WEBVTT 1 00:00:01.510 --> 00:00:05.500 So we learned about the forEach method on arrays. 2 00:00:05.500 --> 00:00:10.500 However, forEach is also available on maps and sets. 3 00:00:10.960 --> 00:00:13.510 And so let's take a small detour now 4 00:00:13.510 --> 00:00:17.793 and see how forEach works with maps and with sets. 5 00:00:19.450 --> 00:00:21.930 And we're gonna start with maps, 6 00:00:21.930 --> 00:00:25.980 and for that let's cut this currencies map 7 00:00:25.980 --> 00:00:27.333 that we have up here. 8 00:00:30.610 --> 00:00:32.020 All right. 9 00:00:32.020 --> 00:00:36.090 And remember that in this array of arrays, 10 00:00:36.090 --> 00:00:39.870 each of these array elements, so this inner array, 11 00:00:39.870 --> 00:00:42.110 will be one entry of the map, 12 00:00:42.110 --> 00:00:46.330 where this here is the key, and this is the value. 13 00:00:46.330 --> 00:00:47.600 All right. 14 00:00:47.600 --> 00:00:52.600 And so we can call forEach also on a map. 15 00:00:54.460 --> 00:00:59.460 So forEach, and then again, the callback function, 16 00:00:59.640 --> 00:01:03.700 and now this callback function also has three parameters. 17 00:01:03.700 --> 00:01:05.900 So when the forEach method calls it, 18 00:01:05.900 --> 00:01:09.220 it will call this function with three arguments. 19 00:01:09.220 --> 00:01:12.193 So the first one will be the current value, 20 00:01:13.220 --> 00:01:16.010 so the current value in the current iteration, 21 00:01:16.010 --> 00:01:19.500 the second one is the key, and the third one 22 00:01:19.500 --> 00:01:23.820 is the entire map that is being looped over. 23 00:01:23.820 --> 00:01:26.600 And so you see, this is similar to the array, 24 00:01:26.600 --> 00:01:28.530 where in the array, 25 00:01:28.530 --> 00:01:31.990 the first parameter is the current element of the array, 26 00:01:31.990 --> 00:01:33.720 the second one is the index 27 00:01:33.720 --> 00:01:36.350 and the third is the entire array. 28 00:01:36.350 --> 00:01:40.040 And so there is a nice correspondence between these, 29 00:01:40.040 --> 00:01:42.603 and so this is quite easy to memorize here. 30 00:01:43.540 --> 00:01:47.040 And so let's just log a very quick string here. 31 00:01:49.720 --> 00:01:51.780 So this is not really that important, 32 00:01:51.780 --> 00:01:55.810 but I wanted to show you that forEach also exists 33 00:01:55.810 --> 00:01:58.003 on these two new data structures. 34 00:01:59.200 --> 00:02:02.420 Now, okay, and indeed it worked. 35 00:02:02.420 --> 00:02:05.640 So we got the key of each of the map entries 36 00:02:05.640 --> 00:02:08.570 and then also the value. 37 00:02:08.570 --> 00:02:12.703 Alright, and so now let's try the same with a set. 38 00:02:14.070 --> 00:02:18.933 So this is with a map and this is now with a set. 39 00:02:19.780 --> 00:02:23.183 So let's create currenciesUnique. 40 00:02:25.550 --> 00:02:29.500 Let's say, so new set. 41 00:02:29.500 --> 00:02:33.150 And remember that in here, we need to pass an Iterable. 42 00:02:33.150 --> 00:02:36.310 And so I will use an array here. 43 00:02:36.310 --> 00:02:40.283 So let's just create a couple of strings here, 44 00:02:42.030 --> 00:02:46.173 another US dollar, and then some Euros. 45 00:02:49.330 --> 00:02:50.523 Let's just take a look, 46 00:02:52.920 --> 00:02:56.033 and indeed it only has the unique values. 47 00:02:56.900 --> 00:02:58.820 So just as expected, 48 00:02:58.820 --> 00:03:02.903 and so let's now call forEach on this set as well. 49 00:03:04.270 --> 00:03:05.870 So the currenciesUnique.forEach. 50 00:03:08.960 --> 00:03:12.120 And now once again, the callback function 51 00:03:12.120 --> 00:03:16.600 has the parameters of value, key and map, 52 00:03:18.570 --> 00:03:22.283 and we will see in a second, if that actually makes sense. 53 00:03:23.510 --> 00:03:25.183 So let's look to the console. 54 00:03:26.430 --> 00:03:28.030 Actually the same thing as here. 55 00:03:30.960 --> 00:03:33.310 So let's see what's gonna happen. 56 00:03:33.310 --> 00:03:38.013 So remember we are logging the key and the value, okay? 57 00:03:38.960 --> 00:03:43.960 But now we get USD, USD, and here GBP, GBP. 58 00:03:45.670 --> 00:03:48.620 And so what this means is that the key here 59 00:03:48.620 --> 00:03:52.060 is exactly the same as the value. 60 00:03:52.060 --> 00:03:54.240 So why is that? 61 00:03:54.240 --> 00:03:57.880 Well, a set doesn't have keys, right? 62 00:03:57.880 --> 00:04:00.040 And it doesn't have indexes either. 63 00:04:00.040 --> 00:04:04.410 And so there is no value that would make sense for the key. 64 00:04:04.410 --> 00:04:07.460 All right, so essentially this key here 65 00:04:07.460 --> 00:04:09.050 makes no sense at all. 66 00:04:09.050 --> 00:04:11.450 It wouldn't even have to be there. 67 00:04:11.450 --> 00:04:16.450 And so the people who designed this forEach method for sets, 68 00:04:16.520 --> 00:04:20.760 they could have simply omitted the second argument, right? 69 00:04:20.760 --> 00:04:22.860 Well, if they did that, 70 00:04:22.860 --> 00:04:26.600 then this forEach would have been different from the others. 71 00:04:26.600 --> 00:04:30.400 And so that would then create confusion in developers, 72 00:04:30.400 --> 00:04:33.980 and therefore it was decided to keep the same signature. 73 00:04:33.980 --> 00:04:38.080 So basically to keep the same three parameters 74 00:04:38.080 --> 00:04:40.330 in this callback function and simply 75 00:04:40.330 --> 00:04:44.210 to set the second one to the first one. 76 00:04:44.210 --> 00:04:47.523 So we can just write value here as well, 77 00:04:49.470 --> 00:04:51.433 just to avoid that confusion. 78 00:04:53.030 --> 00:04:53.863 All right. 79 00:04:55.250 --> 00:04:56.550 Now, and of course, 80 00:04:56.550 --> 00:05:00.010 we cannot have the duplicate parameter name. 81 00:05:00.010 --> 00:05:02.460 And so we can just use an underscore, 82 00:05:02.460 --> 00:05:05.750 which in JavaScript means a throwaway variable. 83 00:05:05.750 --> 00:05:09.550 So that means a variable that is completely unnecessary. 84 00:05:09.550 --> 00:05:12.460 So it's just a convention which we will see again 85 00:05:12.460 --> 00:05:13.573 a little bit later. 86 00:05:15.610 --> 00:05:17.200 Okay, and so now it should work 87 00:05:18.480 --> 00:05:20.640 and yeah, that's all you need to know 88 00:05:20.640 --> 00:05:24.360 about the forEach method for maps and sets. 89 00:05:24.360 --> 00:05:27.810 So it's very straightforward if you already understood 90 00:05:27.810 --> 00:05:29.493 how it works for arrays.