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These are the user uploaded subtitles that are being translated: 1 1 00:00:01,180 --> 00:00:04,670 As we move towards the end of this section, 2 2 00:00:04,670 --> 00:00:06,940 we need to learn about the big difference 3 3 00:00:06,940 --> 00:00:10,030 between the way primitive types and objects 4 4 00:00:10,030 --> 00:00:12,467 are stored and memory. 5 5 00:00:12,467 --> 00:00:15,760 And this is actually a very practical aspect 6 6 00:00:15,760 --> 00:00:20,290 and one that causes a lot of confusion in beginners. 7 7 00:00:20,290 --> 00:00:24,230 So let's actually start by writing some code this time 8 8 00:00:24,230 --> 00:00:27,660 to understand what the confusion actually is, 9 9 00:00:27,660 --> 00:00:29,280 so that I can then show you 10 10 00:00:29,280 --> 00:00:31,483 how it all works behind the scenes. 11 11 00:00:33,200 --> 00:00:35,240 So right now I'm not gonna explain 12 12 00:00:35,240 --> 00:00:38,290 how the code we will right now works, 13 13 00:00:38,290 --> 00:00:43,040 but I will just show you what the source of confusion is. 14 14 00:00:43,040 --> 00:00:44,470 But for now let's start with 15 15 00:00:44,470 --> 00:00:47,480 a simple example with primitives. 16 16 00:00:47,480 --> 00:00:50,310 And so remember primitives are like numbers, 17 17 00:00:50,310 --> 00:00:52,503 strings, Boolean's, et cetera. 18 18 00:00:53,720 --> 00:00:56,273 So simply setting the age to 30, 19 19 00:00:57,609 --> 00:01:01,453 and then I'm creating a old age variable. 20 20 00:01:02,370 --> 00:01:04,620 And this one I will set to the age 21 21 00:01:04,620 --> 00:01:07,920 because now it is my birthday basically. 22 22 00:01:07,920 --> 00:01:10,890 And so my age changes to 31, 23 23 00:01:10,890 --> 00:01:14,130 but I still wanted to preserve my old age here 24 24 00:01:14,130 --> 00:01:15,490 in this variable. 25 25 00:01:15,490 --> 00:01:16,600 Okay? 26 26 00:01:16,600 --> 00:01:20,217 So let's log the results to the console now. 27 27 00:01:20,217 --> 00:01:25,217 And they should be pretty much what we expect at this point. 28 28 00:01:25,770 --> 00:01:30,350 So you see that my current age is 31 because, well, 29 29 00:01:30,350 --> 00:01:34,640 I changed the original one from 30 to 31. 30 30 00:01:34,640 --> 00:01:38,190 But the old age is still 30 here. 31 31 00:01:38,190 --> 00:01:41,880 And that's because I set it here at this point of the code 32 32 00:01:42,800 --> 00:01:46,770 to age, which was still 30 at this point. 33 33 00:01:46,770 --> 00:01:51,410 And so then changing the age here from 30 to 31 34 34 00:01:51,410 --> 00:01:55,120 did of course not affect the old age variable again. 35 35 00:01:55,120 --> 00:01:59,773 Again because at this point here age was still 30. 36 36 00:02:01,063 --> 00:02:06,063 All right, so here, hopefully there is not much confusion, 37 37 00:02:06,120 --> 00:02:09,000 everything works just as expected. 38 38 00:02:09,000 --> 00:02:12,913 But now let's create another scenario which has an object. 39 39 00:02:14,470 --> 00:02:17,563 So I'm simply going to create an object for me, 40 40 00:02:19,080 --> 00:02:23,443 with the name of Jonas and age 30. 41 41 00:02:25,220 --> 00:02:28,000 And now let's copy this object 42 42 00:02:28,000 --> 00:02:30,560 because let's say that I have a friend 43 43 00:02:30,560 --> 00:02:32,660 who's also called Jonas. 44 44 00:02:32,660 --> 00:02:36,510 And so instead of creating a brand new object from scratch, 45 45 00:02:36,510 --> 00:02:39,080 I would just copy the me object. 46 46 00:02:39,080 --> 00:02:43,637 So const Friend equals Me. 47 47 00:02:45,680 --> 00:02:46,530 Okay? 48 48 00:02:46,530 --> 00:02:50,240 And so now both the name and the age are the same, 49 49 00:02:50,240 --> 00:02:52,980 but let's say that we have different ages. 50 50 00:02:52,980 --> 00:02:56,120 And so let's change the age of my friend 51 51 00:02:57,220 --> 00:03:00,920 and that's pretty straightforward, right? 52 52 00:03:00,920 --> 00:03:04,610 So let's say that he is 27 years old. 53 53 00:03:04,610 --> 00:03:09,433 But of course we did not change me.age, right? 54 54 00:03:10,410 --> 00:03:15,410 So let's now take a look at both of them in the console. 55 55 00:03:16,980 --> 00:03:18,023 So first my friend, 56 56 00:03:20,850 --> 00:03:25,743 so that's the Friend object and then also Me, 57 57 00:03:26,730 --> 00:03:29,850 or we could say Jonas, it doesn't really matter. 58 58 00:03:29,850 --> 00:03:34,190 But what matters is that we will see the Me object 59 59 00:03:34,190 --> 00:03:37,610 and what do we think will happen now? 60 60 00:03:37,610 --> 00:03:39,800 So let me show it to you. 61 61 00:03:39,800 --> 00:03:43,290 And now we get that both me and my friend 62 62 00:03:43,290 --> 00:03:45,847 have the age of 27. 63 63 00:03:45,847 --> 00:03:49,530 And that looks a little bit strange because all we did 64 64 00:03:49,530 --> 00:03:53,540 was to change the age of the friend, right? 65 65 00:03:53,540 --> 00:03:57,760 Nowhere here, I have me.age equals 27, 66 66 00:03:57,760 --> 00:04:02,140 but still for myself, so here at the Me object, 67 67 00:04:02,140 --> 00:04:04,840 the age is also 27. 68 68 00:04:04,840 --> 00:04:08,980 And so that is what I mean by source of confusion. 69 69 00:04:08,980 --> 00:04:10,200 And so in this video, 70 70 00:04:10,200 --> 00:04:13,233 let's now find out why it works this way. 71 71 00:04:14,720 --> 00:04:18,700 Now, before we can understand the code that we just wrote, 72 72 00:04:18,700 --> 00:04:21,450 we need to review some basics here. 73 73 00:04:21,450 --> 00:04:23,363 First, we need to remember about 74 74 00:04:23,363 --> 00:04:25,900 JavaScripts primitive data types, 75 75 00:04:25,900 --> 00:04:30,900 which are number, string, Boolean, undefined, null, symbol, 76 76 00:04:31,060 --> 00:04:32,860 and BigInt. 77 77 00:04:32,860 --> 00:04:36,660 Then everything else are basically objects. 78 78 00:04:36,660 --> 00:04:40,900 So objects created with the object literal, arrays 79 79 00:04:40,900 --> 00:04:44,670 and even functions are all objects. 80 80 00:04:44,670 --> 00:04:48,660 Now, when we're talking about memory and memory management, 81 81 00:04:48,660 --> 00:04:52,010 it's usual to call primitives, primitive types 82 82 00:04:52,010 --> 00:04:56,260 and objects reference types because of the different way 83 83 00:04:56,260 --> 00:04:58,573 in which they are stored in memory. 84 84 00:04:59,410 --> 00:05:03,810 Next, we need to remember about the JavaScript engine. 85 85 00:05:03,810 --> 00:05:07,330 So the engine has two components, the call stack, 86 86 00:05:07,330 --> 00:05:11,470 where functions are executed and to heap where objects 87 87 00:05:11,470 --> 00:05:13,780 are stored in memory. 88 88 00:05:13,780 --> 00:05:16,710 And that's right, all of objects, 89 89 00:05:16,710 --> 00:05:19,510 or in other words, reference types 90 90 00:05:19,510 --> 00:05:23,020 will get stored right in the memory heap. 91 91 00:05:23,020 --> 00:05:26,780 And I mentioned that when we first talked about the engine, 92 92 00:05:26,780 --> 00:05:31,100 but now you will finally learn how that actually works. 93 93 00:05:31,100 --> 00:05:35,010 On the other hand, primitives or primitive types 94 94 00:05:35,010 --> 00:05:37,640 are stored in the call stack. 95 95 00:05:37,640 --> 00:05:40,650 And with that, I mean that primitive types 96 96 00:05:40,650 --> 00:05:43,680 are stored in the execution contexts 97 97 00:05:43,680 --> 00:05:45,830 in which they are declared. 98 98 00:05:45,830 --> 00:05:47,960 But for the sake of simplicity, 99 99 00:05:47,960 --> 00:05:50,220 let's ignore that detail now. 100 100 00:05:50,220 --> 00:05:52,600 And simply say that primitive types 101 101 00:05:52,600 --> 00:05:54,890 are stored in a call stack 102 102 00:05:54,890 --> 00:05:58,850 because that's where execution context run. 103 103 00:05:58,850 --> 00:06:03,030 All right, but now how does all that actually work? 104 104 00:06:03,030 --> 00:06:06,060 And why did our code example earlier behave 105 105 00:06:06,060 --> 00:06:08,220 in that weird way? 106 106 00:06:08,220 --> 00:06:09,953 Well, let's find out. 107 107 00:06:11,380 --> 00:06:14,860 So here we have the two code examples from earlier, 108 108 00:06:14,860 --> 00:06:19,380 as well as the engine with call stack and heap. 109 109 00:06:19,380 --> 00:06:24,020 And let's start by looking at the primitive values example. 110 110 00:06:24,020 --> 00:06:28,580 So when we declare a variable like age equals 30, 111 111 00:06:28,580 --> 00:06:32,060 what actually happens inside the JavaScript engine 112 112 00:06:32,060 --> 00:06:33,453 and the computer's memory? 113 113 00:06:34,390 --> 00:06:37,460 Well, first JavaScript will create 114 114 00:06:37,460 --> 00:06:42,140 a so-called unique identifier with the variable name. 115 115 00:06:42,140 --> 00:06:45,450 Then a piece of memory will be allocated 116 116 00:06:45,450 --> 00:06:49,778 with a certain address, so 0001 in this example, 117 117 00:06:49,778 --> 00:06:54,180 and finally the value would be stored in memory 118 118 00:06:54,180 --> 00:06:56,310 at the specified address. 119 119 00:06:56,310 --> 00:06:58,260 So in this case, the value 30 120 120 00:06:58,260 --> 00:07:02,463 will be specified at memory address 0001. 121 121 00:07:03,360 --> 00:07:06,400 And remember this all happens in a call stack 122 122 00:07:06,400 --> 00:07:08,653 where primitive values are stored. 123 123 00:07:09,570 --> 00:07:12,830 Now what's extremely important to understand here 124 124 00:07:12,830 --> 00:07:17,070 is that the identifier actually points to the address 125 125 00:07:17,070 --> 00:07:19,690 and not to the value itself. 126 126 00:07:19,690 --> 00:07:24,010 So we would say that the age variable is equal to 30, 127 127 00:07:24,010 --> 00:07:28,680 but in fact, age is equal to the memory address 0001, 128 128 00:07:30,420 --> 00:07:33,730 which holds the value of 30, all right? 129 129 00:07:33,730 --> 00:07:35,310 And this subtle distinction 130 130 00:07:35,310 --> 00:07:38,380 is very important to keep in mind. 131 131 00:07:38,380 --> 00:07:41,420 Now, in the next line, we declare old age 132 132 00:07:41,420 --> 00:07:43,540 to be equal to age. 133 133 00:07:43,540 --> 00:07:48,240 So knowing that a variable actually holds a memory address, 134 134 00:07:48,240 --> 00:07:51,200 what should old age look like? 135 135 00:07:51,200 --> 00:07:55,290 Well, it will simply point to the same memory address 136 136 00:07:55,290 --> 00:07:57,080 as the age variable. 137 137 00:07:57,080 --> 00:08:02,080 And so it will look like old age, is simply 30 as well. 138 138 00:08:02,140 --> 00:08:07,140 Great, but now in the next line, we set eight to 31. 139 139 00:08:07,160 --> 00:08:09,060 So what will happen then? 140 140 00:08:09,060 --> 00:08:12,070 The value at address 0001 141 141 00:08:12,070 --> 00:08:15,270 will certainly not become 31 142 142 00:08:15,270 --> 00:08:18,560 because that would change old age as well, 143 143 00:08:18,560 --> 00:08:21,560 since they both point to the same address. 144 144 00:08:21,560 --> 00:08:24,480 So that would make no sense at all. 145 145 00:08:24,480 --> 00:08:28,590 Also the value at a certain memory address is immutable, 146 146 00:08:28,590 --> 00:08:32,000 or in other words, it cannot be changed. 147 147 00:08:32,000 --> 00:08:34,630 So instead what's going to happen here 148 148 00:08:34,630 --> 00:08:38,190 is that a new piece of memory is allocated. 149 149 00:08:38,190 --> 00:08:42,477 So it's created and the age identifier now simply points 150 150 00:08:42,477 --> 00:08:47,023 to the new address, which is holding the new value of 31, 151 151 00:08:47,920 --> 00:08:48,753 all right? 152 152 00:08:48,753 --> 00:08:51,060 And that's why when we lock both 153 153 00:08:51,060 --> 00:08:53,720 or variables to the console in the end, 154 154 00:08:53,720 --> 00:08:58,050 they both return exactly values that we expect. 155 155 00:08:58,050 --> 00:09:02,200 Now with reference values, things work a bit differently, 156 156 00:09:02,200 --> 00:09:05,230 which is the reason why this example gave us 157 157 00:09:05,230 --> 00:09:09,910 that unexpected, weird behavior earlier in practice. 158 158 00:09:09,910 --> 00:09:14,590 So what's the origin of this weird unexpected result? 159 159 00:09:14,590 --> 00:09:19,590 Well, when a new object is created such as this Me object, 160 160 00:09:19,790 --> 00:09:21,930 it is stored in the heap. 161 161 00:09:21,930 --> 00:09:25,030 And such as before there is a memory address 162 162 00:09:25,030 --> 00:09:27,360 and then the value itself. 163 163 00:09:27,360 --> 00:09:29,710 Now in the case of reference values 164 164 00:09:29,710 --> 00:09:33,300 like this Me object the Me identifier 165 165 00:09:33,300 --> 00:09:35,920 does actually not point directly 166 166 00:09:35,920 --> 00:09:39,790 to this newly created memory address in the heap. 167 167 00:09:39,790 --> 00:09:43,360 So in this example, D30F, 168 168 00:09:43,360 --> 00:09:46,890 instead, it will point to a new piece of memory 169 169 00:09:46,890 --> 00:09:49,350 that's created in the stack. 170 170 00:09:49,350 --> 00:09:53,247 And this new piece of memory will then point to the object 171 171 00:09:53,247 --> 00:09:58,247 that's in the heap by using the memory address as its value. 172 172 00:09:59,220 --> 00:10:02,620 In other words, the piece of memory in the call stack 173 173 00:10:02,620 --> 00:10:06,290 has a reference to the piece of memory in the heap, 174 174 00:10:06,290 --> 00:10:09,810 which holds or Me object, okay? 175 175 00:10:09,810 --> 00:10:13,670 And that's the reason why we call objects reference types 176 176 00:10:13,670 --> 00:10:15,300 in this context. 177 177 00:10:15,300 --> 00:10:19,650 So again, when we declare a variable as an object, 178 178 00:10:19,650 --> 00:10:21,930 an identifier is created, 179 179 00:10:21,930 --> 00:10:25,130 which points to a piece of memory in the stack, 180 180 00:10:25,130 --> 00:10:29,010 which in turn points to a piece of memory in the heap. 181 181 00:10:29,010 --> 00:10:32,770 And that is where the object is actually stored. 182 182 00:10:32,770 --> 00:10:35,870 And it works this way because objects might be 183 183 00:10:35,870 --> 00:10:39,230 too large to be stored in the stack. 184 184 00:10:39,230 --> 00:10:42,210 Instead they are stored in the heap, 185 185 00:10:42,210 --> 00:10:45,620 which is like an almost unlimited memory pool. 186 186 00:10:45,620 --> 00:10:48,060 And the stack just keeps a reference 187 187 00:10:48,060 --> 00:10:51,610 to where the object is actually stored in the heap 188 188 00:10:51,610 --> 00:10:54,363 so that it can find it whenever necessary. 189 189 00:10:55,260 --> 00:10:57,390 Now, moving on in the code, 190 190 00:10:57,390 --> 00:11:00,540 we create a new variable called Friend 191 191 00:11:00,540 --> 00:11:03,900 that we set equal to the Me object. 192 192 00:11:03,900 --> 00:11:06,340 So what will happen here? 193 193 00:11:06,340 --> 00:11:09,570 Well, just like with primitive values, 194 194 00:11:09,570 --> 00:11:11,760 the Friend identifier will point 195 195 00:11:11,760 --> 00:11:16,560 to the exact same memory address as the Me identifier. 196 196 00:11:16,560 --> 00:11:20,000 And again, that address contains the reference, 197 197 00:11:20,000 --> 00:11:23,280 which then points to the object itself. 198 198 00:11:23,280 --> 00:11:27,010 And like this the Friend object is now essentially 199 199 00:11:27,010 --> 00:11:30,290 the exact same as the Me object. 200 200 00:11:30,290 --> 00:11:32,313 Do you see that here in the diagram? 201 201 00:11:33,230 --> 00:11:35,690 So here comes the interesting part 202 202 00:11:35,690 --> 00:11:38,870 because now we're actually gonna change a property 203 203 00:11:38,870 --> 00:11:43,870 in the Friend object by setting friend.age to 27. 204 204 00:11:44,420 --> 00:11:46,840 So what happens then is that the object 205 205 00:11:46,840 --> 00:11:51,800 is found in the heap, and the 30 is changed to 27. 206 206 00:11:51,800 --> 00:11:52,970 Great. 207 207 00:11:52,970 --> 00:11:56,410 And by the way, even though we defined the Friend variable 208 208 00:11:56,410 --> 00:12:01,170 as a constant, we can actually still manipulate the object 209 209 00:12:01,170 --> 00:12:03,100 without problems. 210 210 00:12:03,100 --> 00:12:06,180 And when we think about that, it makes sense 211 211 00:12:06,180 --> 00:12:09,840 because we're actually not changing the value in memory 212 212 00:12:09,840 --> 00:12:14,810 for the Friend identifier, it is still D30F. 213 213 00:12:14,810 --> 00:12:17,440 So the reference to the object. 214 214 00:12:17,440 --> 00:12:20,800 All we did was to change the value in the heap, 215 215 00:12:20,800 --> 00:12:22,690 and that's not a problem. 216 216 00:12:22,690 --> 00:12:26,230 So it's a misconception that all variables declared 217 217 00:12:26,230 --> 00:12:28,920 with const are immutable. 218 218 00:12:28,920 --> 00:12:32,610 In fact, that is only true for primitive values, 219 219 00:12:32,610 --> 00:12:35,200 but not for reference values. 220 220 00:12:35,200 --> 00:12:39,440 So keep that in mind, whenever you're working with const. 221 221 00:12:39,440 --> 00:12:43,000 Anyway, as we lock the Friend variable to the console, 222 222 00:12:43,000 --> 00:12:47,690 we get the age of 27, just as we said it before. 223 223 00:12:47,690 --> 00:12:50,350 But then when we lock the Me object, 224 224 00:12:50,350 --> 00:12:52,580 we get that weird behavior 225 225 00:12:52,580 --> 00:12:56,690 that we could previously not explain and not understand. 226 226 00:12:56,690 --> 00:12:59,750 But with everything that we learned in this lecture, 227 227 00:12:59,750 --> 00:13:01,520 it actually now makes sense 228 228 00:13:01,520 --> 00:13:05,950 that in the Me object, age is now also 27, 229 229 00:13:05,950 --> 00:13:10,100 even though we never changed me.age directly. 230 230 00:13:10,100 --> 00:13:13,410 And the reason for this, as we can see in this slide 231 231 00:13:13,410 --> 00:13:16,430 is the fact that Me and Friend 232 232 00:13:16,430 --> 00:13:21,400 actually point to the exact same object in the memory heap. 233 233 00:13:21,400 --> 00:13:24,370 So whenever we change something in this object, 234 234 00:13:24,370 --> 00:13:28,490 it will always be reflected in Friend and in Me. 235 235 00:13:28,490 --> 00:13:30,430 So in both these objects. 236 236 00:13:30,430 --> 00:13:34,040 So these are basically just two different identifiers 237 237 00:13:34,040 --> 00:13:36,960 pointing to the exact same value. 238 238 00:13:36,960 --> 00:13:40,907 And once again, that value is the memory address D30F 239 239 00:13:42,750 --> 00:13:46,800 which points to the reference in the memory heap. 240 240 00:13:46,800 --> 00:13:49,430 And one important implication of this 241 241 00:13:49,430 --> 00:13:53,330 is that whenever you think that you're copying an object, 242 242 00:13:53,330 --> 00:13:55,950 you're really just creating a new variable 243 243 00:13:55,950 --> 00:13:59,050 that points to the exact same object. 244 244 00:13:59,050 --> 00:14:01,380 And this has huge implications 245 245 00:14:01,380 --> 00:14:04,280 for the way JavaScript works in practice. 246 246 00:14:04,280 --> 00:14:06,760 And we will see that in the next video 247 247 00:14:06,760 --> 00:14:08,670 and throughout the course. 248 248 00:14:08,670 --> 00:14:11,450 Now there are actually ways around this, 249 249 00:14:11,450 --> 00:14:13,600 as we will also learn later. 250 250 00:14:13,600 --> 00:14:17,020 but in general, this is how reference values work 251 251 00:14:17,020 --> 00:14:18,580 in JavaScript. 252 252 00:14:18,580 --> 00:14:21,370 So make sure to really understand this, 253 253 00:14:21,370 --> 00:14:25,170 as well as the implications that this behavior has, 254 254 00:14:25,170 --> 00:14:28,620 even if that means that you have to re-watch this lecture 255 255 00:14:28,620 --> 00:14:33,570 or at least the explanation of this slide, all right? 256 256 00:14:33,570 --> 00:14:36,570 Then once you really understand what happened here, 257 257 00:14:36,570 --> 00:14:40,260 in this example, let's understand primitive values 258 258 00:14:40,260 --> 00:14:43,430 and reference values even better in practice 259 259 00:14:43,430 --> 00:14:44,843 in the next lecture. 260 260 00:14:45,900 --> 00:14:49,750 But before we do that, let me just quickly take a second 261 261 00:14:49,750 --> 00:14:52,920 and mention a three more, really big topics 262 262 00:14:52,920 --> 00:14:56,100 about how JavaScript works behind the scenes 263 263 00:14:56,100 --> 00:14:58,250 that will not be in the section, 264 264 00:14:58,250 --> 00:15:02,400 but closer to where we actually need to learn about them. 265 265 00:15:02,400 --> 00:15:06,220 For example, one fundamental concept of JavaScript 266 266 00:15:06,220 --> 00:15:08,710 is prototypal inheritance. 267 267 00:15:08,710 --> 00:15:10,690 But we will only talk about that 268 268 00:15:10,690 --> 00:15:14,610 in the object oriented programming section of this course. 269 269 00:15:14,610 --> 00:15:18,110 Because it doesn't make sense to learn about this now 270 270 00:15:18,110 --> 00:15:20,550 only to then forget it all 271 271 00:15:20,550 --> 00:15:23,083 until we finally reach that section. 272 272 00:15:23,960 --> 00:15:28,930 The same is true for a detailed lecture on the event loop. 273 273 00:15:28,930 --> 00:15:32,720 So I already introduced the event loop in this section, 274 274 00:15:32,720 --> 00:15:36,080 but in the section about asynchronous JavaScript, 275 275 00:15:36,080 --> 00:15:38,010 we will dive really deep into 276 276 00:15:38,010 --> 00:15:40,370 how exactly the event loop works 277 277 00:15:40,370 --> 00:15:42,910 and why it's such a fundamental piece 278 278 00:15:42,910 --> 00:15:44,393 of the JavaScript engine. 279 279 00:15:45,270 --> 00:15:47,390 Finally, we will have lectures on 280 280 00:15:47,390 --> 00:15:51,100 how the DOM actually works behind the scenes too. 281 281 00:15:51,100 --> 00:15:52,430 And that's going to be 282 282 00:15:52,430 --> 00:15:55,530 in the advanced DOM and events section 283 283 00:15:55,530 --> 00:15:57,160 so that you can then apply 284 284 00:15:57,160 --> 00:16:00,000 what you learned there right away. 285 285 00:16:00,000 --> 00:16:01,190 Okay? 286 286 00:16:01,190 --> 00:16:03,893 And with that being said, let's now move on. 25358