Instanceof and Type Predicates

hey there everyone it's here back again with another video and in this video we're going to be talking about the two more topics which are again belongs to the same category of type narrowing and the first one is easy the second one takes a couple of minutes to actually adjust but it's also easy not to worry I'm here everything is going to be easy so not to worry on that part I hope you are enjoying the series if yes do share it up with all of your friends and hit that subscribe in case you want to okay let's go ahead and talk about the two more type of how we can narrow down the type this entire portion of the series is about how we can go into the more preciseness of the type so that we can take better action and that's what the typescript is all about getting the types all narrowed down so let me walk you through with the documentation again and these all examples that we'll be talking through some portion of these examples come from the documentation but I've modified them a little bit for the easier understanding but yes the source is always documentation so the next in the line is the instance off so this is how further down you can narrow wrote down the things and they just do mention the Prototype and all of that but the idea is the focus of your attention should be on anything that is or that can be constructed with the new keyword that's where this instance off actually comes into the picture so uh this is a common keyword it's something that is obviously uh there so you can just go ahead and check out anything is X is an instant of date so uh anything just you can use new keyword like we create a lot of classes we can also create arrays with the new keyword we can create objects with a new keyword all of that that can be find out I'll just borrow this code into the code reader so I can just show you a little bit more so we have gone through with this one so this is an instance off now again in the value they says hey this is a parameter and all that we'll just get rid of that because we don't need this parameter we don't need this part Let's uh let's adjust this code a little bit so it's easier to understand and there we go all right so what this function is doing it's a log value now notice here this x can be a date or can be a string so how can we find a date because remember date can be easily created any type of variable can be created just by saying new and then we create a date just like that a new date is being created similar to that you can actually go ahead and create an array like this so anything that uses this queue new keyword is uh there and this keyword instance of is almost like not literally but almost like type of type of checks you for the default types instance of check you whether this object was an instance of some class or maybe some uh something like that so here we checked whether the x is an instance of date so it just returns it true if that returns it true we are able to further narrow it down now notice here if I come up here we are 100 sure that X is going to be date in this case and in the other case it's going to be a string so yes this is also a valuable keyword where the type of doesn't really cut through there you can check whether this is instance off and again remember from the docs it can only be used where there is a new keyword that there is a potential of having a new keyword so this is the easy part now coming up on the part which is little bit tricky which is the type predicates okay uh this is where typescript has nothing to do it's a bit of a logical flaw not flaw I wouldn't say but a logical uh kind of a flow that goes through and which sometimes matches and it introduces a new set of keyword let me show you and again we're going to be using the same example that is given up here pet fish and all of that so we'll just just gonna literally line by line write this keyword but then we're only going to write this much and then to understand it we are going to use our own functions okay but first let's go ahead and use this one so we're gonna go ahead and say function and this function there we go in this function we're going to say get food there we go and before we actually go ahead and Define this get food and stuff we need to actually have a fish so again this is all coming up from the documentation not making anything up so there we go so this is a fish and we're defining a type of it which is going to be like this and we're going to say that hey you are going to have a method of swim and this is going to be a method like that this is not going to return anything so we're going to go ahead and say wide so this is really simple we're going to go ahead and put a semicolon duplicate that this is not a fish this is a bird and a bird don't swim they actually fly so we're going to have a fly method now we have two types just like we had interface we could have type again both the same values and based on this now we can use an example that was given to us in documentation so we're going to go ahead and create a method which says is fish so obviously just like we had method like is admin this is a method which is going to validate whether the input value was given is a Fisher really simple uh it could be any variable well whatever you want to say it could be either a fish or it could be a bird so these are two values now what we want that this function should return as a true or false value now how they do it in the documentation is a little bit of a typecasting pet as fish and all of that so let me walk you through so what they do is first they say that pet let's cast this as fish so just like that let's wrap this up inside a parenthesis once you actually do this then they try to check the method so fish has a method of dot swim and if that method exists that means it cannot be unidentified so we're gonna say if that is not equal to undefined did I say it unidentified I mean to say undefined okay so what we're going to do is we're gonna simply go ahead and say return there all right so really simple yet a little bit more of the code that is involved but here we are saying pet is a fish if it has a method of dot swim that is not undefined then true it is a fish so that is exactly what they are doing and they are exactly saying like this but things actually change a little bit so let's try to use this this much so far we have got the code from the documentation but let's try to implement that so further down the road I try to get some food for the fish so let's go ahead and get some food for the fish and I know that the food is different for fish and for birds so Pat is going to be type of either fish that might come or it could be a bird all right now since I have the access of this method which is is fish I can use an if and else condition here I can go ahead and simply say is fish which is a method available to me and I can pass on this pet here okay so far no problem and if that is the case then here if I go on to line 66 I'm 100 sure that whatever the value of the pet comes up that has already been identified for true for the fish case now let's go ahead and say that if this is the case then we are going to return as a fish food and if not then obviously it is a bird so we're going to go ahead and return this is a bird food I don't know what to call fission but I'll also say this pet here now in theory if we have defined it so well and we have done this so much this is coming up directly from the documentation by the time we reach line 66 the pet should be identified as truly as fish and on the line 69 it should be identified truly as bird but that is not the case if I hover over this pet it is still confused typescript is still confused whether this is a fish or a bird here also goes same that it is a fish or a bird so still the value or the type of the value is not being identified although it is not giving us any error it will still work as it is fine but this method is truly not behaving because what this method is returning is a true and false value which is a Boolean in this in the case so returning a Boolean that is fine notice here it is returning a Boolean it is not returning me a type of fish or a bird okay so what does the documentation says documentation says that you can use pet is fish so you can instead of returning a Boolean you can use the syntax of pet is fish that means you are typecasting it as a particular type for example I can come up here since here on the line number 60. I'm 100 sure that if this returns as true then I'm sure the return type is fish and again you have to use a colon here my bad there we go so this is a bit of a new syntax but now you get the idea the True Value returning Boolean is not going to cut through in this special case but if you go ahead and return the type which could be fish or anything else in your case is now a guarantee that we are returning truly not as true but actually as a fish when I come here now we are identifying that we are returning a fish or we are returning a bird I guess this is a little bit of a weird syntax but this is fantastic how they have implemented it mind boggling but really really I absolutely love that how they have defined it yes the documentation is also almost like that but I have just tried to break it down this one is not so great explanatory I've just modified this example but again everything is coming from the documentation we don't learn anything from elsewhere it's just documentation so I hope you have enjoyed this one I'm super happy that you are hit that subscribe button and let's catch up in the next one

Channel: Hitesh Choudhary

Views: 7,299

Keywords: Programming, python, javascript, devops, cloud

Id: 0JZ6EW-QrJw

Length: 9min 24sec (564 seconds)

Published: Wed Oct 26 2022