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So we get the basics about generics down hopefully.

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Now let's dive a bit deeper into constraints or into one specific type of constraint which we can add

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retail trade yet a no function of generic function which will name extract and convert the idea of this

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function here is dead.

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We get an object as a first argument and the second parameter is in the end a key now want to return

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to value for that key in the object which we can do with this default javascript notation using the

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square brackets to access a property in object.

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But that's nothing typescript specific.

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That's vanilla javascript in the end.

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Now Therefore here we could say that here we want to have an object.

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And for the key we want to have a string.

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Let's say now you're getting an error though because what we don't know here in the end is whether the

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object we're getting here really will have that key.

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That's the end what typescript is telling us here with this strange error.

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For example here I could console lock the result of calling this with let's say an empty object and

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then name and here by the way I'm calling this extract and convert because I want to call this without

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console log and simply say value colon plus.

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But that's just a side note I'm returning a string here and the end with that retrieve data.

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But the main problem here is I could call it like this and it would pass in an object as a first parameter

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and then a string as a second but correctly typescript tells me that in the end it's not guaranteed

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that this key exists in this object now could guarantee us we can again use generic types here.

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We could say we got a type T and it will be our object here we can say extends Object but we also got

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a type you are a key here let's say.

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Which in the end has to be a key of our t type though.

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So of this object we want to guarantee that what we get here is a second parameter is a key is a property

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of that first type.

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And for Dad we can sake extends key of

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and now you see the error is gone here and instead of got the error down there because now I'm informing

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typescript here that the first parameter should be any kind of object.

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And a second parameter should be any kind of key in that object.

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And here.

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Yes we're passing in any kind of object but we don't have a name key in there.

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This error will only go away as soon as I add a name key.

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So this is all something we can use generic types for with this key off keyword here also to tell typescript

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that we want to ensure that we have this correct structure and that's of course really useful because

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it allows us to ensure that we don't make dumb mistakes where we tried to call this function which in

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the end would try to access a property that doesn't exist.

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So this works but if I tried to access h here I again get an error.

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For example let's switch just back to name them for.