1 00:00:00,520 --> 00:00:01,880 Hello again, everyone. 2 00:00:02,500 --> 00:00:09,370 My friends, we have now completed the topic of data types, so in this lesson I want to talk about 3 00:00:09,370 --> 00:00:10,380 functions. 4 00:00:10,840 --> 00:00:15,430 So for this, I will create a new file and continue with it. 5 00:00:16,000 --> 00:00:21,910 And I want different topics to be as tidy as can be. 6 00:00:22,990 --> 00:00:29,950 As you will remember, we were typing functions using the function keyword in JavaScript. 7 00:00:31,700 --> 00:00:34,730 And I'd say this function adds three different numbers and. 8 00:00:44,410 --> 00:00:47,530 Returns this total result to us. 9 00:00:50,100 --> 00:00:52,620 So that's how you write a simple function, right? 10 00:00:53,760 --> 00:01:00,030 In fact, when we look here, we're going to have trouble with the parameters that come in here, have 11 00:01:00,720 --> 00:01:02,100 another type, the number. 12 00:01:02,100 --> 00:01:02,370 Right. 13 00:01:03,060 --> 00:01:09,330 So let's say that we give string values for the first variable here and number of values for the others. 14 00:01:10,060 --> 00:01:14,820 Well, this is going to be a big problem for us down the road while we're working. 15 00:01:15,270 --> 00:01:19,410 So again, here, typescript, offers a great solution. 16 00:01:21,030 --> 00:01:26,370 So likewise, I'll define the second function, of course, with a different function name. 17 00:01:27,460 --> 00:01:32,760 And in the same way will create a function that adds three different numbers and returns a result, 18 00:01:33,490 --> 00:01:38,290 but in order to avoid the previous problem, I can specify that the. 19 00:01:41,730 --> 00:01:44,040 Next parameters will be numbers. 20 00:01:48,370 --> 00:01:55,070 So I specify that the variables will be numbers and the result will be number just like that. 21 00:01:55,750 --> 00:02:00,010 So when I write a different type for the variable to come in here. 22 00:02:02,110 --> 00:02:04,060 The program warns me. 23 00:02:05,030 --> 00:02:06,410 You can use no here. 24 00:02:09,250 --> 00:02:11,500 As if it's not as string or boolean value. 25 00:02:13,330 --> 00:02:19,030 For example, as you can see, when we write the first function here, the variable types appear as 26 00:02:19,030 --> 00:02:19,360 any. 27 00:02:22,730 --> 00:02:27,230 However, it is obvious that these values will be numbers for the second function. 28 00:02:28,240 --> 00:02:32,800 And as you can see, everything is just a little clearer and TypeScript. 29 00:02:34,160 --> 00:02:41,150 And actually, that's what I was trying to tell you in the first lesson, so we can see the error in 30 00:02:41,150 --> 00:02:44,620 our code while writing, not while working. 31 00:02:45,530 --> 00:02:49,460 So it's possible to fix errors quickly on the fly. 32 00:02:51,930 --> 00:02:58,200 So, yes, my friends, we can define these functions like this, so for another way, we can define 33 00:02:58,200 --> 00:03:00,330 them by using the let key word. 34 00:03:01,620 --> 00:03:08,040 So we assign this function to a variable, we have already seen this article before. 35 00:03:12,440 --> 00:03:15,920 And here again, in the same way will add three numbers and. 36 00:03:22,590 --> 00:03:24,990 Return the sum as a result. 37 00:03:28,260 --> 00:03:35,460 So now when defining this function, I stated that the result would be a number, so if I delete the 38 00:03:35,460 --> 00:03:36,420 return here. 39 00:03:37,950 --> 00:03:39,840 TypeScript will return an error. 40 00:03:41,200 --> 00:03:47,610 Because I stated that when I was defining the function, a value would be returned. 41 00:03:50,150 --> 00:03:55,730 All right, so I want to show you one more thing that can be done with functions. 42 00:03:57,180 --> 00:04:03,780 So while we're doing these function, definitions always define three numbers and three values as parameters, 43 00:04:04,590 --> 00:04:07,380 and I want this last number to be optional. 44 00:04:08,350 --> 00:04:14,170 So I'll just send two parameters and normally there's no problems with JavaScript, but. 45 00:04:18,370 --> 00:04:23,770 When you look at how I'm writing like this, of course, it'll give an error with TypeScript. 46 00:04:29,240 --> 00:04:34,970 But if I come in here and put in a question mark, the error goes away, why is that? 47 00:04:36,110 --> 00:04:43,550 Because I've stated that this variable is optional and sometimes only two parameters need to be entered. 48 00:04:45,390 --> 00:04:48,690 So we can reinforce this with a small example. 49 00:04:50,730 --> 00:04:57,720 So now we have an optional parameter within this function, first, we'll add two numbers in total and 50 00:04:57,720 --> 00:05:05,190 I'll put two to the variable count then if the third number is not defined. 51 00:05:15,420 --> 00:05:16,320 We added. 52 00:05:17,500 --> 00:05:20,020 At total and we increase. 53 00:05:23,330 --> 00:05:24,530 The count by one. 54 00:05:34,450 --> 00:05:36,760 Then we can go and print these values on a console. 55 00:05:45,960 --> 00:05:49,500 So are you ready for another of usage of this function? 56 00:05:51,530 --> 00:05:52,640 So as we're. 57 00:05:53,780 --> 00:05:59,480 Giving the parameters here later, we can also give default values when defining the function. 58 00:06:01,610 --> 00:06:04,760 For example, as you see, I'll define the last number here. 59 00:06:05,850 --> 00:06:08,130 I'm sending the other numbers later. 60 00:06:08,280 --> 00:06:13,200 So in total, these two numbers and this number will be. 61 00:06:14,130 --> 00:06:17,340 And if I come in here and send three numbers again. 62 00:06:18,570 --> 00:06:20,490 This third number will receive. 63 00:06:21,760 --> 00:06:23,110 The last number that we sent. 64 00:06:24,690 --> 00:06:34,320 We don't get any errors both the first time and this time, so here the values we determine in this 65 00:06:34,320 --> 00:06:36,990 way are default values. 66 00:06:40,230 --> 00:06:41,310 Terrific. 67 00:06:41,340 --> 00:06:48,180 So now there's another feature that I want to tell you about that we can use and functions, rest parameters. 68 00:06:49,590 --> 00:06:56,040 I don't know if you're aware of it, but we've talked over a number of parameters so far. 69 00:06:56,760 --> 00:07:02,760 But sometimes we might not know the exact number of parameters while preparing our projects. 70 00:07:03,480 --> 00:07:10,500 For example, the number of customers visiting a site or the number of invitees participating, you 71 00:07:10,500 --> 00:07:15,120 know, in an invitation and event invitation. 72 00:07:17,390 --> 00:07:22,460 You get the idea, but in such cases, we can use rest parameters. 73 00:07:23,420 --> 00:07:27,410 So now let's see how to use rest parameters just by defining a new function. 74 00:07:28,700 --> 00:07:33,680 For example, I want to find the average age of people who log in to any site in this function. 75 00:07:34,010 --> 00:07:40,970 So here, while defining the parameter that will go to the function, I'll just put three dots in parentheses 76 00:07:40,970 --> 00:07:41,320 first. 77 00:07:42,860 --> 00:07:51,680 Now, specify the variable name and what type it is, so I'm using a number array now here it is important 78 00:07:51,680 --> 00:07:54,650 that all the following parameters are of the same type. 79 00:07:55,800 --> 00:08:05,280 So the values will be numbers also, if the first value is certain, then we can define it here so we 80 00:08:05,280 --> 00:08:09,870 can find the rest parameter as the second parameter. 81 00:08:11,630 --> 00:08:19,220 So let's calculate the average operation right now, so for this, I define two variables named total 82 00:08:19,430 --> 00:08:20,270 and count. 83 00:08:21,190 --> 00:08:24,580 I'll just make zero as the default. 84 00:08:30,120 --> 00:08:31,530 Then I'll just create a for loop. 85 00:08:33,050 --> 00:08:39,950 The first value starts and zero and continues until the last element of the number, right, and we'll 86 00:08:39,950 --> 00:08:42,260 add the next variable with the total. 87 00:08:49,000 --> 00:08:51,370 And then the count increases by one. 88 00:08:55,590 --> 00:09:00,990 So as a result, I can be dividing the total by count and get. 89 00:09:08,290 --> 00:09:10,480 All right, so now that defines our function. 90 00:09:11,940 --> 00:09:15,480 And now we can send as many parameters as we want to hear. 91 00:09:18,450 --> 00:09:23,190 So if we send two, three or four parameters to this function, it's not going to have a Brahma. 92 00:09:32,270 --> 00:09:39,590 So do you see that then we can easily perform operations in uncertain situations just by using the rest 93 00:09:39,590 --> 00:09:40,310 parameter? 94 00:09:42,680 --> 00:09:49,220 Now, I want to show you one more thing, so the function definition here is the same as JavaScript 95 00:09:49,850 --> 00:09:51,980 and there's not really a problem with that. 96 00:09:52,100 --> 00:09:59,330 But in order to be more professional, we can use the typescript const keyword. 97 00:10:00,600 --> 00:10:03,930 So we can define the function in this way. 98 00:10:05,740 --> 00:10:07,750 All right, my friend, so that's the end of the lesson. 99 00:10:08,080 --> 00:10:15,850 As you can see, we're making faster expressions, OK, especially now that you're more familiar with 100 00:10:15,850 --> 00:10:18,280 some of these topics because of JavaScript. 101 00:10:20,480 --> 00:10:26,490 So here we examined some of the differences in TypeScript, because the logic is pretty much the same. 102 00:10:26,900 --> 00:10:29,150 We can clarify the parts that are different. 103 00:10:29,540 --> 00:10:29,960 All right. 104 00:10:29,970 --> 00:10:31,580 So I want to see in the next lesson.