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.