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Now a special type of type guard you could say or something that helps you with type guards is discriminated

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union.

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Now what's dead.

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It's a pattern which you can use when working with union types that makes implementing type guards easier.

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It is available when you work with object types.

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Let me show you how it works let's say we have a couple of interfaces and this could be classes as well.

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But here all go for interfaces to show that it works with those as well.

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And there I have my Bert which has a flying speed which should be of type no let's say.

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And I also have my horse which has a ground speed or a running speed which should be of type no so if

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these two interfaces here now we create our own union type animal which is either a bird or a horse

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and justice before we might have a function move.

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Animal that takes an animal as an input which is of type animal.

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So using our union type you now in their I want to say moving with speed and now of course I need two

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axes either flying speed or running speed.

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Now of course we could have just named that speed but this is just a simplified example of a real use

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case you might face where you have some interfaces or objects in general that are kind of related but

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have different properties or methods.

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So here it's the speed property which actually is different.

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So I can't ex is flying speed here because of course not all animals have a flying speed.

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Now we can do what we learned before we can check if flying speed is in animal here and if that is the

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case we can execute this code now this is an approach we can take but the more potential animals we

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get there the more things we have to check.

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We also can mis type here in this string.

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We can forget an E for example and wonder why it is won't work.

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Now we can't use instance off here because I'm breaking with interfaces as I explained.

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Animal instance of bird will not work because data is an interface which is not compiled to javascript.

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So at runtime this will not be available as a constructor function.

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So the effort is does not work.

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Now we can build a discriminated union here by giving every interface so every object which should be

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part of that union an extra property you can use any name you want of you use kind or type here.

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I'll go for type and name this bird now important we're trading an interface here.

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So this is actually not a value for the type property.

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This instead is a literal type.

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As you learned where type must hold a string which must be bird.

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So this is a type assignment where we narrowed down the value that may be stored in type to exactly

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this string value and I do the same for horse just with horse as a literal type.

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Now here in our function we can use a switch statement and switch on animal dots type every animal has

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a type property because we added it to all interfaces that make up our animal.

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And then here we can have a couple of cases where we even get auto completion because TypeScript and

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our I.T. understands that type can only ever be bird or horse and if it's bird then we could set some

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internal variable speed to let's say animal dot flying speed and then brake thereafter otherwise if

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the case is horse we said speed equal to animal not running speed and there after we can console log

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moving at speed plus speed and if I now call move animal with an animal I create on the fly here where

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I set to type two bird and I'm only allowed to use bird or horse then I have to set my flying speed

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and by the way types could even recognizes that a concept running speed here to try to do that and set

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us to 10 it complains that this is not a way lable for type bird.

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If I said two flying speed however it works and if we save that it outputs this correctly and also compiles

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without errors.

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Now this is a discriminated union because we have one common property in every object that makes up

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our union which describes Stat object so that we can use this property that describes this object in

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our check to have 100 percent type safety and understand which properties are available for such an

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object and which properties are not.

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So this is a no useful pattern which you can use when working with objects and with union types and

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it even works with interfaces because this interface forces every object you built based on this interface

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to have this type so instead of checking for the existence of a given property or instead of using instance

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of where you as a property we note it exists to check which type of object we're working with we also

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eliminate the danger of mis typing because typescript understands that the only cases we can have here

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for animal type are bird and horse and hence it gives us does auto completion and if we would introduce

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a typo we would immediately get an error.

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So therefore it is a very useful pattern when working with objects and union types.