1 00:00:01,690 --> 00:00:03,280 Let's now take a look at 2 00:00:03,280 --> 00:00:06,270 the three different ways of declaring variables 3 00:00:06,270 --> 00:00:07,543 in JavaScript. 4 00:00:08,810 --> 00:00:11,910 So, up until this point, we have always used 5 00:00:11,910 --> 00:00:14,890 the let keyword to declare variables. 6 00:00:14,890 --> 00:00:18,650 However, there are also const and var. 7 00:00:18,650 --> 00:00:22,570 Now, let and const were introduced in ES6, 8 00:00:22,570 --> 00:00:25,270 and so they are modern JavaScript. 9 00:00:25,270 --> 00:00:29,960 While the var keyword is the old way of declaring variables. 10 00:00:29,960 --> 00:00:33,340 So, let's learn how they are different and which one to use 11 00:00:33,340 --> 00:00:35,640 in which situations. 12 00:00:35,640 --> 00:00:40,300 And let's, again, start by getting rid of this code. 13 00:00:40,300 --> 00:00:43,680 Actually, all I will do is move this to the end 14 00:00:44,540 --> 00:00:47,470 and in the future, I will just do this before recording 15 00:00:47,470 --> 00:00:48,303 the next video. 16 00:00:49,530 --> 00:00:53,690 So anyway, we use the let keyword to declare variables 17 00:00:53,690 --> 00:00:57,291 that can change later so basically during the execution 18 00:00:57,291 --> 00:01:01,890 of our program and that's exactly what we did here. 19 00:01:01,890 --> 00:01:05,460 So, we used let to declare JavaScript is fun, 20 00:01:05,460 --> 00:01:09,693 and then we changed that later from true to yes. 21 00:01:11,140 --> 00:01:16,070 But for example, or another example I would say is 22 00:01:17,290 --> 00:01:22,030 let's set the age to 30 at one point of the program. 23 00:01:22,030 --> 00:01:24,240 And then in another point of the program, 24 00:01:24,240 --> 00:01:27,040 we can change it to something else. 25 00:01:27,040 --> 00:01:31,000 For example, when the person turns 31 26 00:01:31,000 --> 00:01:35,160 and then the age of course changes from 30 to 31. 27 00:01:35,160 --> 00:01:38,250 And so, it's perfectly okay to declare a variable 28 00:01:38,250 --> 00:01:41,210 with let at one point in the program, 29 00:01:41,210 --> 00:01:44,580 and then later assign a new value to it. 30 00:01:44,580 --> 00:01:48,400 In technical terms, we call this reassigning a value to 31 00:01:48,400 --> 00:01:51,638 a variable, or also we say that we mutate 32 00:01:51,638 --> 00:01:54,760 the age variable in this case. 33 00:01:54,760 --> 00:01:57,790 So, it was 30 and now it's 31, 34 00:01:57,790 --> 00:02:01,200 and so we mutated the variable, okay? 35 00:02:01,200 --> 00:02:04,420 So, that's a term that you will hear all the time 36 00:02:04,420 --> 00:02:06,640 in the JavaScript world. 37 00:02:06,640 --> 00:02:08,940 So, when we need to mutate a variable, 38 00:02:08,940 --> 00:02:12,340 that's the perfect use case for using let 39 00:02:12,340 --> 00:02:15,170 and that also counts for the case that we want 40 00:02:15,170 --> 00:02:17,040 to declare empty variables. 41 00:02:17,040 --> 00:02:20,150 So for example, as we did here, where we declared 42 00:02:20,150 --> 00:02:24,430 an empty year and then later reassigned that variable 43 00:02:24,430 --> 00:02:28,600 to 1991, and sometimes we actually do this in practice. 44 00:02:28,600 --> 00:02:31,950 For example, when we want to declare all the variables 45 00:02:31,950 --> 00:02:33,560 at the top of a file, 46 00:02:33,560 --> 00:02:37,450 but only assign actual values to them later in the program, 47 00:02:37,450 --> 00:02:40,160 for example, based on some condition. 48 00:02:40,160 --> 00:02:43,920 Now, on the other hand, we use the const keyword 49 00:02:43,920 --> 00:02:47,280 to declare variables that are not supposed to change 50 00:02:47,280 --> 00:02:49,810 at any point in the future. 51 00:02:49,810 --> 00:02:54,250 So, the value in a const variable cannot be changed. 52 00:02:54,250 --> 00:02:59,250 For example, let's say the birth year, 1991 for example. 53 00:03:05,820 --> 00:03:10,820 If we then tried to reassign that let's say to 1990, 54 00:03:11,190 --> 00:03:12,473 it should not work. 55 00:03:13,570 --> 00:03:18,400 So, let's test that, and indeed we get a type error. 56 00:03:18,400 --> 00:03:22,210 So, assignment to constant variable. 57 00:03:22,210 --> 00:03:25,470 And so, that's exactly what the const keyword does. 58 00:03:25,470 --> 00:03:28,770 It creates a variable that we cannot reassign 59 00:03:28,770 --> 00:03:32,250 or in technical terms, an immutable variable. 60 00:03:32,250 --> 00:03:36,580 So, a variable that cannot be mutated, okay? 61 00:03:36,580 --> 00:03:39,500 And the birth year is a perfect example for that 62 00:03:39,500 --> 00:03:43,850 because in fact, the birth year of a person can never change 63 00:03:43,850 --> 00:03:46,470 while the age of course can change. 64 00:03:46,470 --> 00:03:49,850 And so that's why we used let for the age 65 00:03:49,850 --> 00:03:53,100 and const for the birth year, okay? 66 00:03:53,100 --> 00:03:54,790 That makes sense? 67 00:03:54,790 --> 00:03:56,940 Now the fact that variables created 68 00:03:56,940 --> 00:03:59,840 with const are immutable also means 69 00:03:59,840 --> 00:04:03,720 that we cannot declare empty const variables. 70 00:04:03,720 --> 00:04:07,163 For example, this is not legal. 71 00:04:08,390 --> 00:04:11,500 So, let's comment out this one here, again with command 72 00:04:11,500 --> 00:04:15,253 or control slash so that it doesn't get in our way here. 73 00:04:17,270 --> 00:04:19,750 And then reload, and then we get 74 00:04:19,750 --> 00:04:23,760 an error missing initializer in const declaration. 75 00:04:23,760 --> 00:04:26,010 And so this error message means that 76 00:04:26,010 --> 00:04:30,360 when using const, we need basically an initial value. 77 00:04:30,360 --> 00:04:31,973 Alright, so just like here. 78 00:04:33,050 --> 00:04:36,550 Now with these two different ways of declaring variables, 79 00:04:36,550 --> 00:04:39,460 you will probably ask, should I use let, 80 00:04:39,460 --> 00:04:42,990 or should I use const to declare a new variable? 81 00:04:42,990 --> 00:04:46,850 Well, as a best practice for writing clean code, 82 00:04:46,850 --> 00:04:51,350 I always recommend to use const by default and let only 83 00:04:51,350 --> 00:04:54,850 when you are really sure that the variable needs to change 84 00:04:54,850 --> 00:04:57,200 at some point in the future. 85 00:04:57,200 --> 00:04:59,240 For example, when you have a variable 86 00:04:59,240 --> 00:05:03,260 that's really never supposed to change like this birth year, 87 00:05:03,260 --> 00:05:05,580 you should always use const. 88 00:05:05,580 --> 00:05:07,100 Also, if you are sure 89 00:05:07,100 --> 00:05:10,890 that the age variable is never changing inside your program, 90 00:05:10,890 --> 00:05:13,940 you should declare it using const as well. 91 00:05:13,940 --> 00:05:16,650 The reason for this is that it's a good practice 92 00:05:16,650 --> 00:05:19,390 to have as little variable mutations 93 00:05:19,390 --> 00:05:22,040 or variable changes as possible 94 00:05:22,040 --> 00:05:25,650 because changing variables introduces a potential 95 00:05:25,650 --> 00:05:27,090 to create bugs. 96 00:05:27,090 --> 00:05:30,010 So, basically errors in your code. 97 00:05:30,010 --> 00:05:34,560 So again, by default, always just use const and let, 98 00:05:34,560 --> 00:05:37,500 when you're sure that the value of the variable needs 99 00:05:37,500 --> 00:05:40,570 to change at some point in your code. 100 00:05:40,570 --> 00:05:44,270 And so from now on, I will probably use the const way 101 00:05:44,270 --> 00:05:48,090 of declaring variables as much as possible. 102 00:05:48,090 --> 00:05:50,810 Now, there's also a third way in JavaScript 103 00:05:50,810 --> 00:05:54,610 of declaring variables, which is the var keyword, 104 00:05:54,610 --> 00:05:58,910 but this one should actually be completely avoided, okay? 105 00:05:58,910 --> 00:06:01,760 However, we should still know how it works 106 00:06:01,760 --> 00:06:05,400 for legacy reasons because you will see this maybe 107 00:06:05,400 --> 00:06:08,770 in older code basis or some older tutorials 108 00:06:08,770 --> 00:06:10,670 that you'll find online. 109 00:06:10,670 --> 00:06:13,083 So, let's take a look at how it works. 110 00:06:14,330 --> 00:06:19,230 So, var is basically the old way of defining variables, 111 00:06:19,230 --> 00:06:21,140 prior to ES6. 112 00:06:21,140 --> 00:06:24,280 And at first sight, it works actually pretty much 113 00:06:24,280 --> 00:06:25,963 the same as let. 114 00:06:27,180 --> 00:06:32,180 So, we can write var job equals programmer, 115 00:06:34,250 --> 00:06:37,880 and then just like with let, we can change the value of 116 00:06:37,880 --> 00:06:41,893 the variable later, just like this. 117 00:06:43,270 --> 00:06:47,433 Let's take this one out and let's just reload, 118 00:06:48,370 --> 00:06:50,000 and we didn't get any errors, 119 00:06:50,000 --> 00:06:53,040 which means that it's basically allowed 120 00:06:53,040 --> 00:06:58,040 to mutate this job variable when we use var, right? 121 00:06:58,340 --> 00:07:02,460 Now, although it looks like var and let are very similar, 122 00:07:02,460 --> 00:07:05,510 below the surface, they're actually pretty different. 123 00:07:05,510 --> 00:07:07,540 And actually there are also many differences 124 00:07:07,540 --> 00:07:10,550 between led const and var. 125 00:07:10,550 --> 00:07:12,760 So, between all three of them, 126 00:07:12,760 --> 00:07:15,930 and you will learn all about these differences later 127 00:07:15,930 --> 00:07:17,240 in section seven, 128 00:07:17,240 --> 00:07:20,380 because this is actually really important stuff. 129 00:07:20,380 --> 00:07:23,690 And I know that I keep saying that you will learn 130 00:07:23,690 --> 00:07:25,660 some topics later in the course, 131 00:07:25,660 --> 00:07:29,110 but the truth is that learning is really not linear. 132 00:07:29,110 --> 00:07:32,120 It cannot happen in a linear way. 133 00:07:32,120 --> 00:07:35,840 So, you sometimes need more advanced things right now, 134 00:07:35,840 --> 00:07:37,850 even without understanding them. 135 00:07:37,850 --> 00:07:38,870 But that's not a problem 136 00:07:38,870 --> 00:07:41,540 because you will understand them eventually. 137 00:07:41,540 --> 00:07:45,240 So, right now it would really not be useful for you to know 138 00:07:45,240 --> 00:07:47,470 that the let is block scoped 139 00:07:47,470 --> 00:07:50,380 and that VAR is function-scoped, right? 140 00:07:50,380 --> 00:07:52,590 Because you don't even know what a block 141 00:07:52,590 --> 00:07:54,510 or what a function really is. 142 00:07:54,510 --> 00:07:56,930 And so, we will learn about these differences 143 00:07:56,930 --> 00:07:58,750 in section seven. 144 00:07:58,750 --> 00:08:01,810 For now, what matters is that you should basically 145 00:08:01,810 --> 00:08:05,020 never use var, okay? 146 00:08:05,020 --> 00:08:08,560 Now just to finish, some people will probably point out 147 00:08:08,560 --> 00:08:12,820 that we actually don't even have to declare variables at all 148 00:08:12,820 --> 00:08:15,810 because it's actually not mandatory. 149 00:08:15,810 --> 00:08:18,730 We can just write something like this, 150 00:08:18,730 --> 00:08:23,730 last name equals Schmedtmann in my case. 151 00:08:28,760 --> 00:08:32,460 And then we can just use this variable name in this case 152 00:08:32,460 --> 00:08:34,860 by logging it to the console 153 00:08:34,860 --> 00:08:38,980 and JavaScript will happily execute this script, 154 00:08:38,980 --> 00:08:42,720 even without us declaring a variable, right? 155 00:08:42,720 --> 00:08:47,420 We didn't use let nor const nor a var in this case, 156 00:08:47,420 --> 00:08:49,240 but it still worked. 157 00:08:49,240 --> 00:08:52,950 However, this is actually a pretty terrible idea 158 00:08:52,950 --> 00:08:55,130 because this doesn't create a variable 159 00:08:55,130 --> 00:08:57,940 in the current so-called scope. 160 00:08:57,940 --> 00:09:00,840 Instead, JavaScript will create a property 161 00:09:00,840 --> 00:09:02,760 on the global object. 162 00:09:02,760 --> 00:09:05,520 And once again, you will understand a little bit later 163 00:09:05,520 --> 00:09:08,860 what all of this means, but what matters is 164 00:09:08,860 --> 00:09:13,110 that you should always properly declare variables, okay? 165 00:09:13,110 --> 00:09:15,520 Never just write a variable like this 166 00:09:15,520 --> 00:09:17,942 without really declaring it. 167 00:09:17,942 --> 00:09:19,120 Great. 168 00:09:19,120 --> 00:09:21,300 You're doing great progress here, 169 00:09:21,300 --> 00:09:24,590 even though our code doesn't really do anything yet, 170 00:09:24,590 --> 00:09:28,380 and we're not really writing any real applications yet. 171 00:09:28,380 --> 00:09:32,290 Right, but we still need to get some fundamentals down 172 00:09:32,290 --> 00:09:33,710 before we can do that. 173 00:09:33,710 --> 00:09:36,960 And so that's really important, and that's why I'm stressing 174 00:09:36,960 --> 00:09:39,840 all the little details about everything here. 175 00:09:39,840 --> 00:09:42,620 And I hope you see how important that is 176 00:09:42,620 --> 00:09:44,490 for your progress, okay? 177 00:09:44,490 --> 00:09:47,800 And don't worry, we will get to the cool stuff even 178 00:09:47,800 --> 00:09:51,300 a bit later in this first section, okay? 179 00:09:51,300 --> 00:09:52,520 But for now in the next video, 180 00:09:52,520 --> 00:09:55,843 let's talk about operators in JavaScript.