WEBVTT 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.