1 00:00:00,820 --> 00:00:04,860 Whenever you and I write typescript code, we're going to rely upon type inference quite heavily. 2 00:00:05,140 --> 00:00:08,920 This is where TypeScript is going to figure out the different types that are flowing around our application 3 00:00:08,920 --> 00:00:09,650 for us. 4 00:00:10,180 --> 00:00:13,690 However, there are three different scenarios that we're going to look at, where we're going to have 5 00:00:13,690 --> 00:00:15,730 to add in type annotations manually. 6 00:00:15,730 --> 00:00:19,630 Instead, remember, these type annotations are going to help TypeScript along. 7 00:00:20,350 --> 00:00:24,610 The first example we're going to look at is whenever we have a function that returns the any type and 8 00:00:24,610 --> 00:00:28,140 we need to clarify it and we haven't really discussed the any type just yet. 9 00:00:28,240 --> 00:00:32,140 So we're going to write out first just a little bit of code, then we'll review what the any type really 10 00:00:32,140 --> 00:00:36,700 means and then we'll get a better idea of why we need to use a type annotation in this case. 11 00:00:36,940 --> 00:00:37,810 So let's get to it. 12 00:00:38,400 --> 00:00:42,640 I'm going to flip back over to my code editor and I'm going to go down to the very bottom of the file 13 00:00:43,180 --> 00:00:43,600 down here. 14 00:00:43,600 --> 00:00:47,920 We're going to add in our three different examples on type annotations so I can put in a little comment 15 00:00:47,920 --> 00:00:51,190 that says, when do you use annotations? 16 00:00:52,140 --> 00:00:54,630 So case number one is when we have a function. 17 00:00:56,150 --> 00:00:57,260 That returns. 18 00:00:58,590 --> 00:01:00,030 The any type. 19 00:01:01,060 --> 00:01:05,650 OK, so for this example, I want to define a little snippet of Jason as a string. 20 00:01:05,860 --> 00:01:08,040 No, just going to try to pass that, Jason. 21 00:01:08,770 --> 00:01:14,650 So I'm going to define a new variable called lowercase Jason, and I'm going to assign it a JSON string. 22 00:01:14,920 --> 00:01:15,940 So I'll put down a string. 23 00:01:15,940 --> 00:01:18,670 And then inside of here, we're going to write out just a little bit of JSON. 24 00:01:19,490 --> 00:01:20,680 So let's say curly brace. 25 00:01:21,070 --> 00:01:24,610 I'll put down X and double quotes, a colon and then 10. 26 00:01:25,740 --> 00:01:31,500 Then why, in double quotes, a colon and 20, and then I'll close off the curly brace like so. 27 00:01:32,770 --> 00:01:38,140 So now I could use that string with the JSON, parse function and turn the string into an actual object, 28 00:01:39,020 --> 00:01:41,890 so I'm going to create a new variable called How about coordinates? 29 00:01:42,900 --> 00:01:49,350 In a sign that the result of Jason that pass with that Jason variable, so in theory, if we now ran 30 00:01:49,350 --> 00:01:58,020 this code and console logged out coordinates, we would see something like X is 10 and Y is 20 like 31 00:01:58,020 --> 00:01:58,320 so. 32 00:02:00,150 --> 00:02:04,770 Now, there's just one little gotcha inside of here, if you mouse over it coordinates, you'll notice 33 00:02:04,770 --> 00:02:06,750 that it has a type connotation of any. 34 00:02:06,840 --> 00:02:11,039 So this means that TypeScript thinks that coordinates is of type any. 35 00:02:12,110 --> 00:02:16,790 In addition, if we mouseover pass right here, you'll notice a lot of nasty syntax up here, but at 36 00:02:16,790 --> 00:02:22,070 the very end of it is something that says Colon, any that means that the JSON parts function returns 37 00:02:22,070 --> 00:02:23,090 the any type. 38 00:02:24,070 --> 00:02:26,340 So clearly, this anything is something pretty important. 39 00:02:26,530 --> 00:02:32,230 Let's have a quick pause, a little aside here and discuss what the any type means and then we'll understand, 40 00:02:32,230 --> 00:02:36,880 based on that discussion, why we would need to add in a type annotation here, even though it looks 41 00:02:36,880 --> 00:02:39,250 like there's no airs or warnings with our code. 42 00:02:40,090 --> 00:02:40,420 All right. 43 00:02:40,420 --> 00:02:41,440 So quick aside here. 44 00:02:42,220 --> 00:02:47,450 So the first thing I want to review is how the adjacent parts function works in this diagram. 45 00:02:47,500 --> 00:02:51,490 I've got some different values on the left hand side, and then we're going to imagine that we feed 46 00:02:51,490 --> 00:02:56,710 each of these different values into JSON pass and we'll take a guess at what the different types are 47 00:02:56,710 --> 00:02:57,850 that will come out of the function. 48 00:02:58,540 --> 00:03:01,340 Now, in reality, this diagram is just a little bit misleading right here. 49 00:03:01,360 --> 00:03:06,560 In reality, any time we call Jason Pass, we get back a value of the any type. 50 00:03:07,180 --> 00:03:13,180 So this column over here on the right hand side, I'm just saying this is what the like maybe the type 51 00:03:13,180 --> 00:03:13,570 would be, right? 52 00:03:13,580 --> 00:03:17,530 This is like we know that the type of false ones we pass, it would be a boolean. 53 00:03:17,540 --> 00:03:18,870 That's not what we get with TypeScript. 54 00:03:19,000 --> 00:03:20,560 This is just like the ideal world. 55 00:03:21,870 --> 00:03:27,210 So if we fed in a string of falls to Jason Pass, we get back a billion again, that's not what really 56 00:03:27,210 --> 00:03:27,630 happens. 57 00:03:27,630 --> 00:03:30,060 We get back any but hey, we're just saying we get back a billion. 58 00:03:30,780 --> 00:03:33,030 If we pass in a string of four, we would get back a number. 59 00:03:33,510 --> 00:03:38,610 If we pass in something that looks like an object with value and then five, we would get back a type 60 00:03:38,610 --> 00:03:40,410 that is an object with value. 61 00:03:40,410 --> 00:03:41,070 That is a number. 62 00:03:41,520 --> 00:03:46,860 And likewise, if we have a object with name that is pointing out a string, we would get back an object 63 00:03:46,860 --> 00:03:48,660 with a name, property that is a string. 64 00:03:49,170 --> 00:03:51,120 And you can see each of these examples very easily. 65 00:03:51,120 --> 00:03:56,850 If you open up a JavaScript console really quickly and you call, say, JSON pass with a string of like 66 00:03:56,850 --> 00:03:57,930 five, we get back a number. 67 00:03:57,930 --> 00:03:58,230 Right. 68 00:03:58,260 --> 00:03:58,950 Makes sense. 69 00:03:59,670 --> 00:04:05,940 Now, the kind of common theme here is that we can clearly pass in very different strings into this 70 00:04:05,940 --> 00:04:10,440 function and we can possibly get back very, very different value types. 71 00:04:11,220 --> 00:04:14,910 In some cases, we might get a boolean, in other cases we might get a number. 72 00:04:14,940 --> 00:04:18,420 And then in some other cases we might get these really complicated objects. 73 00:04:19,370 --> 00:04:23,810 So the goal, this diagram right here is just to help you understand that any time we call Jason Pass, 74 00:04:23,930 --> 00:04:27,990 we get back possibly some very, very different value types. 75 00:04:28,610 --> 00:04:32,540 Now, the problem here is that TypeScript cannot predict these different types. 76 00:04:32,810 --> 00:04:38,240 TypeScript doesn't know what we're going to get out of Jason Pass because it depends entirely on the 77 00:04:38,240 --> 00:04:40,490 string that we put into that function. 78 00:04:41,430 --> 00:04:47,130 So when you and I are writing code inside of our code editor and we call Jason Pass typescript, just 79 00:04:47,130 --> 00:04:53,370 plain can't predict what we get back from the function because it will be entirely different based upon 80 00:04:53,370 --> 00:04:54,300 the string that we put in. 81 00:04:54,630 --> 00:04:59,760 And TypeScript is not going to try to like read the string and figure out what it would return if we 82 00:04:59,760 --> 00:05:01,010 call Jason parts. 83 00:05:01,680 --> 00:05:07,770 So as a shortcut, typescript instead decides to simply say, you know what, I can't guess all these 84 00:05:07,770 --> 00:05:09,390 different types, that's too complicated. 85 00:05:09,690 --> 00:05:15,210 So I'm going to say, if you ever call Jason Pass, you're going to get back something called the any 86 00:05:15,210 --> 00:05:15,630 type. 87 00:05:17,220 --> 00:05:23,430 The any type essentially means that typescript has no idea what type of value is being returned from 88 00:05:23,430 --> 00:05:28,410 JSON parts, has no clue at all, and that's what we see inside of our code editor. 89 00:05:29,310 --> 00:05:34,140 So right here, when we mouseover coordinates, TypeScript is saying this is the any type, that means 90 00:05:34,140 --> 00:05:37,710 that TypeScript has no idea what type coordinates is. 91 00:05:39,530 --> 00:05:43,700 So that's what the any type indicates now, a couple more notes here on the any type. 92 00:05:44,780 --> 00:05:50,870 So any is a type, just as string or boolean are, any time we have to write out a type of a variable 93 00:05:50,870 --> 00:05:55,490 like, for example, in a type annotation, we could in theory put in any just as easily. 94 00:05:56,470 --> 00:06:00,640 Any time we see something that is marked with any type, that means that TypeScript has no idea what's 95 00:06:00,640 --> 00:06:05,040 going on and in general, that is a very bad thing to have inside of application. 96 00:06:05,650 --> 00:06:10,840 Remember the entire idea behind TypeScript and all these types and all this kind of stuff is that we're 97 00:06:10,840 --> 00:06:14,140 using TypeScript to catch errors inside of our code editor. 98 00:06:14,530 --> 00:06:20,080 And we able to do that because of these types like types like string and number and boolean and whatnot. 99 00:06:20,710 --> 00:06:25,810 If TypeScript knows the different types inside of application, it can take a look at any time we say 100 00:06:26,080 --> 00:06:30,880 a function on an object or reference a property on an object in typescript can figure out whether or 101 00:06:30,880 --> 00:06:35,410 not we are referencing a property that actually exists or if it's one that we just made a typo on. 102 00:06:36,120 --> 00:06:42,580 So any time we have a value, that is the any type typescript cannot do any er checking whatsoever around 103 00:06:42,580 --> 00:06:43,270 that value. 104 00:06:44,020 --> 00:06:47,350 So for example, right now if I go back over to my code editor. 105 00:06:48,670 --> 00:06:53,830 I can write out coordinates, dot, blah, blah, blah, blah, blah, like, so this is clearly a property 106 00:06:53,830 --> 00:06:58,990 that does not exist, but you'll see that TypeScript does nothing to help me out and tell me that I 107 00:06:58,990 --> 00:07:00,230 might have a mistake right here. 108 00:07:00,970 --> 00:07:05,500 However, if I go back up to any of these other examples where I'm working with variables that have 109 00:07:05,500 --> 00:07:08,680 known type like let's say how about speed right here? 110 00:07:09,490 --> 00:07:13,990 If I reference speed, blah, blah, blah, TypeScript is going to jump in very quickly and say, hey, 111 00:07:13,990 --> 00:07:15,730 that's not a property that exists. 112 00:07:16,210 --> 00:07:21,340 It's only going to allow me to call properties that actually exist like, let's say to upper case like. 113 00:07:21,340 --> 00:07:22,450 So that's fine. 114 00:07:22,600 --> 00:07:25,480 And it's only fine because TypeScript knows that speed is a string. 115 00:07:26,380 --> 00:07:32,020 So, again, any time we have a variable of type, any, it is generally a bad thing because TypeScript 116 00:07:32,020 --> 00:07:33,570 can't do its job. 117 00:07:34,690 --> 00:07:39,040 All right, so I think that we've gone into enough detail on this any type, so let's have another quick 118 00:07:39,040 --> 00:07:39,570 pause right here. 119 00:07:39,580 --> 00:07:44,050 We're going to come back to the next video and understand how we can use a type annotation to fix this 120 00:07:44,050 --> 00:07:44,950 thing up right here. 121 00:07:45,070 --> 00:07:47,010 So quick pause and I'll see you in just a minute.