Declaration merging can be used to modify existing interfaces in the global namespace.

Consider the following interface:

```typescript
declare global {
  namespace React {
    interface MyAwesomeInterface {
      foo: string;
    }
  }
}
```

If we wanted to add a property to the interface, the easiest way would be to add a `bar: string;` like this:

```typescript
declare global {
  namespace React {
    interface MyAwesomeInterface {
      foo: string;
      bar: string;
    }
  }
}
```

But how can we do this without directly changing the existing code?

## Challenge

Your task is to use declaration merging to add `bar: string;` to the `MyAwesomeInterface` without modifying the existing code.

Hint: To do this, you will need to use `declare global`, `namespace`, and `interface` again.

Declaration merging can be used to modify existing interfaces in the global namespace.

You can have multiple `declare global` blocks in the same file, even if they both are going to modify the same namespace.

In order to solve this challenge, we will duplicate the existing `MyAwesomeSolutionInterface` and add the `bar: string;` property to the second block:

```typescript
declare global {
  namespace React {
    interface MyAwesomeSolutionInterface {
      foo: string;
    }
  }
}

declare global {
  namespace React {
    interface MyAwesomeSolutionInterface {
      bar: string;
    }
  }
}

type test = Expect<
  Equal<React.MyAwesomeSolutionInterface, { foo: string; bar: string }>
>;
```

Both of the `MyAwesomeSolutionInterface` interface declarations are in the same scope, so they get merged.

Note that this wouldn't work if these were types. In that case, TypeScript would give us an error because of the duplicate identifier:

```typescript
declare global {
  namespace React {
    type MyAwesomeSolutionInterface {
      foo: string;
    }
  }
}

declare global {
  namespace React {
    type MyAwesomeSolutionInterface { // Error!
      bar: string;
    }
  }
}
```

Declaration merging also wouldn't work if the interfaces were in different namespaces:

```typescript
declare global {
  namespace React {
    type MyAwesomeSolutionInterface {
      foo: string;
    }
  }
}

declare global {
  type MyAwesomeSolutionInterface { // different namespace!
    bar: string;
  }
  namespace React {
    type MyAwesomeSolutionInterface { 
      foo: string;
    }
  }
}
```

This is how you change code that is already available in the global scope.

Learn how to merge interfaces in TypeScript to combine and extend their properties in the same global namespace.

Extend the Global React namespace with Declaration Merging in TypeScript

[00:00] So the solution here is basically to copy this over to add a new declareGlobal because you can have multiple per file. And now, instead of having foo string, we can actually just add bar string. And those interfaces, because they're in the same scope, they get merged. So this is declaration merging of interfaces.

[00:19] This wouldn't work if these were types, right? So if we have my awesome solution up here, these aren't going to work because we have duplica identifiers in the same scope. And it also wouldn't work if they were in different scopes. So if this one was just in declareGlobal up here, so just a globally available interface,

[00:35] this doesn't work because it's not in the same namespace as this. So here we can see that you can do all of this cool little kind of like declaration merging or all this interesting stuff to actually change things that are already available in the global scope. And we're going to put this into practice in the next exercise.

Modify Existing Interfaces in the Global React Namespace

[00:00] we can use something called declaration merging in TypeScript to actually change stuff in the global namespace that's already there. So I don't want you to change this code whatsoever, right? The really easy way to solve this exercise is just to add bar string up onto this interface, right? This means that this will work down here.

[00:19] But we now have a global inside the namespace React on an interface of my awesome interface. And what I'd like you to do is I'd like you just to kind of like modify that, but with a separate declaration below this comment here. And the idea of what you're trying to do

[00:37] is you're trying to make it also have a bar string on it. That's your goal. So this, again, we're in the sort of like abstract here, but I just want you to see if you can figure this out using the clues here and using this link here that might help too. Good luck.

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Transcript

Modify Existing Interfaces in the Global React Namespace

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