Typescript Class inference https://stackoverflow.com/questions/58035634/typescript-class-inference/58037767

currently.ts

type NonFunctionPropertyNames<T> = { [K in keyof T]: T[K] extends Function ? never : K }[keyof T];
type NonFunctionProperties<T> = Pick<T, NonFunctionPropertyNames<T>>;

type MakeSomeOptional<T, K extends keyof T> = Omit<T, K> & Partial<Pick<T, K>>;

class Base {
  public _id: string;
  
  public static async create<K extends typeof Base>(
    this: K,
    value: NonFunctionProperties<MakeSomeOptional<InstanceType<K>, "_id">>
  ): Promise<InstanceType<K>> {
    const doc = (new this(value)) as InstanceType<K>;
    await doc.save();

    return doc;
  }
}
                             
// please note that i cant just do "public _id?: string" because it will always be there, only on the creation / factory it is optional

solved.ts

type NonFunctionPropertyNames<T> = { [K in keyof T]: T[K] extends Function ? never : K }[keyof T];
type NonFunctionProperties<T> = Pick<T, NonFunctionPropertyNames<T>>;

type MakeSomeOptional<T, K extends keyof T> = Omit<T, K> & Partial<Pick<T, K>>;

abstract class Base {
  public _id: string;
  
  public static async create<T extends Base<any>>(this: new (...a: any[]) => T, value: NonFunctionProperties<MakeSomeOptional<T, "_id">>) {
    const doc = new this(value);
    await doc.save();

    return doc;
  }
}

wanted.ts

// wanted is to have the class abstract to protect accidental usage

type NonFunctionPropertyNames<T> = { [K in keyof T]: T[K] extends Function ? never : K }[keyof T];
type NonFunctionProperties<T> = Pick<T, NonFunctionPropertyNames<T>>;

type MakeSomeOptional<T, K extends keyof T> = Omit<T, K> & Partial<Pick<T, K>>;

abstract class Base {
  public _id: string;
  
  public static async create<T extends Base<any>>(this: new (...a: any[]) => T, value: NonFunctionProperties<MakeSomeOptional<keyof T, "_id">>) {
    const doc = new this(value);
    await doc.save();

    return doc;
  }
}

// currently complains about "'_id' does not exist in type 'toString' | 'valueOf'"