1 00:00:01,480 --> 00:00:03,620 There is an almost mystical feature 2 00:00:03,620 --> 00:00:05,390 of Java script functions 3 00:00:05,390 --> 00:00:09,438 that many developers fail to fully understand. 4 00:00:09,438 --> 00:00:13,572 And what I'm talking about is something called closures. 5 00:00:13,572 --> 00:00:15,440 So when I asked my students, 6 00:00:15,440 --> 00:00:18,753 what's the hardest JavaScript concept to understand, 7 00:00:18,753 --> 00:00:22,290 then many people say that it's closures. 8 00:00:22,290 --> 00:00:25,400 However, I believe that with the right explanation, 9 00:00:25,400 --> 00:00:27,830 it's actually not that hard, 10 00:00:27,830 --> 00:00:30,740 especially when you already understood everything 11 00:00:30,740 --> 00:00:33,020 that you learned before in this course, 12 00:00:33,020 --> 00:00:35,120 such as execution context, 13 00:00:35,120 --> 00:00:37,800 the call stack, and the sculpt chain, 14 00:00:37,800 --> 00:00:40,810 because closures kind of bring all of these concepts 15 00:00:40,810 --> 00:00:45,370 together in a beautiful, almost magical way. 16 00:00:45,370 --> 00:00:49,463 So enough talk, let's see what closures are all about. 17 00:00:51,400 --> 00:00:54,561 So I'm gonna start by creating a new function here 18 00:00:54,561 --> 00:00:57,853 called secure booking. 19 00:01:00,450 --> 00:01:04,810 And it is this function that will create the closure. 20 00:01:04,810 --> 00:01:08,036 Now, the first thing that I need to tell you about closures 21 00:01:08,036 --> 00:01:13,036 is that a closure is not a feature that we explicitly use. 22 00:01:13,710 --> 00:01:16,292 So we don't create closures manually, 23 00:01:16,292 --> 00:01:20,310 like we create a new array or a new function. 24 00:01:20,310 --> 00:01:22,741 So a closure simply happens automatically 25 00:01:22,741 --> 00:01:25,540 in certain situations, we just need 26 00:01:25,540 --> 00:01:28,310 to recognize those situations. 27 00:01:28,310 --> 00:01:31,600 And so that's what we're gonna do here in this example. 28 00:01:31,600 --> 00:01:33,908 So we will create one of those situations 29 00:01:33,908 --> 00:01:37,773 so that we can then take a look at a closure. 30 00:01:38,730 --> 00:01:41,430 So anyway, let's now continue writing this example 31 00:01:43,730 --> 00:01:45,110 and I'm calling this one passengerCount 32 00:01:46,530 --> 00:01:49,410 and it will start at zero, but we will be 33 00:01:49,410 --> 00:01:51,720 able to manipulate it. 34 00:01:51,720 --> 00:01:53,240 And I'm calling this function here, 35 00:01:53,240 --> 00:01:55,947 secure booking because this passengerCount variable 36 00:01:55,947 --> 00:02:00,947 cannot be manipulated and accessed from the outside. 37 00:02:02,690 --> 00:02:05,360 So, and now what's special about this function 38 00:02:05,360 --> 00:02:09,163 is that it will return a new function. 39 00:02:12,600 --> 00:02:15,773 And what we do in this function is to update 40 00:02:15,773 --> 00:02:18,303 the passengerCount variable. 41 00:02:19,220 --> 00:02:22,803 So the variable that is defined in the parent function. 42 00:02:23,800 --> 00:02:25,670 So that's important. 43 00:02:25,670 --> 00:02:28,810 And then let's just log the new passengerCount 44 00:02:28,810 --> 00:02:29,943 to the console. 45 00:02:32,800 --> 00:02:37,480 So passengerCount, alright, 46 00:02:37,480 --> 00:02:41,130 and now let's call the secure booking function 47 00:02:41,130 --> 00:02:44,333 and then store the result in a variable called Booker. 48 00:02:46,820 --> 00:02:49,100 And so this is actually pretty similar to what we did 49 00:02:49,100 --> 00:02:52,600 previously in the lecture of functions 50 00:02:52,600 --> 00:02:54,490 returning other functions. 51 00:02:54,490 --> 00:02:56,752 So we have one function here that we call 52 00:02:56,752 --> 00:03:00,970 and this function will return this new function. 53 00:03:00,970 --> 00:03:02,669 And so as we call secure booking, 54 00:03:02,669 --> 00:03:05,700 it will return exactly this function 55 00:03:05,700 --> 00:03:09,090 and it will then be stored inside this Booker. 56 00:03:09,090 --> 00:03:12,673 And so this here is gonna be now a function as well, right? 57 00:03:13,660 --> 00:03:15,290 So let's analyze in detail 58 00:03:15,290 --> 00:03:17,660 what happens when this line of code here 59 00:03:17,660 --> 00:03:20,180 is executed using all the concepts 60 00:03:20,180 --> 00:03:21,633 that we already know about. 61 00:03:23,130 --> 00:03:26,700 So this is exactly the code that we just wrote. 62 00:03:26,700 --> 00:03:28,410 Now, before we start running 63 00:03:28,410 --> 00:03:30,760 the secure booking function down here, 64 00:03:30,760 --> 00:03:34,720 our code is running in the global execution context. 65 00:03:34,720 --> 00:03:36,970 And in there, we currently only have 66 00:03:36,970 --> 00:03:39,180 this secure booking function. 67 00:03:39,180 --> 00:03:42,260 And so we can also say that the global scope 68 00:03:42,260 --> 00:03:44,970 now contains secure booking. 69 00:03:44,970 --> 00:03:48,530 Then when secure booking is actually executed, 70 00:03:48,530 --> 00:03:50,718 a new execution context is put 71 00:03:50,718 --> 00:03:53,820 on top of the execution stack. 72 00:03:53,820 --> 00:03:56,233 Now, remember, each execution context 73 00:03:56,233 --> 00:03:58,730 has a variable environment, 74 00:03:58,730 --> 00:04:01,900 which contains all its local variables. 75 00:04:01,900 --> 00:04:04,831 In this case, it only contains the passengerCount 76 00:04:04,831 --> 00:04:06,363 set to zero. 77 00:04:07,200 --> 00:04:09,120 This variable environment is also 78 00:04:09,120 --> 00:04:11,470 the scope of this function. 79 00:04:11,470 --> 00:04:14,441 And so the scope chain of this execution context 80 00:04:14,441 --> 00:04:16,191 looks like this. 81 00:04:16,191 --> 00:04:19,315 So passengerCount is in the local scope, 82 00:04:19,315 --> 00:04:22,480 but of course this scope also gets access 83 00:04:22,480 --> 00:04:25,540 to all variables of the parent's scopes. 84 00:04:25,540 --> 00:04:28,960 And in this case, just a global scope. 85 00:04:28,960 --> 00:04:32,161 Anyway, in the next line of the secure booking function, 86 00:04:32,161 --> 00:04:36,060 a new function is returned and it will be stored 87 00:04:36,060 --> 00:04:37,960 in the Booker variable. 88 00:04:37,960 --> 00:04:42,710 So the global context now also contains the Booker variable. 89 00:04:42,710 --> 00:04:44,710 And now what else happens when 90 00:04:44,710 --> 00:04:47,600 the secure booking function returns? 91 00:04:47,600 --> 00:04:48,990 Well, that's right. 92 00:04:48,990 --> 00:04:53,268 Its execution context pops off the stack and disappears. 93 00:04:53,268 --> 00:04:56,740 So the secure booking function has done its job 94 00:04:56,740 --> 00:04:59,700 and has now finished execution. 95 00:04:59,700 --> 00:05:03,980 It really is gone now and that's important to be aware of 96 00:05:03,980 --> 00:05:05,770 and to keep in mind. 97 00:05:05,770 --> 00:05:08,860 And for now, that's actually all we did. 98 00:05:08,860 --> 00:05:11,914 So all this is nothing new at this point, right? 99 00:05:11,914 --> 00:05:14,530 All we did was to analyze the call stack 100 00:05:14,530 --> 00:05:18,620 and the scope chain as we call the secure booking function. 101 00:05:18,620 --> 00:05:20,870 And this is gonna be important to later on 102 00:05:20,870 --> 00:05:22,950 understand the closure. 103 00:05:22,950 --> 00:05:26,320 So as of yet, we didn't see the closure yet. 104 00:05:26,320 --> 00:05:29,357 All we did was use the knowledge that we already have 105 00:05:29,357 --> 00:05:33,600 to understand how this Booker function was created, 106 00:05:33,600 --> 00:05:37,610 because that's gonna be important for the next step. 107 00:05:37,610 --> 00:05:39,357 So let's now go back to our code 108 00:05:39,357 --> 00:05:42,400 to actually use the Booker function, 109 00:05:42,400 --> 00:05:45,313 and then finally see the closure in action. 110 00:05:46,950 --> 00:05:49,880 So now that we understand how the Booker function 111 00:05:49,880 --> 00:05:53,003 was created, let's now actually call it here. 112 00:05:54,860 --> 00:05:58,790 So calling it a couple of times here, and as we can see, 113 00:05:58,790 --> 00:06:00,930 it doesn't need any arguments. 114 00:06:00,930 --> 00:06:03,613 There's no list of parameters, right? 115 00:06:05,150 --> 00:06:07,080 So let's call it here three times 116 00:06:09,280 --> 00:06:13,120 and now let's reload the page here. 117 00:06:13,120 --> 00:06:17,670 And indeed we get one, two, three passengers. 118 00:06:17,670 --> 00:06:20,682 And so what this means is that the Booker function 119 00:06:20,682 --> 00:06:24,910 was in fact able to increment the passengerCount 120 00:06:24,910 --> 00:06:28,810 to one, then to two and then to three. 121 00:06:28,810 --> 00:06:30,920 But now if we think about this, 122 00:06:30,920 --> 00:06:33,300 then how is this even possible? 123 00:06:33,300 --> 00:06:35,143 How can the Booker function update 124 00:06:35,143 --> 00:06:38,196 this passengerCount variable that's defined 125 00:06:38,196 --> 00:06:40,200 in a secure booking function 126 00:06:40,200 --> 00:06:44,030 that actually has already finished executing. 127 00:06:44,030 --> 00:06:45,670 And so, as I just said, 128 00:06:45,670 --> 00:06:48,970 this function has already finished its execution. 129 00:06:48,970 --> 00:06:50,090 It is gone. 130 00:06:50,090 --> 00:06:53,980 So its execution context is no longer on the stack, 131 00:06:53,980 --> 00:06:56,290 as we just saw in the slide, 132 00:06:56,290 --> 00:06:59,030 but still this inner function here, 133 00:06:59,030 --> 00:07:01,180 which is the Booker function, 134 00:07:01,180 --> 00:07:04,911 is still able to access the passengerCount variable 135 00:07:04,911 --> 00:07:07,700 that's inside of the Booker function 136 00:07:07,700 --> 00:07:10,380 that should no longer exist. 137 00:07:10,380 --> 00:07:13,708 And maybe you can guess that what makes this possible 138 00:07:13,708 --> 00:07:17,120 is a closure, but before I explain 139 00:07:17,120 --> 00:07:19,400 exactly how the closure works, 140 00:07:19,400 --> 00:07:21,760 I want you to appreciate once more, 141 00:07:21,760 --> 00:07:24,870 how strange this actually is. 142 00:07:24,870 --> 00:07:27,529 So again, this Booker function here 143 00:07:27,529 --> 00:07:30,150 is simply a function that exists 144 00:07:30,150 --> 00:07:32,193 out here in the global environment 145 00:07:32,193 --> 00:07:35,200 or in the global scope, right? 146 00:07:35,200 --> 00:07:38,383 And the environment in which the function was created. 147 00:07:38,383 --> 00:07:41,870 So this year basically, this environment 148 00:07:41,870 --> 00:07:44,150 is no longer active. 149 00:07:44,150 --> 00:07:46,080 It is in fact gone. 150 00:07:46,080 --> 00:07:49,110 But still the Booker function somehow continues 151 00:07:49,110 --> 00:07:50,980 to have access to the variables 152 00:07:50,980 --> 00:07:55,300 that were present at the time that the function was created. 153 00:07:55,300 --> 00:07:59,300 And in particular, this passengerCount variable here. 154 00:07:59,300 --> 00:08:02,237 And so that's exactly what the closure does. 155 00:08:02,237 --> 00:08:05,520 So we can say that a closure makes a function 156 00:08:05,520 --> 00:08:08,466 remember all the variables that existed 157 00:08:08,466 --> 00:08:12,490 at the function's birthplace essentially, right? 158 00:08:12,490 --> 00:08:15,960 So we can imagine the secure booking 159 00:08:15,960 --> 00:08:19,510 as being the birthplace of this function. 160 00:08:19,510 --> 00:08:22,070 So of the Booker function, essentially. 161 00:08:22,070 --> 00:08:25,829 And so this function remembers everything at its birthplace, 162 00:08:25,829 --> 00:08:28,540 by the time it was created. 163 00:08:28,540 --> 00:08:31,000 And this cannot simply be explained 164 00:08:31,000 --> 00:08:33,160 with the scope chain alone. 165 00:08:33,160 --> 00:08:35,769 So we need to also understand the closure. 166 00:08:35,769 --> 00:08:39,743 And so let me now, really explain how it actually works. 167 00:08:40,940 --> 00:08:43,550 So this is how we left the call stack 168 00:08:43,550 --> 00:08:46,820 and the sculpt chain after the last slide. 169 00:08:46,820 --> 00:08:49,310 And the most important thing to notice here 170 00:08:49,310 --> 00:08:52,322 is that the execution context of secure booking 171 00:08:52,322 --> 00:08:55,140 is no longer on call stack, 172 00:08:55,140 --> 00:08:58,210 because again, this function has finished 173 00:08:58,210 --> 00:09:00,600 execution long ago. 174 00:09:00,600 --> 00:09:04,129 So now it's time to finally run the Booker function 175 00:09:04,129 --> 00:09:07,520 and see exactly what's gonna happen here. 176 00:09:07,520 --> 00:09:11,540 And note that Booker is really this function here, 177 00:09:11,540 --> 00:09:13,583 located in the global scope. 178 00:09:14,540 --> 00:09:17,016 Anyway, the first thing that's gonna happen 179 00:09:17,016 --> 00:09:20,382 is that a new execution context is created 180 00:09:20,382 --> 00:09:23,410 and put on top of the call stack 181 00:09:23,410 --> 00:09:27,036 and the variable environment of this context is emptied 182 00:09:27,036 --> 00:09:29,480 simply because there are no variables 183 00:09:29,480 --> 00:09:31,990 declared in this function. 184 00:09:31,990 --> 00:09:34,680 Now what about the scope chain? 185 00:09:34,680 --> 00:09:37,539 Well, since Booker is in the global context, 186 00:09:37,539 --> 00:09:41,270 it's simply a child's scope of the global scope, 187 00:09:41,270 --> 00:09:44,480 just like this, but maybe now 188 00:09:44,480 --> 00:09:46,870 you're starting to see the problem. 189 00:09:46,870 --> 00:09:49,509 So how will the Booker function access 190 00:09:49,509 --> 00:09:52,510 the passengerCount variable? 191 00:09:52,510 --> 00:09:56,460 It's nowhere to be found in the scope chain, right? 192 00:09:56,460 --> 00:09:58,580 So this is where we start to unveil 193 00:09:58,580 --> 00:10:01,330 the secret of the closure 194 00:10:01,330 --> 00:10:04,100 and the secret is basically this. 195 00:10:04,100 --> 00:10:07,541 Any function always has access to the variable environment 196 00:10:07,541 --> 00:10:12,183 of the execution context in which the function was created. 197 00:10:13,040 --> 00:10:16,719 Now, in the case of Booker, this function was created. 198 00:10:16,719 --> 00:10:21,719 It was born in the execution context of secure booking, 199 00:10:22,060 --> 00:10:26,360 which was popped off the stack previously, remember? 200 00:10:26,360 --> 00:10:28,920 So, therefore the Booker function 201 00:10:28,920 --> 00:10:32,470 will get access to this variable environment, 202 00:10:32,470 --> 00:10:35,623 which contains the passengerCount variable. 203 00:10:35,623 --> 00:10:39,170 And this is how the function will be able to read 204 00:10:39,170 --> 00:10:42,840 and manipulate the passengerCount variable. 205 00:10:42,840 --> 00:10:46,273 And so it's this connection that we call closure. 206 00:10:47,690 --> 00:10:51,700 So let's say all that again, to make this really clear. 207 00:10:51,700 --> 00:10:55,658 So a function always has access to the variable environment 208 00:10:55,658 --> 00:10:59,980 of the execution context in which it was created, 209 00:10:59,980 --> 00:11:03,920 even after a debt execution context is gone. 210 00:11:03,920 --> 00:11:06,800 And this last part is really important. 211 00:11:06,800 --> 00:11:10,400 The closure is then basically this variable environment 212 00:11:10,400 --> 00:11:12,500 attached to the function, 213 00:11:12,500 --> 00:11:15,530 exactly as it was at the time and place 214 00:11:15,530 --> 00:11:18,110 that the function was created. 215 00:11:18,110 --> 00:11:22,290 And this probably still sounds confusing, but don't worry. 216 00:11:22,290 --> 00:11:26,025 I have some more familiar analogies in the next slide. 217 00:11:26,025 --> 00:11:29,397 For now, we are just trying to understand the mechanism 218 00:11:29,397 --> 00:11:33,525 behind the closure, so how it all works behind the scenes. 219 00:11:33,525 --> 00:11:37,610 So what matters the most here is that the Booker function 220 00:11:37,610 --> 00:11:40,472 has access to the passengerCount variable 221 00:11:40,472 --> 00:11:43,435 because it's basically defined in the scope 222 00:11:43,435 --> 00:11:47,141 in which the Booker function was actually created. 223 00:11:47,141 --> 00:11:51,068 So in a sense, the scope chain is actually preserved 224 00:11:51,068 --> 00:11:54,210 through the closure, even when a scope 225 00:11:54,210 --> 00:11:56,260 has already been destroyed 226 00:11:56,260 --> 00:11:59,384 because its execution context is gone. 227 00:11:59,384 --> 00:12:03,080 This means that even though the execution context 228 00:12:03,080 --> 00:12:04,742 has actually been destroyed, 229 00:12:04,742 --> 00:12:07,659 the variable environment somehow keeps living 230 00:12:07,659 --> 00:12:10,210 somewhere in the engine. 231 00:12:10,210 --> 00:12:13,820 Now we can say that the Booker function closed over 232 00:12:13,820 --> 00:12:18,272 its parents scope or over its parent variable environment. 233 00:12:18,272 --> 00:12:21,080 And this includes all function arguments. 234 00:12:21,080 --> 00:12:24,650 Even though in this example, we don't have any. 235 00:12:24,650 --> 00:12:28,656 And now this attached or closed over variable environment 236 00:12:28,656 --> 00:12:31,920 stays with the function forever. 237 00:12:31,920 --> 00:12:36,350 It will carry it around and be able to use it forever. 238 00:12:36,350 --> 00:12:38,750 To make it a bit more digestible, 239 00:12:38,750 --> 00:12:41,435 we can also say that thanks to the closure, 240 00:12:41,435 --> 00:12:45,015 a function does not lose connection to variables 241 00:12:45,015 --> 00:12:48,120 that existed at the function's birthplace. 242 00:12:48,120 --> 00:12:50,619 That's a bit more intuitive, right? 243 00:12:50,619 --> 00:12:53,520 But anyway, let's see what happens now 244 00:12:53,520 --> 00:12:56,890 with execution of the Booker function. 245 00:12:56,890 --> 00:12:59,490 So the function attempts to increase 246 00:12:59,490 --> 00:13:01,830 the passengerCount variable. 247 00:13:01,830 --> 00:13:05,980 However, this variable is not in the current scope. 248 00:13:05,980 --> 00:13:09,454 And so JavaScript will immediately look into the closure 249 00:13:09,454 --> 00:13:12,830 and see if it can find the variable there. 250 00:13:12,830 --> 00:13:16,677 And it does this even before looking at the scope chain. 251 00:13:16,677 --> 00:13:20,570 For example, if there was a global passengerCount variable 252 00:13:20,570 --> 00:13:25,016 set to 10, it would still first use the one in the closure. 253 00:13:25,016 --> 00:13:30,000 So the closure basically has priority over the scope chain. 254 00:13:30,000 --> 00:13:31,970 And so after running this function, 255 00:13:31,970 --> 00:13:34,190 the passengerCount becomes one. 256 00:13:34,190 --> 00:13:35,940 This message is logged. 257 00:13:35,940 --> 00:13:40,350 And then the execution context is popped off the stack. 258 00:13:40,350 --> 00:13:42,990 Then execution moves to the next line. 259 00:13:42,990 --> 00:13:46,990 We get a new execution context and a closure is still there, 260 00:13:46,990 --> 00:13:51,790 still attached to the function and the value is still one. 261 00:13:51,790 --> 00:13:53,849 And so now this function executes, 262 00:13:53,849 --> 00:13:56,546 increasing the passengerCount to two 263 00:13:56,546 --> 00:13:59,790 and logging a message again. 264 00:13:59,790 --> 00:14:02,670 Okay, and that's what closures are 265 00:14:02,670 --> 00:14:04,942 and how they work behind the scenes. 266 00:14:04,942 --> 00:14:08,720 And I know that this is all quite complex. 267 00:14:08,720 --> 00:14:11,420 So let me give you a couple different definitions 268 00:14:11,420 --> 00:14:14,313 of closure now, some more formal ones 269 00:14:14,313 --> 00:14:19,080 and some more intuitive and maybe easier to grasp. 270 00:14:19,080 --> 00:14:21,750 And the most formal definition of closure 271 00:14:21,750 --> 00:14:23,690 is the one we already saw, 272 00:14:23,690 --> 00:14:27,320 which is that a closure is the closed over variable 273 00:14:27,320 --> 00:14:30,060 environment of the execution context 274 00:14:30,060 --> 00:14:32,385 in which a function was created 275 00:14:32,385 --> 00:14:36,700 even after that execution context is gone, 276 00:14:36,700 --> 00:14:39,610 or in other words, even after the function 277 00:14:39,610 --> 00:14:43,563 to which the execution context belongs has returned. 278 00:14:44,500 --> 00:14:47,138 Next and a bit easier to understand, 279 00:14:47,138 --> 00:14:51,400 a closure gives a function access to all the variables 280 00:14:51,400 --> 00:14:53,340 of its parent function. 281 00:14:53,340 --> 00:14:56,860 So the function in which it is defined 282 00:14:56,860 --> 00:15:00,900 even after that parent function has returned. 283 00:15:00,900 --> 00:15:04,224 So the function keeps a reference to its outer scope 284 00:15:04,224 --> 00:15:07,530 even after that outer scope is gone, 285 00:15:07,530 --> 00:15:11,193 which basically preserves the scope chain throughout time. 286 00:15:12,200 --> 00:15:14,230 All right, another definition, 287 00:15:14,230 --> 00:15:18,500 or let's say analogy is that a closure makes sure 288 00:15:18,500 --> 00:15:21,320 that a function does never lose connection 289 00:15:21,320 --> 00:15:25,310 to the variables that existed at the function's birthplace. 290 00:15:25,310 --> 00:15:27,070 It remembers the variables, 291 00:15:27,070 --> 00:15:30,200 even after the birthplace is gone. 292 00:15:30,200 --> 00:15:32,900 It's like a person who doesn't lose connection 293 00:15:32,900 --> 00:15:35,030 to their hometown. 294 00:15:35,030 --> 00:15:38,233 In this analogy, the person is the function 295 00:15:38,233 --> 00:15:42,040 and the hometown is the function's parents scope, 296 00:15:42,040 --> 00:15:44,520 and the function then doesn't lose the connection 297 00:15:44,520 --> 00:15:48,400 to the variables stored in this parent's scope. 298 00:15:48,400 --> 00:15:51,013 And I hope that makes sense. 299 00:15:51,870 --> 00:15:54,236 Finally, some people like to think of 300 00:15:54,236 --> 00:15:58,540 this attached variable environment as a backpack. 301 00:15:58,540 --> 00:16:02,130 So in this analogy, a function has a backpack, 302 00:16:02,130 --> 00:16:05,470 which it carries around wherever it goes. 303 00:16:05,470 --> 00:16:08,250 And this backpack contains all the variables 304 00:16:08,250 --> 00:16:10,670 that were present in the environment 305 00:16:10,670 --> 00:16:13,300 in which the function was created. 306 00:16:13,300 --> 00:16:16,000 Then whenever a variable can't be found 307 00:16:16,000 --> 00:16:17,720 in the function scope, 308 00:16:17,720 --> 00:16:20,092 JavaScript will look into the backpack 309 00:16:20,092 --> 00:16:23,460 and take the missing variable from there. 310 00:16:23,460 --> 00:16:26,358 So kind of similar to the other definitions, 311 00:16:26,358 --> 00:16:30,110 but maybe a little bit more visual. 312 00:16:30,110 --> 00:16:34,050 So these are some different ways of defining closure, 313 00:16:34,050 --> 00:16:36,350 but they all mean the same thing. 314 00:16:36,350 --> 00:16:38,973 So they all represent the same idea. 315 00:16:39,890 --> 00:16:43,010 Finally, we need to understand that we do not 316 00:16:43,010 --> 00:16:45,520 have to create closures manually. 317 00:16:45,520 --> 00:16:47,859 And this is also what I already touched on 318 00:16:47,859 --> 00:16:50,690 at the beginning of the lecture. 319 00:16:50,690 --> 00:16:53,880 So instead, this is something that JavaScript 320 00:16:53,880 --> 00:16:58,520 does completely automatically, we don't have to do anything. 321 00:16:58,520 --> 00:17:02,430 Also, there is no way for us to explicitly access 322 00:17:02,430 --> 00:17:04,500 closed over variables. 323 00:17:04,500 --> 00:17:08,290 That's because closures are not like a tangible thing. 324 00:17:08,290 --> 00:17:12,130 They're not like an object or so that we can access. 325 00:17:12,130 --> 00:17:14,544 So we cannot just reach into a closure 326 00:17:14,544 --> 00:17:17,040 and take variables from it. 327 00:17:17,040 --> 00:17:19,660 That's impossible because a closure 328 00:17:19,660 --> 00:17:23,520 is just an internal property of a function. 329 00:17:23,520 --> 00:17:26,073 We can observe that a closure happens 330 00:17:26,073 --> 00:17:29,400 because functions magically keep having access 331 00:17:29,400 --> 00:17:32,730 to variables that should no longer exist, 332 00:17:32,730 --> 00:17:36,850 but we cannot directly access these variables. 333 00:17:36,850 --> 00:17:40,182 However, what we can do is to actually take a look 334 00:17:40,182 --> 00:17:42,440 at this internal property. 335 00:17:42,440 --> 00:17:46,960 So at this backpack, so to say, in a console. 336 00:17:46,960 --> 00:17:50,183 So let's quickly do that before we finish this lecture. 337 00:17:51,930 --> 00:17:55,320 And we can do this by using console.dir 338 00:17:58,282 --> 00:18:01,270 and then of the Booker function itself. 339 00:18:01,270 --> 00:18:03,440 So similar to console.log 340 00:18:03,440 --> 00:18:06,100 but this one is a bit different. 341 00:18:06,100 --> 00:18:09,433 And so here we now get this function itself. 342 00:18:11,020 --> 00:18:14,290 So we can get the arguments, the name property 343 00:18:14,290 --> 00:18:17,170 that we already took a look at before. 344 00:18:17,170 --> 00:18:18,003 And then down here, 345 00:18:18,003 --> 00:18:20,993 we have this scope's internal property. 346 00:18:21,860 --> 00:18:24,430 And this internal scope's property here 347 00:18:24,430 --> 00:18:26,930 is basically the variable environment 348 00:18:26,930 --> 00:18:28,393 of the Booker function. 349 00:18:29,270 --> 00:18:31,774 Now in here, we can actually see the closure 350 00:18:31,774 --> 00:18:35,460 coming from secure booking, all right? 351 00:18:35,460 --> 00:18:38,520 And so this is where we see the passengerCount, 352 00:18:38,520 --> 00:18:41,540 which currently stands at three. 353 00:18:41,540 --> 00:18:44,267 And so this closure here basically, 354 00:18:44,267 --> 00:18:48,453 is the variable environment of this secure booking. 355 00:18:50,100 --> 00:18:53,793 So that's the one that is being preserved by the closure. 356 00:18:55,340 --> 00:18:56,970 All right? 357 00:18:56,970 --> 00:19:01,650 And by the way, whenever you see these double brackets here, 358 00:19:01,650 --> 00:19:04,390 that means that it is an internal property, 359 00:19:04,390 --> 00:19:06,603 which we cannot access from our code. 360 00:19:07,570 --> 00:19:11,920 All right, this was a long video about closures. 361 00:19:11,920 --> 00:19:13,200 Now in the next lecture, 362 00:19:13,200 --> 00:19:16,080 we're gonna take a look at three more examples 363 00:19:16,080 --> 00:19:19,940 of closures and also analyze how they work, 364 00:19:19,940 --> 00:19:23,120 because it's really important that you understand 365 00:19:23,120 --> 00:19:25,600 this concept of closures. 366 00:19:25,600 --> 00:19:29,071 It's a feature that's used all the time in JavaScript 367 00:19:29,071 --> 00:19:33,080 and many times, even without us realizing 368 00:19:33,080 --> 00:19:35,180 that closures are happening. 369 00:19:35,180 --> 00:19:38,272 So if you want to become confident as a programmer, 370 00:19:38,272 --> 00:19:40,730 you always need to know how exactly 371 00:19:40,730 --> 00:19:43,100 everything in your code works. 372 00:19:43,100 --> 00:19:45,453 And that of course includes closures.