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Update Object Types.md with interface extension details #3481

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19 changes: 19 additions & 0 deletions packages/documentation/copy/en/handbook-v2/Object Types.md
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Expand Up @@ -617,8 +617,27 @@ declare const staffer: Staff;
staffer.name;
// ^?
```

In this case, Staff would require the name property to be both a string and a number, which results in property being of type `never`.



In comparison, extending interfaces with incompatible properties produces a compile-time error:
```ts twoslash
interface Person1 {
name: string;
}

interface Person2 {
name: number;
}

interface Staff extends Person1 , Person2 {}
```
Unlike intersection types, interface extension requires properties with the same name to have compatible types.



## Generic Object Types

Let's imagine a `Box` type that can contain any value - `string`s, `number`s, `Giraffe`s, whatever.
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