 00:09:03.040 --> 00:09:07.140 And now we can log JavaScriptIsFun instead 168 00:09:08.240 --> 00:09:10.700 but of course the variable declaration here needs 169 00:09:10.700 --> 00:09:12.860 to happen before the lock. 170 00:09:12.860 --> 00:09:15.810 And that's because JavaScript programs are executed 171 00:09:15.810 --> 00:09:19.260 from top to bottom like this 172 00:09:19.260 --> 00:09:22.030 and more about that a bit later, okay. 173 00:09:22.030 --> 00:09:24.630 What matters is that this need to be at the end. 174 00:09:24.630 --> 00:09:27.600 So let's cut it and put it here 175 00:09:28.620 --> 00:09:31.703 and also get rid of this one, save it. 176 00:09:33.400 --> 00:09:35.410 And here we still get true. 177 00:09:35.410 --> 00:09:37.590 And so here we created a nice 178 00:09:37.590 --> 00:09:42.590 and descriptive variable name holding a Boolean value. 179 00:09:43.260 --> 00:09:45.460 And remember that it's actually the value 180 00:09:45.460 --> 00:09:49.490 that holds the data type and not the variable 181 00:09:49.490 --> 00:09:52.200 because I could now actually go ahead and change this 182 00:09:52.200 --> 00:09:54.600 to a value with another type. 183 00:09:54.600 --> 00:09:57.000 And actually I will do that in a second 184 00:09:57.000 --> 00:09:58.700 but for now let's take a look 185 00:09:58.700 --> 00:10:01.920 at a special operator called type of. 186 00:10:01.920 --> 00:10:04.989 So type of it's an operator just like the plus 187 00:10:04.989 --> 00:10:06.989 or the minus operator 188 00:10:06.989 --> 00:10:11.050 that we can use to show the type of a value. 189 00:10:11.050 --> 00:10:12.433 So let me show you how. 190 00:10:16.680 --> 00:10:20.160 So again, we use console.log because we actually want to see 191 00:10:20.160 --> 00:10:23.728 the result of this operator in the console. 192 00:10:23.728 --> 00:10:28.728 So type of and then let's say just true, okay. 193 00:10:30.050 --> 00:10:32.730 And more about how operators actually work 194 00:10:32.730 --> 00:10:35.760 and what they are a bit later in the section. 195 00:10:35.760 --> 00:10:39.600 For now, let's just see what happens when we reload this 196 00:10:40.640 --> 00:10:43.060 and we see that it's a Boolean. 197 00:10:43.060 --> 00:10:47.100 So just as I said, the result of using this operator here 198 00:10:47.100 --> 00:10:49.780 on a value will be a new value 199 00:10:49.780 --> 00:10:54.780 which is a string with the type of this value here, okay. 200 00:10:55.410 --> 00:10:57.143 So let's do that again here. 201 00:10:57.143 --> 00:10:59.290 I will copy it just a couple of times 202 00:11:02.460 --> 00:11:05.330 just like this to make it a bit faster. 203 00:11:05.330 --> 00:11:08.337 And so here let's use JavaScriptIsFun. 204 00:11:09.990 --> 00:11:12.270 And so this should also be a Boolean 205 00:11:12.270 --> 00:11:15.710 but just to make sure. 206 00:11:15.710 --> 00:11:20.710 Here let's now use a number and finally a string. 207 00:11:23.030 --> 00:11:24.530 So just to prove you 208 00:11:24.530 --> 00:11:28.000 that these are actually numbers and strings. 209 00:11:28.000 --> 00:11:32.160 All right, and indeed we see that JavaScript 210 00:11:32.160 --> 00:11:37.160 is fun is a Boolean, 23 is a number and Jonas is a string. 211 00:11:38.910 --> 00:11:40.890 And speaking about strings, 212 00:11:40.890 --> 00:11:44.380 I just want to quickly emphasize that we really always need 213 00:11:44.380 --> 00:11:48.961 to use quotes in order to create a string, okay. 214 00:11:48.961 --> 00:11:51.660 Let's actually show that here in the console. 215 00:11:51.660 --> 00:11:54.540 So we can use double quotes or single quotes 216 00:11:54.540 --> 00:11:58.530 but we really do need quotes in order to create a string. 217 00:11:58.530 --> 00:12:03.530 So Jonah's, for example is a string, okay, 218 00:12:03.640 --> 00:12:06.090 and so is Jonah's with single quotes 219 00:12:07.230 --> 00:12:11.280 but if we do it like this then it's not gonna work. 220 00:12:11.280 --> 00:12:15.320 So it needs to be the same at the beginning and the end. 221 00:12:15.320 --> 00:12:17.340 but without any quotes at all, 222 00:12:17.340 --> 00:12:21.230 JavaScript will see this as a variable. 223 00:12:21.230 --> 00:12:23.690 And so then we get this reference error 224 00:12:23.690 --> 00:12:27.010 which is saying that Jonah's is not defined. 225 00:12:27.010 --> 00:12:29.930 And that's because it's looking for a variable 226 00:12:29.930 --> 00:12:31.340 that does not exist. 227 00:12:31.340 --> 00:12:35.090 We never declared a variable called Jonas, right. 228 00:12:35.090 --> 00:12:38.390 And so that's why we get this error here. 229 00:12:38.390 --> 00:12:41.600 So when you need a string, don't forget the quotes. 230 00:12:41.600 --> 00:12:45.440 This is a mistake that I see beginners make all the time. 231 00:12:45.440 --> 00:12:48.890 And so really be careful about that. 232 00:12:48.890 --> 00:12:53.890 Now moving on, let's actually see dynamic typing in action. 233 00:12:53.940 --> 00:12:56.473 And remember that dynamic typing simply means 234 00:12:56.473 --> 00:13:00.003 that we can easily change the type of a value 235 00:13:00.003 --> 00:13:02.360 that is hold by a variable. 236 00:13:02.360 --> 00:13:05.190 So that sounds a bit fancy 237 00:13:05.190 --> 00:13:07.513 but it's really as simple as this. 238 00:13:08.760 --> 00:13:13.760 So we already have a variable called JavaScriptIsFun 239 00:13:14.270 --> 00:13:17.300 and so let's now reassign that variable. 240 00:13:17.300 --> 00:13:20.140 So we do that like this. 241 00:13:20.140 --> 00:13:24.452 So we write the name of the variable then equal 242 00:13:24.452 --> 00:13:27.270 and then a new value. 243 00:13:27.270 --> 00:13:29.830 And what matters is that we do not write 244 00:13:29.830 --> 00:13:31.890 the let here again. 245 00:13:31.890 --> 00:13:33.570 So this let keyword. 246 00:13:33.570 --> 00:13:36.230 So the first time that we declare a new variable 247 00:13:36.230 --> 00:13:38.610 we need to do it using let 248 00:13:38.610 --> 00:13:42.330 but then when we want to change the value of the variable, 249 00:13:42.330 --> 00:13:46.830 we simply write it again like this but without the let. 250 00:13:46.830 --> 00:13:49.530 So we simply assign a new value here 251 00:13:49.530 --> 00:13:52.050 to this already existing variable. 252 00:13:52.050 --> 00:13:55.000 And this is actually the first time that we did that 253 00:13:55.000 --> 00:13:58.020 and so it's important to understand what happened here. 254 00:13:58.020 --> 00:14:03.020 Okay, so again, we first declared a new variable 255 00:14:03.360 --> 00:14:07.240 with this name and assigned it this true value. 256 00:14:07.240 --> 00:14:10.690 And then down here, we changed the value data's 257 00:14:10.690 --> 00:14:11.883 in the variable. 258 00:14:12.720 --> 00:14:15.610 So using the box analogy from before, 259 00:14:15.610 --> 00:14:18.980 it's like initially putting a book in a box 260 00:14:18.980 --> 00:14:22.100 and then later taking the book out of the box 261 00:14:22.100 --> 00:14:25.160 and instead putting a phone in a box. 262 00:14:25.160 --> 00:14:29.170 And so just like here, we changed the content of the box. 263 00:14:29.170 --> 00:14:32.497 In our example here, we changed from true 264 00:14:32.497 --> 00:14:35.663 to the string of yes. 265 00:14:36.760 --> 00:14:41.360 Okay, and now let's just grab this piece of code here 266 00:14:41.360 --> 00:14:45.560 so that we can see that dynamic typing actually works. 267 00:14:45.560 --> 00:14:47.293 So if I reload now, 268 00:14:48.520 --> 00:14:50.440 then let's see. 269 00:14:50.440 --> 00:14:54.550 Well, we get here from line 34. 270 00:14:54.550 --> 00:14:56.823 Let's actually simplify this a little bit. 271 00:14:58.170 --> 00:15:01.860 So commenting out this code and I'm using command 272 00:15:01.860 --> 00:15:06.860 or control slash, that's a reload again. 273 00:15:08.170 --> 00:15:12.040 And so you'll see that initially the type of JavaScript 274 00:15:12.040 --> 00:15:15.010 is fun is a bullion, right. 275 00:15:15.010 --> 00:15:18.460 But then we change it to a string down here. 276 00:15:18.460 --> 00:15:20.920 And so indeed when we then test the type 277 00:15:20.920 --> 00:15:24.480 of the variable again in line 39, 278 00:15:24.480 --> 00:15:26.810 then we get the result of a string. 279 00:15:26.810 --> 00:15:31.020 All right, and that basically is dynamic typing. 280 00:15:31.020 --> 00:15:35.000 Great, and now let's see an example of undefined. 281 00:15:35.000 --> 00:15:38.560 So remember that a number string and bullion 282 00:15:38.560 --> 00:15:40.720 are the most used data types 283 00:15:40.720 --> 00:15:43.980 but there are more like undefined and null 284 00:15:43.980 --> 00:15:47.250 and also symbol in BigInt but in this lecture, 285 00:15:47.250 --> 00:15:51.370 I'm only gonna talk about the three most important ones 286 00:15:51.370 --> 00:15:54.290 and then also undefined and null. 287 00:15:54.290 --> 00:15:57.410 So remember that undefined is the value taken 288 00:15:57.410 --> 00:16:00.600 by a variable that is not yet defined. 289 00:16:00.600 --> 00:16:04.430 So basically undefined means an empty value. 290 00:16:04.430 --> 00:16:08.490 So it's perfectly legal in JavaScript to do this. 291 00:16:08.490 --> 00:16:11.713 So to define a variable without value. 292 00:16:12.930 --> 00:16:17.410 And if we now take a look at this variable, 293 00:16:18.750 --> 00:16:21.320 then we should see undefined. 294 00:16:21.320 --> 00:16:23.230 And actually let's also check the type 295 00:16:24.250 --> 00:16:27.623 so that I can show you something, here. 296 00:16:33.540 --> 00:16:37.690 Okay, and so you get both undefined. 297 00:16:37.690 --> 00:16:40.680 What this means is that whenever we declare 298 00:16:40.680 --> 00:16:43.960 an empty variable, the value of the variable 299 00:16:43.960 --> 00:16:47.250 will be undefined, enter type of the variable 300 00:16:47.250 --> 00:16:49.400 is also undefined. 301 00:16:49.400 --> 00:16:53.460 Now that maybe sounds a bit confusing but it's really not. 302 00:16:53.460 --> 00:16:57.260 So simply put undefined is both the value 303 00:16:57.260 --> 00:17:00.040 and the type of the value and undefined 304 00:17:00.040 --> 00:17:03.210 is different than other types in this way. 305 00:17:03.210 --> 00:17:05.460 But this detail is not really important. 306 00:17:05.460 --> 00:17:07.860 What matters is that when you declare 307 00:17:07.860 --> 00:17:10.190 an empty variable like this 308 00:17:10.190 --> 00:17:13.190 it will automatically hold the value of undefined. 309 00:17:13.190 --> 00:17:16.000 And that's why we get undefined here. 310 00:17:16.000 --> 00:17:20.343 Then just like before, we can also reassign 311 00:17:20.343 --> 00:17:21.993 this variable basically. 312 00:17:23.740 --> 00:17:28.330 So we can now go ahead and say year 1991. 313 00:17:28.330 --> 00:17:30.830 And once more without the let 314 00:17:30.830 --> 00:17:33.730 because we are not creating a new variable, 315 00:17:33.730 --> 00:17:37.350 we are simply assigning a new value to you. 316 00:17:37.350 --> 00:17:41.630 So it used to be undefined, right, as we just discussed 317 00:17:41.630 --> 00:17:44.820 and now I'm changing it to 1991. 318 00:17:44.820 --> 00:17:49.200 And so if I test for the type now again, 319 00:17:49.200 --> 00:17:52.470 it should now be a number, right. 320 00:17:52.470 --> 00:17:54.050 So let's check that. 321 00:17:54.050 --> 00:17:59.050 And indeed we can again see the effect of dynamic typing. 322 00:17:59.130 --> 00:18:03.270 Great, and now finally just to quickly finish, 323 00:18:03.270 --> 00:18:05.800 I want to show you an error that exists 324 00:18:05.800 --> 00:18:08.060 in the type of operator. 325 00:18:08.060 --> 00:18:11.010 So all I'm going to do here is to quickly say, 326 00:18:11.010 --> 00:18:15.640 console.log type of null. 327 00:18:17.120 --> 00:18:21.090 So remember that null is just another data type 328 00:18:21.090 --> 00:18:23.010 and it's similar to undefined. 329 00:18:23.010 --> 00:18:26.210 And it's also similar in the fact that both the value 330 00:18:26.210 --> 00:18:30.330 and the type of the value are null, okay. 331 00:18:30.330 --> 00:18:35.330 Now about this bug, Java script says that a type of null 332 00:18:35.820 --> 00:18:40.050 is an object and this doesn't make any sense at all. 333 00:18:40.050 --> 00:18:45.050 And so this is regarded as a buck or an error in JavaScript. 334 00:18:45.140 --> 00:18:49.800 However, this bug is never corrected for legacy reasons 335 00:18:49.800 --> 00:18:52.620 but now it's of course not an object. 336 00:18:52.620 --> 00:18:54.360 This should return null 337 00:18:54.360 --> 00:18:58.230 just as type of undefined returns undefined. 338 00:18:58.230 --> 00:19:01.670 So just keep that in mind when working with type of 339 00:19:01.670 --> 00:19:04.510 but we will also come back to this later in the course 340 00:19:04.510 --> 00:19:08.270 to make sure that you don't create any accidental bugs 341 00:19:08.270 --> 00:19:11.130 because of this weird behavior. 342 00:19:11.130 --> 00:19:12.730 Okay, and that's all I had 343 00:19:12.730 --> 00:19:16.653 to tell you about data types in JavaScript.