1 00:00:01,490 --> 00:00:03,130 So, we just reviewed 2 00:00:03,130 --> 00:00:05,670 and also implemented some clean 3 00:00:05,670 --> 00:00:08,590 and modern JavaScript practices. 4 00:00:08,590 --> 00:00:12,030 However, there is currently a major trend 5 00:00:12,030 --> 00:00:15,610 and shift to something called declarative code 6 00:00:15,610 --> 00:00:18,670 and functional programming in JavaScript. 7 00:00:18,670 --> 00:00:22,780 And so, let's now take some time to look at what declarative 8 00:00:22,780 --> 00:00:25,393 and functional programming actually are. 9 00:00:26,530 --> 00:00:29,710 So, there are two fundamentally different ways 10 00:00:29,710 --> 00:00:32,090 of writing code in programming, 11 00:00:32,090 --> 00:00:34,540 which we also call paradigms. 12 00:00:34,540 --> 00:00:36,120 And these two paradigms, 13 00:00:36,120 --> 00:00:39,323 are imperative code and declarative code. 14 00:00:40,240 --> 00:00:43,210 Now, whenever we write imperative code, 15 00:00:43,210 --> 00:00:45,560 we basically need to explain to the computer 16 00:00:45,560 --> 00:00:48,660 how to do a certain things. 17 00:00:48,660 --> 00:00:52,570 So, basically, we need to explain every single step 18 00:00:52,570 --> 00:00:54,840 that the computer needs to follow 19 00:00:54,840 --> 00:00:57,770 in order to achieve a certain result. 20 00:00:57,770 --> 00:01:00,770 But, this might sound a little bit abstract, 21 00:01:00,770 --> 00:01:03,740 so let's try a more real world example. 22 00:01:03,740 --> 00:01:08,460 So, let's say that we want someone to bake a cake for us. 23 00:01:08,460 --> 00:01:11,880 And so, if we would do that in an imperative way, 24 00:01:11,880 --> 00:01:16,150 we would tell the person exactly the step by step recipe 25 00:01:16,150 --> 00:01:20,350 that they would have to follow in order to bake that cake, 26 00:01:20,350 --> 00:01:21,630 am I right? 27 00:01:21,630 --> 00:01:24,690 So again, it is telling every single step 28 00:01:24,690 --> 00:01:29,150 that the person has to follow in order to achieve a result. 29 00:01:29,150 --> 00:01:31,940 And now bringing that back into code, 30 00:01:31,940 --> 00:01:33,750 here in this code example, 31 00:01:33,750 --> 00:01:37,000 we are trying to double the R array. 32 00:01:37,000 --> 00:01:39,480 And so, this loop that I have here, 33 00:01:39,480 --> 00:01:43,420 is a purely imperative way of writing that. 34 00:01:43,420 --> 00:01:46,500 So, here we are telling the computer step by step, 35 00:01:46,500 --> 00:01:50,700 to create an empty array to create a counter 36 00:01:50,700 --> 00:01:54,220 that starts at zero, then to increase that counter 37 00:01:54,220 --> 00:01:57,780 until we reach the length of the original array, 38 00:01:57,780 --> 00:02:01,850 and then how exactly to store the new result 39 00:02:01,850 --> 00:02:04,230 in each new position of the array. 40 00:02:04,230 --> 00:02:05,860 So, there's a lot of steps 41 00:02:05,860 --> 00:02:08,190 that we really give the computer here, 42 00:02:08,190 --> 00:02:10,790 in order for us to achieve the result 43 00:02:10,790 --> 00:02:13,003 of doubling that R array. 44 00:02:13,960 --> 00:02:17,100 Okay, so that's imperative programming, 45 00:02:17,100 --> 00:02:20,670 but on the other hand, we also have declarative programming, 46 00:02:20,670 --> 00:02:25,190 where the programmer tells the computer only what to do. 47 00:02:25,190 --> 00:02:28,110 And so, when we write declarative code, 48 00:02:28,110 --> 00:02:31,930 we simply describe the way that the computer should achieve 49 00:02:31,930 --> 00:02:33,570 a certain result. 50 00:02:33,570 --> 00:02:35,590 But the how it should do it, 51 00:02:35,590 --> 00:02:38,460 so basically, the step by step instructions, 52 00:02:38,460 --> 00:02:43,030 they get abstracted away, so we do not care about them. 53 00:02:43,030 --> 00:02:45,807 And going back to our cake example here, 54 00:02:45,807 --> 00:02:50,070 the declarative way of instructing someone to bake the cake 55 00:02:50,070 --> 00:02:54,230 would be to simply describe that cake to the person, 56 00:02:54,230 --> 00:02:56,400 and then the person would have to come up 57 00:02:56,400 --> 00:02:59,900 with the step by step recipe on their own. 58 00:02:59,900 --> 00:03:02,250 So, simply describing the task, 59 00:03:02,250 --> 00:03:04,640 and the result that should be achieved 60 00:03:04,640 --> 00:03:09,010 is the declarative way of doing it, all right? 61 00:03:09,010 --> 00:03:12,270 And now coming back to the code example of duplicating 62 00:03:12,270 --> 00:03:14,060 the values in an array, 63 00:03:14,060 --> 00:03:17,510 this is how we do it in the declarative way. 64 00:03:17,510 --> 00:03:21,410 So, we have R array, and then we simply tell JavaScript, 65 00:03:21,410 --> 00:03:26,130 that it should map the values in the R array to a new array, 66 00:03:26,130 --> 00:03:30,270 and each of these values should be multiplied by two. 67 00:03:30,270 --> 00:03:32,710 And so, if you compare this code example, 68 00:03:32,710 --> 00:03:34,360 with the one on the left, 69 00:03:34,360 --> 00:03:37,230 then you will really see that in this example, 70 00:03:37,230 --> 00:03:40,030 all we are doing is describing the way 71 00:03:40,030 --> 00:03:42,620 that the computer should achieve the result 72 00:03:42,620 --> 00:03:44,210 that we are looking for. 73 00:03:44,210 --> 00:03:47,780 We are simply telling it what to do, which in this case, 74 00:03:47,780 --> 00:03:51,680 is to simply map the original array onto a new array 75 00:03:51,680 --> 00:03:54,120 and doubling all the elements. 76 00:03:54,120 --> 00:03:56,470 But, all these super detailed steps 77 00:03:56,470 --> 00:04:00,440 that we have on the left side, like creating an empty array 78 00:04:00,440 --> 00:04:02,520 and initializing a counter, 79 00:04:02,520 --> 00:04:05,730 all of these steps have been abstracted away, 80 00:04:05,730 --> 00:04:09,550 because we don't really care about them, all right? 81 00:04:09,550 --> 00:04:12,140 And this is pretty important to understand, 82 00:04:12,140 --> 00:04:15,980 because more and more this is how modern JavaScript code 83 00:04:15,980 --> 00:04:17,890 is actually written. 84 00:04:17,890 --> 00:04:21,700 So, the difference between imperative and declarative 85 00:04:21,700 --> 00:04:24,530 is not just some theoretical difference. 86 00:04:24,530 --> 00:04:28,490 So, the declarative paradigm is actually a really big 87 00:04:28,490 --> 00:04:31,040 and popular programming paradigm, 88 00:04:31,040 --> 00:04:35,020 which has even given rise to a sub paradigm called, 89 00:04:35,020 --> 00:04:36,383 functional programming. 90 00:04:37,620 --> 00:04:39,320 And functional programming, 91 00:04:39,320 --> 00:04:42,120 is basically a declarative paradigm, 92 00:04:42,120 --> 00:04:45,780 which is based on the idea of writing software, 93 00:04:45,780 --> 00:04:50,200 simply by combining multiple so called pure functions, 94 00:04:50,200 --> 00:04:54,230 while avoiding side effects and mutating data. 95 00:04:54,230 --> 00:04:56,810 And actually, functional programming 96 00:04:56,810 --> 00:04:58,970 and writing declarative code, 97 00:04:58,970 --> 00:05:03,280 has now basically become the modern way of writing code 98 00:05:03,280 --> 00:05:05,440 in the JavaScript world. 99 00:05:05,440 --> 00:05:09,831 So, you will see declarative and functional code everywhere. 100 00:05:09,831 --> 00:05:14,070 And, in fact, we have even been using it all along, 101 00:05:14,070 --> 00:05:16,730 but without really knowing that this style 102 00:05:16,730 --> 00:05:21,050 was called declarative, and functional, all right. 103 00:05:21,050 --> 00:05:23,730 But let's quickly go back to the definition 104 00:05:23,730 --> 00:05:27,800 of functional programming, and talk about what side effects 105 00:05:27,800 --> 00:05:30,130 and pure functions are. 106 00:05:30,130 --> 00:05:35,130 So, a side effect is basically simply a modification 107 00:05:35,130 --> 00:05:39,350 of any data that's outside of a function. 108 00:05:39,350 --> 00:05:43,680 So, for example, mutating any variable that is external 109 00:05:43,680 --> 00:05:47,370 to the function is causing a side effect. 110 00:05:47,370 --> 00:05:51,070 So basically, any variable that is outside of the scope 111 00:05:51,070 --> 00:05:54,100 of the function, all right? 112 00:05:54,100 --> 00:05:57,526 Now, data does not only refer to variables, 113 00:05:57,526 --> 00:06:01,840 so for example, logging stuff to the console, 114 00:06:01,840 --> 00:06:04,280 or also changing something in the DOM, 115 00:06:04,280 --> 00:06:07,370 is also causing side effects. 116 00:06:07,370 --> 00:06:09,660 Now next up, a pure function, 117 00:06:09,660 --> 00:06:12,800 is a function without side effects. 118 00:06:12,800 --> 00:06:16,070 So, basically a function that does not mutate 119 00:06:16,070 --> 00:06:20,210 any external variables, and that does also not depend 120 00:06:20,210 --> 00:06:22,350 on any external variables. 121 00:06:22,350 --> 00:06:26,700 So basically, if we give the same inputs to a pure function, 122 00:06:26,700 --> 00:06:29,890 it will always return the same output 123 00:06:29,890 --> 00:06:32,870 and again, that's because it does not depend 124 00:06:32,870 --> 00:06:34,880 on any external variables, 125 00:06:34,880 --> 00:06:36,963 and it also does not manipulate them. 126 00:06:37,840 --> 00:06:41,760 And finally, if we look again, at our definition here, 127 00:06:41,760 --> 00:06:44,260 we also see that functional programming 128 00:06:44,260 --> 00:06:47,490 is about avoiding mutating data, 129 00:06:47,490 --> 00:06:51,810 and we do that by using something called immutability. 130 00:06:51,810 --> 00:06:54,920 So, in functional programming state, 131 00:06:54,920 --> 00:06:59,890 which also means basically data is never modified. 132 00:06:59,890 --> 00:07:02,410 So, let's say that we have some application, 133 00:07:02,410 --> 00:07:06,250 and we have an object there to keep track of all the data 134 00:07:06,250 --> 00:07:08,580 that we need in an application. 135 00:07:08,580 --> 00:07:11,130 And so that we usually called state, 136 00:07:11,130 --> 00:07:14,220 and so again, in functional programming, 137 00:07:14,220 --> 00:07:17,040 that state is never modified. 138 00:07:17,040 --> 00:07:21,550 Instead, what we will do is to copy that object, 139 00:07:21,550 --> 00:07:26,160 so that state, and then it is that copy that is mutated, 140 00:07:26,160 --> 00:07:28,210 and can then be returned, 141 00:07:28,210 --> 00:07:32,840 but the original state is never touched, okay? 142 00:07:32,840 --> 00:07:37,020 So, that's what it means for the state being immutable, 143 00:07:37,020 --> 00:07:40,100 and the big upside of immutability is that, 144 00:07:40,100 --> 00:07:43,200 it makes it so much easier to keep track 145 00:07:43,200 --> 00:07:46,971 of how the data flows through our entire application. 146 00:07:46,971 --> 00:07:51,370 And so ultimately, that will allow us to write better code 147 00:07:51,370 --> 00:07:56,070 with less bugs, and code that is also more readable, 148 00:07:56,070 --> 00:07:58,890 which overall, is the entire goal 149 00:07:58,890 --> 00:08:02,510 of using functional programming in the first place. 150 00:08:02,510 --> 00:08:04,420 Now, I'm telling you all this, 151 00:08:04,420 --> 00:08:06,240 not with the goal of turning you 152 00:08:06,240 --> 00:08:08,400 into a functional programmer, 153 00:08:08,400 --> 00:08:12,160 because that would actually be a very hard task, 154 00:08:12,160 --> 00:08:16,320 because this is really just a very high level introduction 155 00:08:16,320 --> 00:08:19,100 to what functional programming actually is. 156 00:08:19,100 --> 00:08:20,520 But behind the surface, 157 00:08:20,520 --> 00:08:23,400 functional programming is a huge paradigm, 158 00:08:23,400 --> 00:08:27,490 which is really difficult to implement in practice. 159 00:08:27,490 --> 00:08:30,360 But it is still very important that you know, 160 00:08:30,360 --> 00:08:33,700 some of these principles, such as side effects, 161 00:08:33,700 --> 00:08:36,370 pure functions, and immutability, 162 00:08:36,370 --> 00:08:38,610 because many of the popular libraries, 163 00:08:38,610 --> 00:08:42,000 such as React or Redux, are actually built 164 00:08:42,000 --> 00:08:44,510 around all of these principles. 165 00:08:44,510 --> 00:08:46,080 So for example, in React, 166 00:08:46,080 --> 00:08:49,190 the state is also completely immutable, 167 00:08:49,190 --> 00:08:52,370 and so if you ever want to learn something like React, 168 00:08:52,370 --> 00:08:54,830 you will need to know about these concepts 169 00:08:54,830 --> 00:08:57,460 in order to use it properly. 170 00:08:57,460 --> 00:09:01,150 However, some principles such as pure functions, 171 00:09:01,150 --> 00:09:05,570 or side effects, can actually be a bit easier to implement 172 00:09:05,570 --> 00:09:07,460 into our own code. 173 00:09:07,460 --> 00:09:09,890 So, what I'm trying to say is that, 174 00:09:09,890 --> 00:09:14,330 we can actually mix imperative and declarative programming 175 00:09:14,330 --> 00:09:19,330 in our own codes, we don't have to go 100% declarative. 176 00:09:19,570 --> 00:09:24,090 Or in other words, we don't have to go 100% in the direction 177 00:09:24,090 --> 00:09:27,260 of making our code completely functional. 178 00:09:27,260 --> 00:09:30,620 And so again, we can already start using, 179 00:09:30,620 --> 00:09:32,840 some of the functional programming techniques 180 00:09:32,840 --> 00:09:35,050 in our own code base. 181 00:09:35,050 --> 00:09:38,820 So for example, you can try to avoid data mutations 182 00:09:38,820 --> 00:09:40,840 as often as possible. 183 00:09:40,840 --> 00:09:43,900 And of course, this will not always be possible, 184 00:09:43,900 --> 00:09:46,930 but it's also not really necessary. 185 00:09:46,930 --> 00:09:50,150 So these are mainly and are just suggestions, 186 00:09:50,150 --> 00:09:53,540 but which will still create more readable 187 00:09:53,540 --> 00:09:57,120 and overall better and cleaner code. 188 00:09:57,120 --> 00:10:00,570 So, another thing that you can do is to always prefer, 189 00:10:00,570 --> 00:10:04,260 built in methods or functions that do not produce 190 00:10:04,260 --> 00:10:07,510 side effects over the ones that do, 191 00:10:07,510 --> 00:10:11,450 and this is really important for data transformations. 192 00:10:11,450 --> 00:10:13,210 So whenever you want to do that, 193 00:10:13,210 --> 00:10:18,210 you should use a method such as Map, Filter and Reduce. 194 00:10:18,230 --> 00:10:20,970 So, this is the functional and modern way 195 00:10:20,970 --> 00:10:24,380 of doing data transformations, and many times, 196 00:10:24,380 --> 00:10:27,620 this is actually the first contact that many people have, 197 00:10:27,620 --> 00:10:29,570 with functional programming. 198 00:10:29,570 --> 00:10:32,860 So, Map, Filter and Reduce are actually present 199 00:10:32,860 --> 00:10:35,550 in all functional programming languages, 200 00:10:35,550 --> 00:10:39,460 and they are very important to implement a functional code 201 00:10:39,460 --> 00:10:42,173 into more declarative code in our code. 202 00:10:43,100 --> 00:10:46,730 And finally, you can also try to avoid side effects 203 00:10:46,730 --> 00:10:49,610 into functions that you write yourself. 204 00:10:49,610 --> 00:10:53,320 And again, this is of course, not always possible, 205 00:10:53,320 --> 00:10:55,970 and also not always necessary. 206 00:10:55,970 --> 00:10:59,390 So, we will never be able to avoid all side effects 207 00:10:59,390 --> 00:11:03,040 in applications, because of course, at some point, 208 00:11:03,040 --> 00:11:05,590 the application needs to do something. 209 00:11:05,590 --> 00:11:08,120 So, it needs to display something on the DOM, 210 00:11:08,120 --> 00:11:10,380 or log something to the console, 211 00:11:10,380 --> 00:11:14,470 or really create some side effect, okay? 212 00:11:14,470 --> 00:11:17,060 But you can still try to think about this, 213 00:11:17,060 --> 00:11:19,940 and to start incorporating side effects 214 00:11:19,940 --> 00:11:21,343 more into your own code. 215 00:11:22,310 --> 00:11:26,420 And now to finish, let's come back to declarative syntax, 216 00:11:26,420 --> 00:11:30,390 because functional programming is only a part of using 217 00:11:30,390 --> 00:11:32,750 and writing declarative code. 218 00:11:32,750 --> 00:11:36,160 So, in order to write code that is more declarative, 219 00:11:36,160 --> 00:11:39,320 you should use array and object destructuring 220 00:11:39,320 --> 00:11:41,310 whenever that's possible. 221 00:11:41,310 --> 00:11:43,740 You should also use the spread operator, 222 00:11:43,740 --> 00:11:47,570 the ternary operator, and also template literals 223 00:11:47,570 --> 00:11:51,300 whenever that is possible, because if you think about it, 224 00:11:51,300 --> 00:11:54,700 then all of these four ways of writing code, 225 00:11:54,700 --> 00:11:57,930 actually makes the code more declarative. 226 00:11:57,930 --> 00:12:01,330 So, these operators are more about telling the code 227 00:12:01,330 --> 00:12:04,270 what to do, and not exactly the steps 228 00:12:04,270 --> 00:12:06,530 that it should take, right? 229 00:12:06,530 --> 00:12:11,530 And that's, again, true for all these four pieces of syntax. 230 00:12:11,880 --> 00:12:15,410 All right, so let's now actually continue working 231 00:12:15,410 --> 00:12:19,060 on the code example from last lecture and implement, 232 00:12:19,060 --> 00:12:23,183 some of these functional programming principles in practice.