[Typescript] The naked type

So what is a nake type?

Example:

type UnionToIntersection<T> = 
  (T extends any ? (x: T) => any : never) //...

 

We check Tin a sub-type condition,

T extends any ?...

And T is not wrapping with anything:

type Naked<T> = T extends ... // naked!

type NotNaked<T> = { o: T } extends ... // not naked!

 

Feature of The naked type

If Tis a union type, Typescript run each item of the Tagainst the condition.

So with a naked type, a conditional of union types becomes a union of conditional types, For example:

type WrapNaked<T> = 
  T extends any ? { o: T } : never; 

type Foo = WrapNaked<string | number | boolean>

// equals to
type Foo = WrapNaked<string> | WrapNaked<number> | WrapNaked<boolean>

// equals to

type Foo = 
  string extends any ? { o: string } : never |
  number extends any ? { o: number } : never |
  boolean extends any ? { o: boolean } : never

type Foo = 
  { o: string } | { o: number } | { o: boolean }

 

As compared to the non-naked version:

type WrapNaked<T> = 
  { o: T } extends any ? { o: T } : never

type Foo = WrapNaked<string | number | boolean>

// A non Naked type, so this equals to
type Foo = { o: string | number | boolean } extends any ? { o: string | number | boolean } : never

type Foo = { o: string | number | boolean }