1 00:00:01,130 --> 00:00:02,100 Now that we're getting 2 00:00:02,100 --> 00:00:04,580 more comfortable with JavaScript, 3 00:00:04,580 --> 00:00:07,880 it's time to learn about a more theoretical concept. 4 00:00:07,880 --> 00:00:08,830 And that's the difference 5 00:00:08,830 --> 00:00:11,980 between statements and expressions. 6 00:00:11,980 --> 00:00:15,510 And this is gonna be just a very high level overview, 7 00:00:15,510 --> 00:00:18,933 just so that the future lecturers make more sense. 8 00:00:20,210 --> 00:00:23,730 So essentially, an expression is a piece of code 9 00:00:23,730 --> 00:00:26,070 that produces a value. 10 00:00:26,070 --> 00:00:29,890 For example, three plus four is an expression 11 00:00:29,890 --> 00:00:33,840 because this is gonna produce a value, okay? 12 00:00:33,840 --> 00:00:36,790 So I don't even mean the entire line of code. 13 00:00:36,790 --> 00:00:39,890 What I mean is really just this three plus four. 14 00:00:39,890 --> 00:00:42,440 This is an expression, okay? 15 00:00:42,440 --> 00:00:46,060 And again, because it does produce a value. 16 00:00:46,060 --> 00:00:50,120 The same goes, for example for just any number. 17 00:00:50,120 --> 00:00:52,310 This for example is an expression too, 18 00:00:52,310 --> 00:00:54,720 even though it's just a simple value. 19 00:00:54,720 --> 00:00:57,300 But it's still also an expression, 20 00:00:57,300 --> 00:00:59,230 because again, this itself 21 00:00:59,230 --> 00:01:01,673 will produce a value in JavaScript. 22 00:01:02,670 --> 00:01:04,110 Or we can do this. 23 00:01:04,110 --> 00:01:08,503 So true and false and, 24 00:01:10,080 --> 00:01:11,770 not false. 25 00:01:11,770 --> 00:01:12,820 So all of this here, 26 00:01:12,820 --> 00:01:14,960 is an expression, right? 27 00:01:14,960 --> 00:01:18,303 Because this will in the end, produce some Boolean value. 28 00:01:19,190 --> 00:01:21,970 Now, on the other hand, we have statements. 29 00:01:21,970 --> 00:01:24,890 And the statement is like a bigger piece of code 30 00:01:24,890 --> 00:01:26,360 that is executed 31 00:01:26,360 --> 00:01:29,840 and which does not produce a value on itself. 32 00:01:29,840 --> 00:01:33,430 And we can compare this with normal spoken language. 33 00:01:33,430 --> 00:01:34,730 And so in this example, 34 00:01:34,730 --> 00:01:37,880 a declaration is like a complete sentence 35 00:01:37,880 --> 00:01:40,340 and expressions are like the words 36 00:01:40,340 --> 00:01:42,680 that make up the sentences. 37 00:01:42,680 --> 00:01:45,430 Now, basically we write our whole programs 38 00:01:45,430 --> 00:01:47,570 as a sequence of actions. 39 00:01:47,570 --> 00:01:50,470 And these actions are statements. 40 00:01:50,470 --> 00:01:53,773 So let's take as an example, the, if else, statement. 41 00:01:54,620 --> 00:01:55,800 So let's say, 42 00:01:55,800 --> 00:01:59,963 if 23 is greater than 10, 43 00:02:01,430 --> 00:02:03,810 then, well just do something. 44 00:02:03,810 --> 00:02:08,787 So create a string called, 23 is bigger. 45 00:02:11,500 --> 00:02:14,230 The code itself doesn't really matter here. 46 00:02:14,230 --> 00:02:18,200 So this, if else statement is in fact a statement. 47 00:02:18,200 --> 00:02:20,350 And so that's the reason why I have called it 48 00:02:20,350 --> 00:02:23,040 the if else statement all the time. 49 00:02:23,040 --> 00:02:26,130 And the same is true for the switch statement. 50 00:02:26,130 --> 00:02:27,510 So this statement here 51 00:02:27,510 --> 00:02:30,430 doesn't really produce a value, does it? 52 00:02:30,430 --> 00:02:32,640 All it does is, in this case, 53 00:02:32,640 --> 00:02:36,810 it simply declares is variable called STR. 54 00:02:36,810 --> 00:02:37,830 But that's it. 55 00:02:37,830 --> 00:02:39,680 It performs some actions, 56 00:02:39,680 --> 00:02:41,650 in this case, declaring this variable, 57 00:02:41,650 --> 00:02:43,540 but it doesn't produce a value. 58 00:02:43,540 --> 00:02:45,130 And so in that, it's different from, 59 00:02:45,130 --> 00:02:48,903 for example, having this here, okay? 60 00:02:49,890 --> 00:02:53,750 Now the string itself, again is an expression. 61 00:02:53,750 --> 00:02:55,780 So this is probably a bit confusing, 62 00:02:55,780 --> 00:02:57,940 but that's not a problem, okay? 63 00:02:57,940 --> 00:03:00,420 My goal here is not that you understand 64 00:03:00,420 --> 00:03:03,300 the exact rules of what an expression is 65 00:03:03,300 --> 00:03:04,750 and what a statement is, 66 00:03:04,750 --> 00:03:08,430 because, in fact, it's all a bit fuzzy. 67 00:03:08,430 --> 00:03:11,130 So my main goal with this lecture is really 68 00:03:11,130 --> 00:03:14,400 that you know that statements and expressions exist, 69 00:03:14,400 --> 00:03:16,630 and that expressions produce values, 70 00:03:16,630 --> 00:03:19,610 and that statements are like full sentences 71 00:03:19,610 --> 00:03:21,770 that translate our actions. 72 00:03:21,770 --> 00:03:25,480 So the actions that we want the program to perform, okay? 73 00:03:25,480 --> 00:03:27,370 So really that's all you need to know, 74 00:03:27,370 --> 00:03:31,080 and all you need to basically memorize from this video. 75 00:03:31,080 --> 00:03:33,210 We don't need to go into all the details. 76 00:03:33,210 --> 00:03:36,760 So there's no need to start asking a bunch of questions 77 00:03:36,760 --> 00:03:40,080 about this in the Q and A section, all right? 78 00:03:40,080 --> 00:03:43,930 So this will all make even more sense as we move on. 79 00:03:43,930 --> 00:03:48,570 So I said that this string here is itself an expression. 80 00:03:48,570 --> 00:03:52,240 And then this whole line of code is a statement, 81 00:03:52,240 --> 00:03:55,000 because this again does not produce a value. 82 00:03:55,000 --> 00:03:58,960 And so basically, whenever something ends with a semicolon, 83 00:03:58,960 --> 00:04:00,970 that's then a statement. 84 00:04:00,970 --> 00:04:03,403 It's like a complete sentence, okay? 85 00:04:04,270 --> 00:04:07,280 Now this difference between expressions and statements 86 00:04:07,280 --> 00:04:08,880 is important to know 87 00:04:08,880 --> 00:04:10,810 because JavaScript expects 88 00:04:10,810 --> 00:04:14,350 statements and expressions in different places. 89 00:04:14,350 --> 00:04:16,680 For example, in a template literal, 90 00:04:16,680 --> 00:04:20,500 we can only insert expressions, but not statements. 91 00:04:20,500 --> 00:04:22,230 So let me just very quickly 92 00:04:23,490 --> 00:04:24,863 demonstrate that to you. 93 00:04:26,180 --> 00:04:28,770 So we can say, I am, 94 00:04:28,770 --> 00:04:31,330 and then our placeholder here, basically. 95 00:04:31,330 --> 00:04:34,260 And so in here, we need to put an expression. 96 00:04:34,260 --> 00:04:36,440 So something that produces a value 97 00:04:36,440 --> 00:04:38,910 which can then be put into the string 98 00:04:38,910 --> 00:04:41,800 that we're building here using the template literal. 99 00:04:41,800 --> 00:04:42,663 So let's say, 100 00:04:43,790 --> 00:04:45,970 just to do some calculations here. 101 00:04:45,970 --> 00:04:49,043 So 2037 minus 1991, 102 00:04:50,020 --> 00:04:51,990 years old. 103 00:04:51,990 --> 00:04:55,023 So again, this is an expression here. 104 00:04:56,000 --> 00:04:57,563 And so this one will work. 105 00:04:58,440 --> 00:05:00,823 However, we cannot do this. 106 00:05:01,860 --> 00:05:03,710 For example, we could not insert 107 00:05:04,870 --> 00:05:07,340 this if statement in here. 108 00:05:07,340 --> 00:05:10,963 It wouldn't make any sense at all, okay? 109 00:05:12,140 --> 00:05:13,380 Let's try that. 110 00:05:13,380 --> 00:05:15,270 And so we actually get right away, 111 00:05:15,270 --> 00:05:17,880 unexpected token, if. 112 00:05:17,880 --> 00:05:21,960 And that's because JavaScript knows that this is a statement 113 00:05:21,960 --> 00:05:24,320 and statements don't make sense 114 00:05:24,320 --> 00:05:27,103 where JavaScript expects an expression. 115 00:05:29,600 --> 00:05:31,773 Now, if we had a variable, for example, 116 00:05:33,000 --> 00:05:35,350 then if we used that here, 117 00:05:35,350 --> 00:05:38,710 then this would actually also be an expression. 118 00:05:38,710 --> 00:05:39,890 Because this variable 119 00:05:39,890 --> 00:05:43,710 will essentially just be replaced with this string. 120 00:05:43,710 --> 00:05:45,430 And so this produces a value, 121 00:05:45,430 --> 00:05:48,883 and so of course this is completely acceptable. 122 00:05:50,820 --> 00:05:53,760 Okay, and that's actually all I had to show you 123 00:05:53,760 --> 00:05:55,410 in this video. 124 00:05:55,410 --> 00:05:57,990 Again, my goal was really just to make sure 125 00:05:57,990 --> 00:05:59,830 that you kind of understand 126 00:05:59,830 --> 00:06:02,610 the difference between expressions and declarations 127 00:06:02,610 --> 00:06:04,080 so that moving forward, 128 00:06:04,080 --> 00:06:06,423 we are really both on the same page here.