How to subclass a promise

angularpromise.js

// ES6

class AngularPromise extends Promise {
    constructor(executor) {
        super((resolve, reject) => {
            // before
            return executor(resolve, reject);
        });
        // after
    }

    then(onFulfilled, onRejected) {
        // before
        const returnValue = super.then(onFulfilled, onRejected);
        // after
        return returnValue;
    }
}

// ES5

function AngularPromise(executor) {
    var p = new Promise(function (resolve, reject) {
        // before
        return executor(resolve, reject);
    });
    // after

    p.__proto__ = AngularPromise.prototype;
    return p;
}

AngularPromise.__proto__ = Promise;
AngularPromise.prototype.__proto__ = Promise.prototype;

AngularPromise.prototype.then = function then(onFulfilled, onRejected) {
    // before
    var returnValue = Promise.prototype.then.call(this, onFulfilled, onRejected);
    // after
    return returnValue;
}

@eKoopmans: my hope would be that we could have some interesting ways to extend Promise that used the extends keyword & class syntax. Alas.

None the less, good job with your hackery of Promise.

Here's an example of a deferred promise, the constructor leverages defaults and might be close to what you wanted? @rektide https://gist.github.com/oliverfoster/00897f4552cef64653ef14d8b26338a6

@rektide here you go;

class YourPromise extends Promise {
	resolve;
	reject;
	init = (resolve, reject) => {
		this.resolve = resolve;
		this.reject  = reject;
		this.init    = null;
		return this;
	};
	
	constructor(callback, ...yourArgs) {
		super(callback);
	}
	
	static async create(...yourArgs) {
		let output;
		const initArgs = await new Promise((resolve) => {
			output = new YourPromise(
				(...initArgs) => { resolve(initArgs); },
				...yourArgs
			);
		});
		//return an array because JS is a dick and unrolls all nested promises with a single `await` keyword
		//INCLUDING the one we're deliberately trying to return AS A PROMISE (YourPromise extends Promise)
		return [output.init(...initArgs)];
	}
}

Instantiate via (await YourPromise.create('your', 'arguments'))[0]

Here's a useful example of creating a Queue (TypeScript)

@rektide, here is an implementation for deferred promise with class syntax:

`
class Deferred extends Promise {
/** @type {Function} /
#resolve;
/* @type {Function} */
#reject;

/**

• Creates new deferred promise.
• To create defered object DO NOT pass executor parameter!!!
• Parameter is only needed for Promise to work correctly.
• Somehow Promise calls constructor twice. First time when you
• create Promise. Second time - during first .then() with
• different executor.
• @param {undefined|null|Function} executor - DO NOT pass anything.
  */
  constructor(executor) {
  let res;
  let rej;
  executor = executor ?? ((resolve, reject) => {
  res = resolve;
  rej = reject;
  });
  super(executor);
  if (res && rej) {
  this.#resolve = res;
  this.#reject = rej;
  }
  }

resolve(value) {
this.#resolve(value);
}

reject(err) {
this.#reject(err);
}
}
`

You can test it with:
`
const d = new Deferred();

d
.then((val) => {
console.log(Resolved with ${val});
}, (err) => {
console.log(Rejected with ${err});
});

d.resolve("RES");
// d.reject("REJ");

console.log("done");
`

@Aleksandras-Novikovas's implementation, with formatting/highlighting:

@rektide, here is an implementation for deferred promise with class syntax:

class Deferred extends Promise {
  /** @type {Function} */ #resolve;
  /** @type {Function} */ #reject;

  /**
   * Creates new deferred promise.
   * To create defered object DO NOT pass executor parameter!!!
   * Parameter is only needed for Promise to work correctly.
   * Somehow Promise calls constructor twice. First time when you
   * create Promise. Second time - during first .then() with
   * different executor.
   * @param {undefined|null|Function} executor - DO NOT pass anything.
   */
  constructor(executor) {
    let res;
    let rej;
    executor = executor ?? ((resolve, reject) => {
      res = resolve;
      rej = reject;
    });
    super(executor);
    if (res && rej) {
      this.#resolve = res;
      this.#reject = rej;
    }

    resolve(value) {
      this.#resolve(value);
    }

    reject(err) {
      this.#reject(err);
    }
  }
}

You can test it with:

const d = new Deferred();

d.then((val) => { console.log(`Resolved with ${val}`); }, (err) => { console.log(`Rejected with ${err}`); });

d.resolve("RES"); // d.reject("REJ");

console.log("done");

I've written much cleaner version: timeout-promise

This is what I came up with after a few hours of debugging and trial+error. It stores the call stack of the location where it is instantiated, allowing rejectWithError() to produce useful errors even when it is called from a parallel asynchronous process, e.g. an event handler.

export class DeferredPromise<T> extends Promise<T> {
  resolve: (value: T | PromiseLike<T>) => void;
  reject: (reason: T | Error) => void;

  initialCallStack: Error['stack'];

  constructor(executor: ConstructorParameters<typeof Promise<T>>[0] = () => {}) {
      let resolver: (value: T | PromiseLike<T>) => void;
      let rejector: (reason: T | Error) => void;

      super((resolve, reject) => {
          resolver = resolve;
          rejector = reject;
          return executor(resolve, reject);   // Promise magic: this line is unexplicably essential
      });

      this.resolve = resolver!;
      this.reject = rejector!;

      // store call stack for location where instance is created
      this.initialCallStack = Error().stack?.split('\n').slice(2).join('\n');
  }

  /** @throws error with amended call stack */
  rejectWithError(error: Error) {
    error.stack = [error.stack?.split('\n')[0], this.initialCallStack].join('\n');
    this.reject(error);
  }
}

You can use it like this:

const deferred = new DeferredPromise();

/* resolve */
deferred.resolve(value);
await deferred;

/* reject */
deferred.reject(Error(errorMessage));
await deferred; // throws Error(errorMessage) with current call stack

/* reject */
deferred.rejectWithError(Error(errorMessage));
await deferred; // throws Error(errorMessage) with amended call stack

/* reject with custom error type */
class CustomError extends Error {}
deferred.rejectWithError( new CustomError(errorMessage) );
await deferred; // throws CustomError(errorMessage) with amended call stack

Example use in my own project:
deferred-promise.ts
usage in badge-usb.ts > BadgeUSB._handlePacket()

You don't have to define constructor argument If you don't need the value returned by .then method to be of your class instance.
Example:

class DeferredPromise extends Promise {
    static get [Symbol.species]() {
        return Promise;
    }
    constructor() {
        let internalResolve = () => { };
        let internalReject = () => { };
        super((resolve, reject) => {
            internalResolve = resolve;
            internalReject = reject;
        });
        this.resolve = internalResolve;
        this.reject = internalReject;
    }
}

This is what I came up with after a few hours of debugging and trial+error. It stores the call stack of the location where it is instantiated, allowing rejectWithError() to produce useful errors even when it is called from a parallel asynchronous process, e.g. an event handler.

export class DeferredPromise<T> extends Promise<T> {
  resolve: (value: T | PromiseLike<T>) => void;
  reject: (reason: T | Error) => void;

  initialCallStack: Error['stack'];

  constructor(executor: ConstructorParameters<typeof Promise<T>>[0] = () => {}) {
      let resolver: (value: T | PromiseLike<T>) => void;
      let rejector: (reason: T | Error) => void;

      super((resolve, reject) => {
          resolver = resolve;
          rejector = reject;
          return executor(resolve, reject);   // Promise magic: this line is unexplicably essential
      });

      this.resolve = resolver!;
      this.reject = rejector!;

      // store call stack for location where instance is created
      this.initialCallStack = Error().stack?.split('\n').slice(2).join('\n');
  }

  /** @throws error with amended call stack */
  rejectWithError(error: Error) {
    error.stack = [error.stack?.split('\n')[0], this.initialCallStack].join('\n');
    this.reject(error);
  }
}

You can use it like this:

const deferred = new DeferredPromise();

/* resolve */
deferred.resolve(value);
await deferred;

/* reject */
deferred.reject(Error(errorMessage));
await deferred; // throws Error(errorMessage) with current call stack

/* reject */
deferred.rejectWithError(Error(errorMessage));
await deferred; // throws Error(errorMessage) with amended call stack

/* reject with custom error type */
class CustomError extends Error {}
deferred.rejectWithError( new CustomError(errorMessage) );
await deferred; // throws CustomError(errorMessage) with amended call stack

Example use in my own project: deferred-promise.ts usage in badge-usb.ts > BadgeUSB._handlePacket()

It works like a magic. Thanks for your work.