Extendable Promise subclass inheritance that works with Babel or TypeScript ES5 target

ES5-compatible extendable ES6 Promise class

// inherits from promise class through the chain
new ExtendablePromise(resolve => resolve()).then() instanceof Promise === true

// inherits from promise subclass through the chain
new ExtendablePromise(resolve => resolve()).then() instanceof ExtendablePromise === true

// allows for direct calls for ES5 inheritance
ExtendablePromise.call(Object.create(ExtendablePromise.prototype), resolve => resolve()).then()

extendable-promise-es6.js

class ExtendablePromise {
  constructor(executor) {
    if (!(this instanceof ExtendablePromise)) {
      throw new TypeError("Constructor ExtendablePromise requires 'new'");
    }

    this._promise = new Promise(executor);
  }

  catch(onRejected) {
    return this.constructor.resolve(this._promise.catch(onRejected));
  }

  then(onFulfilled, onRejected) {
    return this.constructor.resolve(this._promise.then(onFulfilled, onRejected));
  }

  finally(onFinally) {
    return this.constructor.resolve(this._promise.finally(onFinally));
  }
}

Object.setPrototypeOf(ExtendablePromise, Promise);
Object.setPrototypeOf(ExtendablePromise.prototype, Promise.prototype);

extendable-promise-es6.ts

// Extracted from lib es2015.promise
interface ExtendablePromiseConstructor {
  readonly prototype: ExtendablePromise<any>;

  new <T>(executor: (resolve: (value?: T | PromiseLike<T>) => void, reject: (reason?: any) => void) => void): ExtendablePromise<T>;

  all<T1, T2, T3, T4, T5, T6, T7, T8, T9, T10>(values: readonly [T1 | PromiseLike<T1>, T2 | PromiseLike<T2>, T3 | PromiseLike<T3>, T4 | PromiseLike <T4>, T5 | PromiseLike<T5>, T6 | PromiseLike<T6>, T7 | PromiseLike<T7>, T8 | PromiseLike<T8>, T9 | PromiseLike<T9>, T10 | PromiseLike<T10>]): ExtendablePromise<[T1, T2, T3, T4, T5, T6, T7, T8, T9, T10]>;
  all<T1, T2, T3, T4, T5, T6, T7, T8, T9>(values: readonly [T1 | PromiseLike<T1>, T2 | PromiseLike<T2>, T3 | PromiseLike<T3>, T4 | PromiseLike <T4>, T5 | PromiseLike<T5>, T6 | PromiseLike<T6>, T7 | PromiseLike<T7>, T8 | PromiseLike<T8>, T9 | PromiseLike<T9>]): ExtendablePromise<[T1, T2, T3, T4, T5, T6, T7, T8, T9]>;
  all<T1, T2, T3, T4, T5, T6, T7, T8>(values: readonly [T1 | PromiseLike<T1>, T2 | PromiseLike<T2>, T3 | PromiseLike<T3>, T4 | PromiseLike <T4>, T5 | PromiseLike<T5>, T6 | PromiseLike<T6>, T7 | PromiseLike<T7>, T8 | PromiseLike<T8>]): ExtendablePromise<[T1, T2, T3, T4, T5, T6, T7, T8]>;
  all<T1, T2, T3, T4, T5, T6, T7>(values: readonly [T1 | PromiseLike<T1>, T2 | PromiseLike<T2>, T3 | PromiseLike<T3>, T4 | PromiseLike <T4>, T5 | PromiseLike<T5>, T6 | PromiseLike<T6>, T7 | PromiseLike<T7>]): ExtendablePromise<[T1, T2, T3, T4, T5, T6, T7]>;
  all<T1, T2, T3, T4, T5, T6>(values: readonly [T1 | PromiseLike<T1>, T2 | PromiseLike<T2>, T3 | PromiseLike<T3>, T4 | PromiseLike <T4>, T5 | PromiseLike<T5>, T6 | PromiseLike<T6>]): ExtendablePromise<[T1, T2, T3, T4, T5, T6]>;
  all<T1, T2, T3, T4, T5>(values: readonly [T1 | PromiseLike<T1>, T2 | PromiseLike<T2>, T3 | PromiseLike<T3>, T4 | PromiseLike <T4>, T5 | PromiseLike<T5>]): ExtendablePromise<[T1, T2, T3, T4, T5]>;
  all<T1, T2, T3, T4>(values: readonly [T1 | PromiseLike<T1>, T2 | PromiseLike<T2>, T3 | PromiseLike<T3>, T4 | PromiseLike <T4>]): ExtendablePromise<[T1, T2, T3, T4]>;
  all<T1, T2, T3>(values: readonly [T1 | PromiseLike<T1>, T2 | PromiseLike<T2>, T3 | PromiseLike<T3>]): ExtendablePromise<[T1, T2, T3]>;
  all<T1, T2>(values: readonly [T1 | PromiseLike<T1>, T2 | PromiseLike<T2>]): ExtendablePromise<[T1, T2]>;
  all<T>(values: readonly (T | PromiseLike<T>)[]): ExtendablePromise<T[]>;

  race<T>(values: readonly T[]): ExtendablePromise<T extends PromiseLike<infer U> ? U : T>;
  race<T>(values: Iterable<T>): ExtendablePromise<T extends PromiseLike<infer U> ? U : T>;

  reject<T = never>(reason?: any): ExtendablePromise<T>;

  resolve<T>(value: T | PromiseLike<T>): ExtendablePromise<T>;
  resolve(): ExtendablePromise<void>;
}

class ExtendablePromise<T> {
  static all: ExtendablePromiseConstructor['all'];
  static race: ExtendablePromiseConstructor['race'];
  static reject: ExtendablePromiseConstructor['reject'];
  static resolve: ExtendablePromiseConstructor['resolve'];

  [Symbol.toStringTag]: string;
  protected _promise: Promise<T>;

  constructor(executor: (resolve: (value?: T | PromiseLike<T>) => void, reject: (reason?: any) => void) => void) {
    if (!(this instanceof ExtendablePromise)) {
      throw new TypeError("Constructor ExtendablePromise requires 'new'");
    }

    this._promise = new Promise<T>(executor);
  }

  catch<TResult = never>(onRejected?: ((reason: any) => TResult | PromiseLike<TResult>) | undefined | null): ExtendablePromise<T | TResult> {
    return (this.constructor as ExtendablePromiseConstructor).resolve(
      this._promise.catch(onRejected)
    );
  }

  then<TResult1 = T, TResult2 = never>(onFulfilled?: ((value: T) => TResult1 | PromiseLike<TResult1>) | undefined | null, onRejected?: ((reason: any) => TResult2 | PromiseLike<TResult2>) | undefined | null): ExtendablePromise<TResult1 | TResult2> {
    return (this.constructor as ExtendablePromiseConstructor).resolve(
      this._promise.then(onFulfilled, onRejected)
    );
  }

  finally(onFinally?: (() => void) | undefined | null): ExtendablePromise<T> {
    return (this.constructor as ExtendablePromiseConstructor).resolve(
      this._promise.finally(onFinally)
    );
  }
}


Object.setPrototypeOf(ExtendablePromise, Promise);
Object.setPrototypeOf(ExtendablePromise.prototype, Promise.prototype);

extendable-promise-legacy-es5.js

var ExtendablePromise = function ExtendablePromise(executor) {
  if (!(this instanceof ExtendablePromise)) {
    throw new TypeError("Constructor ExtendablePromise requires 'new'");
  }

  this._promise = new Promise(executor);

  return this;
}

Object.getOwnPropertyNames(Promise).forEach(function (key) {
  if (typeof Promise[key] === 'function') {
    ExtendablePromise[key] = Promise[key];
  }
});

ExtendablePromise.prototype = Object.create(Promise.prototype);

ExtendablePromise.prototype.constructor = ExtendablePromise;

ExtendablePromise.prototype.catch = function catch_(onRejected) {
  return this.constructor.resolve(this._promise.catch(onRejected));
};

ExtendablePromise.prototype.then = function then(onFulfilled, onRejected) {
  return this.constructor.resolve(this._promise.then(onFulfilled, onRejected));
};

ExtendablePromise.prototype.finally = function finally_(onFinally) {
  return this.constructor.resolve(this._promise.finally(onFinally));
};

extendable-promise-legacy-es5.ts

// Extracted from lib es2015.promise
interface ExtendablePromiseConstructor {
  prototype: ExtendablePromise<any>;

  new <T>(executor: (resolve: (value?: T | PromiseLike<T>) => void, reject: (reason?: any) => void) => void): ExtendablePromise<T>;

  all<T1, T2, T3, T4, T5, T6, T7, T8, T9, T10>(values: readonly [T1 | PromiseLike<T1>, T2 | PromiseLike<T2>, T3 | PromiseLike<T3>, T4 | PromiseLike <T4>, T5 | PromiseLike<T5>, T6 | PromiseLike<T6>, T7 | PromiseLike<T7>, T8 | PromiseLike<T8>, T9 | PromiseLike<T9>, T10 | PromiseLike<T10>]): ExtendablePromise<[T1, T2, T3, T4, T5, T6, T7, T8, T9, T10]>;
  all<T1, T2, T3, T4, T5, T6, T7, T8, T9>(values: readonly [T1 | PromiseLike<T1>, T2 | PromiseLike<T2>, T3 | PromiseLike<T3>, T4 | PromiseLike <T4>, T5 | PromiseLike<T5>, T6 | PromiseLike<T6>, T7 | PromiseLike<T7>, T8 | PromiseLike<T8>, T9 | PromiseLike<T9>]): ExtendablePromise<[T1, T2, T3, T4, T5, T6, T7, T8, T9]>;
  all<T1, T2, T3, T4, T5, T6, T7, T8>(values: readonly [T1 | PromiseLike<T1>, T2 | PromiseLike<T2>, T3 | PromiseLike<T3>, T4 | PromiseLike <T4>, T5 | PromiseLike<T5>, T6 | PromiseLike<T6>, T7 | PromiseLike<T7>, T8 | PromiseLike<T8>]): ExtendablePromise<[T1, T2, T3, T4, T5, T6, T7, T8]>;
  all<T1, T2, T3, T4, T5, T6, T7>(values: readonly [T1 | PromiseLike<T1>, T2 | PromiseLike<T2>, T3 | PromiseLike<T3>, T4 | PromiseLike <T4>, T5 | PromiseLike<T5>, T6 | PromiseLike<T6>, T7 | PromiseLike<T7>]): ExtendablePromise<[T1, T2, T3, T4, T5, T6, T7]>;
  all<T1, T2, T3, T4, T5, T6>(values: readonly [T1 | PromiseLike<T1>, T2 | PromiseLike<T2>, T3 | PromiseLike<T3>, T4 | PromiseLike <T4>, T5 | PromiseLike<T5>, T6 | PromiseLike<T6>]): ExtendablePromise<[T1, T2, T3, T4, T5, T6]>;
  all<T1, T2, T3, T4, T5>(values: readonly [T1 | PromiseLike<T1>, T2 | PromiseLike<T2>, T3 | PromiseLike<T3>, T4 | PromiseLike <T4>, T5 | PromiseLike<T5>]): ExtendablePromise<[T1, T2, T3, T4, T5]>;
  all<T1, T2, T3, T4>(values: readonly [T1 | PromiseLike<T1>, T2 | PromiseLike<T2>, T3 | PromiseLike<T3>, T4 | PromiseLike <T4>]): ExtendablePromise<[T1, T2, T3, T4]>;
  all<T1, T2, T3>(values: readonly [T1 | PromiseLike<T1>, T2 | PromiseLike<T2>, T3 | PromiseLike<T3>]): ExtendablePromise<[T1, T2, T3]>;
  all<T1, T2>(values: readonly [T1 | PromiseLike<T1>, T2 | PromiseLike<T2>]): ExtendablePromise<[T1, T2]>;
  all<T>(values: readonly (T | PromiseLike<T>)[]): ExtendablePromise<T[]>;

  race<T>(values: readonly T[]): ExtendablePromise<T extends PromiseLike<infer U> ? U : T>;
  race<T>(values: Iterable<T>): ExtendablePromise<T extends PromiseLike<infer U> ? U : T>;

  reject<T = never>(reason?: any): ExtendablePromise<T>;

  resolve<T>(value: T | PromiseLike<T>): ExtendablePromise<T>;
  resolve(): ExtendablePromise<void>;
}

interface ExtendablePromise<T> {
  _promise: Promise<T>;

  [Symbol.toStringTag]: string;

  catch<TResult = never>(onRejected?: ((reason: any) => TResult | PromiseLike<TResult>) | undefined | null): ExtendablePromise<T | TResult>;

  then<TResult1 = T, TResult2 = never>(onFulfilled?: ((value: T) => TResult1 | PromiseLike<TResult1>) | undefined | null, onRejected?: ((reason: any) => TResult2 | PromiseLike<TResult2>) | undefined | null): ExtendablePromise<TResult1 | TResult2>;

  finally(onFinally?: (() => void) | undefined | null): ExtendablePromise<T>;
}


var ExtendablePromise = function ExtendablePromise<T>(this: ExtendablePromise<T>, executor: (resolve: (value?: T | PromiseLike<T>) => void, reject: (reason?: any) => void) => void) {
  if (!(this instanceof ExtendablePromise)) {
    throw new TypeError("Constructor ExtendablePromise requires 'new'");
  }

  this._promise = new Promise(executor);

  return this;
} as unknown as ExtendablePromiseConstructor;
type TExtendablePromiseMethods = keyof ExtendablePromiseConstructor;

(Object.getOwnPropertyNames(Promise) as (keyof ExtendablePromiseConstructor)[])
.forEach(function (key) {
  if (typeof Promise[key] === 'function') {
    ExtendablePromise[key] = Promise[key] as any;
  }
});

ExtendablePromise.prototype = Object.create(Promise.prototype);

ExtendablePromise.prototype.constructor = ExtendablePromise;

ExtendablePromise.prototype.catch = function catch_(onRejected) {
  return (this.constructor as ExtendablePromiseConstructor).resolve(
    this._promise.catch(onRejected)
  );
};

ExtendablePromise.prototype.then = function then(onFulfilled, onRejected) {
  return (this.constructor as ExtendablePromiseConstructor).resolve(
    this._promise.then(onFulfilled, onRejected)
  );
};

ExtendablePromise.prototype.finally = function finally_(onFinally) {
  return (this.constructor as ExtendablePromiseConstructor).resolve(
    this._promise.finally(onFinally)
  );
};