Check out the solution below (3.0)
Pick is simply a type and not a class; a class serves as both a type and an object constructor. Types only exist during compile time, which explains why you are encountering the error.
You have the option to create a function that takes in a constructor and returns a new constructor capable of instantiating an object with fewer fields (or at least claiming it does):
export class Base {
public c: number = 0;
constructor(public a: number, public b: number) {
}
}
function pickConstructor<T extends { new (...args: any[]) : any, prototype: any }>(ctor: T)
: <TKeys extends keyof InstanceType<T>>(...keys: TKeys[]) => ReplaceInstanceType<T, Pick<InstanceType<T>, TKeys>> & { [P in keyof Omit<T, 'prototype'>] : T[P] } {
return function (keys: string) { return ctor as any };
}
export class PartialDescendant extends pickConstructor(Base)("a", "b") {
public constructor(a: number, b: number) {
super(a, b)
}
}
var r = new PartialDescendant(0,1);
type IsValidArg<T> = T extends object ? keyof T extends never ? false : true : true;
type ReplaceInstanceType<T, TNewInstance> = T extends new (a: infer A, b: infer B, c: infer C, d: infer D, e: infer E, f: infer F, g: infer G, h: infer H, i: infer I, j: infer J) => infer R ? (
IsValidArg<J> extends true ? new (a: A, b: B, c: C, d: D, e: E, f: F, g: G, h: H, i: I, j: J) => TNewInstance :
IsValidArg<I> extends true ? new (a: A, b: B, c: C, d: D, e: E, f: F, g: G, h: H, i: I) => TNewInstance :
IsValidArg<H> extends true ? new (a: A, b: B, c: C, d: D, e: E, f: F, g: G, h: H) => TNewInstance :
IsValidArg<G> extends true ? new (a: A, b: B, c: C, d: D, e: E, f: F, g: G) => TNewInstance :
IsValidArg<F> extends true ? new (a: A, b: B, c: C, d: D, e: E, f: F) => TNewInstance :
IsValidArg<E> extends true ? new (a: A, b: B, c: C, d: D, e: E) => TNewInstance :
IsValidArg<D> extends true ? new (a: A, b: B, c: C, d: D) => TNewInstance :
IsValidArg<C> extends true ? new (a: A, b: B, c: C) => TNewInstance :
IsValidArg<B> extends true ? new (a: A, b: B) => TNewInstance :
IsValidArg<A> extends true ? new (a: A) => TNewInstance :
new () => TNewInstance
) : never
When using constructors parameters, keep in mind that things like parameter names, optional parameters, and multiple signatures may be lost.
Edit
Since the original question was answered, TypeScript has introduced an enhanced solution to this issue. With the inclusion of Tuples in rest parameters and spread expressions, there is no longer a need for all the overloads for ReplaceReturnType:
export class Base {
public c: number = 0;
constructor(public a: number, public b: number) {
}
}
type Omit<T, K extends keyof T> = Pick<T, Exclude<keyof T, K>>
function pickConstructor<T extends { new (...args: any[]) : any, prototype: any }>(ctor: T)
: <TKeys extends keyof InstanceType<T>>(...keys: TKeys[]) => ReplaceInstanceType<T, Pick<InstanceType<T>, TKeys>> & { [P in keyof Omit<T, 'prototype'>] : T[P] } {
return function (keys: string| symbol | number) { return ctor as any };
}
export class PartialDescendant extends pickConstructor(Base)("a", "b") {
public constructor(a: number, b: number) {
super(a, b)
}
}
var r = new PartialDescendant(0,1);
type ArgumentTypes<T> = T extends new (... args: infer U ) => any ? U: never;
type ReplaceInstanceType<T, TNewInstance> = T extends new (...args: any[])=> any ? new (...a: ArgumentTypes<T>) => TNewInstance : never;
This revised approach is not only more concise but also addresses various issues:
- Optional parameters remain optional
- Argument names are retained
- Works seamlessly for any number of arguments