1 00:00:02,420 --> 00:00:04,720 So now that we know what pure functions are, 2 00:00:04,730 --> 00:00:10,910 let's dive into factory functions. Pretty unrelated to pure functions but still an important concept. 3 00:00:10,970 --> 00:00:17,620 The idea behind factory functions is that you have a function that produces another function, 4 00:00:17,630 --> 00:00:22,540 now that might sound strange but it makes more sense once we see an example. 5 00:00:22,700 --> 00:00:29,450 Let's say we're building a web application, some accounting software where as part of that software, we 6 00:00:29,450 --> 00:00:32,860 need to show various tax amounts 7 00:00:33,140 --> 00:00:40,190 and of course as you know, we got different taxes based on which amount we're working with, for our 8 00:00:40,190 --> 00:00:47,420 revenue we might need the value added tax, for our profit before tax we might need the income tax or the 9 00:00:47,420 --> 00:00:52,520 corporate tax. Now the tax calculation typically always is the same. 10 00:00:52,520 --> 00:01:01,070 If we had a function, calculate tax, then we probably would have a function that receives a tax rate and 11 00:01:01,160 --> 00:01:06,710 an amount and then we just return amount times tax, 12 00:01:06,740 --> 00:01:12,980 something like that, that could be a function that calculates a tax, of course it's a trivial function 13 00:01:13,220 --> 00:01:15,090 but you get the idea. 14 00:01:15,140 --> 00:01:24,860 Now the problem is if we have our value added tax amount, we might need to call calculate tax with our 15 00:01:24,860 --> 00:01:26,150 amount, 100 16 00:01:26,420 --> 00:01:35,810 and the tax rate of let's say 90%, so 0.9. For our income tax, we might need to call calculate tax 17 00:01:35,810 --> 00:01:41,060 with an amount of 100 and 0.25 or whatever. 18 00:01:41,060 --> 00:01:46,640 Now of course it's fine to do it like that and because it's a trivial function, it definitely seems doable 19 00:01:46,880 --> 00:01:48,560 to call it like that 20 00:01:48,560 --> 00:01:54,230 but what if you needed that amount in different parts of your software? Then you need to call the functions 21 00:01:54,230 --> 00:01:57,740 with these tax rates in different parts of the software. 22 00:01:57,740 --> 00:02:00,480 Now that's where a factory function can be helpful. 23 00:02:00,560 --> 00:02:08,420 You would create another function which you could call create tax calculator, so I try to make it clear 24 00:02:08,420 --> 00:02:13,710 by the function name that this function will return something which then does something for us 25 00:02:13,850 --> 00:02:17,440 and in this function, we create another function, as you learned 26 00:02:17,480 --> 00:02:23,540 you can have functions in functions, so we can move that calculates tax function into the create tax 27 00:02:23,540 --> 00:02:27,520 calculator function. So now we have that function in there 28 00:02:27,540 --> 00:02:34,680 and now the interesting thing is the create tax calculator function could take our tax parameter and 29 00:02:34,680 --> 00:02:38,100 we can therefore actually remove it from that internal function 30 00:02:38,100 --> 00:02:43,110 because of that scope thing too which I'll come back in the next lectures as well, a function in a 31 00:02:43,110 --> 00:02:47,400 function can access everything which is available outside of that inner function, 32 00:02:47,400 --> 00:02:52,940 so that tax parameter which we have in the outer function can be used in the inner function as well 33 00:02:53,070 --> 00:02:58,740 and therefore now here, we have our tax calculator function as part of that outer function. 34 00:02:58,740 --> 00:03:03,550 Now here in the outer function, we can return calculate tax 35 00:03:03,600 --> 00:03:05,900 and I don't return the result of it, 36 00:03:05,940 --> 00:03:06,500 so I don't 37 00:03:06,510 --> 00:03:07,310 execute it, 38 00:03:07,380 --> 00:03:10,770 instead I just return a pointer at this function. 39 00:03:10,770 --> 00:03:12,930 Now what's the benefit of that? 40 00:03:12,930 --> 00:03:20,010 Well now we return a function which expects an amount as an input but which doesn't need the tax percentage 41 00:03:20,130 --> 00:03:21,420 as an input. 42 00:03:21,420 --> 00:03:31,590 So now what we can do is we can create a vat calculator or name it calculate vat amount and that 43 00:03:31,590 --> 00:03:39,090 simply is a call to create tax calculator where we pass in the vat percentage and then we might have 44 00:03:39,090 --> 00:03:49,660 our income tax amount which we create with create tax calculator, .25 as an argument. 45 00:03:49,660 --> 00:03:56,260 So now what I in the end store here and I should name this calculate income tax amount, 46 00:03:56,290 --> 00:03:58,500 so what I store here are in the end 47 00:03:58,510 --> 00:04:05,460 two functions, this constant holds a function, it holds this inner function configured for this percentage 48 00:04:05,650 --> 00:04:11,830 and this here also holds the inner function but configured with a different percentage because I call 49 00:04:11,830 --> 00:04:13,500 the outer function twice, 50 00:04:13,510 --> 00:04:18,140 so it runs separately two times, with different arguments for the tax, 51 00:04:18,160 --> 00:04:23,980 so different values for the tax argument and therefore this inner function is recreated two times, 52 00:04:24,190 --> 00:04:26,780 once for every execution of the outer function 53 00:04:26,920 --> 00:04:33,850 and every time it logs in the tax percentage of that outer function which differs for the two executions 54 00:04:33,850 --> 00:04:39,880 and therefore the function which we return is this preconfigured inner function with the logged in 55 00:04:39,970 --> 00:04:42,450 tax amount and hence 56 00:04:42,460 --> 00:04:50,260 now if I console log calculate vat amount for 100 and then again I repeat this for let's say 57 00:04:50,290 --> 00:04:56,270 200, I just need to call this calculate vat amount function like this without passing in the 58 00:04:56,290 --> 00:05:02,770 value added tax percentage over and over again because I configured this once here with the help of 59 00:05:02,770 --> 00:05:06,100 this factory function and we're good to go thereafter. 60 00:05:06,130 --> 00:05:10,890 So if we save that and we reload, I get my different value added tax amounts. 61 00:05:11,500 --> 00:05:14,170 So that's the idea behind factory functions, 62 00:05:14,170 --> 00:05:20,620 if you have a certain function which you call multiple times in different parts of your app and it can 63 00:05:20,620 --> 00:05:23,170 be preconfigured in a certain way 64 00:05:23,260 --> 00:05:29,200 so that calling it is easier and you don't need to repeat yourself as often because our second argument 65 00:05:29,230 --> 00:05:35,500 would always be the same here if we would expect two arguments as we did earlier, in such a case, a factory 66 00:05:35,500 --> 00:05:39,370 function can be convenient and can save you time and code.