TypeScript: filtering in mapped types

Question

Is it possible to create a mapped type, where the mapping would depend on the type of a property. For instance, assume that I want to map all String properties to a type Foo and all other to Bar . So, I'd like to do something like:

type Mapped<T> = {
   [P in keyof T]: T[P] === String ? Foo : Bar
}

Is there any syntax to achieve this?

Answer 1

You will have to install typescript@next or wait for version 2.8 to be officially released.

You can then achieve what you want like this:

type Mapped<T> = {
   [P in keyof T]: T[P] extends string ? Foo : Bar
}