TypeScript: how to extract the generic parameter from a type?

Question

Say I have a type like React.ComponentClass<Props> and I want to reference the Props part but it is unnamed in my current context.

To illustrate:

const x = makeComponent(); // typeof x = React.ComponentClass<Something>

In this situation, I can use typeof x but that doesn't give me direct access to the Something type.

What I want is something like this:

type GetPropsType<C extends React.ComponentClass<P>> = P

So that I can extract Something with the type GetPropsType<typeof x>

Anything equivalent would be great.

Answer 1

You can use infer:

type TypeWithGeneric<T> = T[]
type extractGeneric<Type> = Type extends TypeWithGeneric<infer X> ? X : never

type extracted = extractGeneric<TypeWithGeneric<number>>
// extracted === number

Playground

Answer 2

You can use pattern matching for that:

namespace React {
    export class ComponentClass<T> {}
}

function doSomethingWithProps<T>(x : React.ComponentClass<T>) : T {
    return null as T;
}

class Something {}
let comp = new React.ComponentClass<Something>();
const instanceOfSomething = doSomethingWithProps(comp);