Callable Objects

Historically I've done:

``` ts
// Declaration
interface Foo {
(): string; // Callable
displayName: string; // Some object property
}
```

Which is great for a `.d.ts` file:

``` ts
declare var foo: Foo
```

But awful looking for when you have to do your own creation:

``` ts
// Creation using assertion (eww)
const foo: Foo = function() {
return "hello";
} as any;
foo.displayName = "foo";
```

@pksorensen just pointed out https://twitter.com/pksorensen/status/767128252326506500

``` ts
function foo() {
return "hello";
}
namespace foo {
export const displayName = "foo"
}
```

> Add to Tips

Locate the book's Tips section and review nearby entries for placement and formatting. Add a tip explaining callable objects, including the provided declaration, assertion-based creation, and function/namespace pattern; done means the example is included clearly in the book.

Written by the indexing model from the issue text.