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

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);

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