Implementation of Promise.any using Promise.all

promiseAnyFallback.ts

/**
 * As of writing, Promise.any is Stage 4 (Finished)
 * This code is just a fun reminder of how Promise.any can be implemented using Promise.all
 * Proposal: https://github.com/tc39/proposal-promise-any
 */

class AggregateErrorFallback extends Error {
  errors: Array<Error>;
  constructor(message: string, errors: Array<Error>) {
    super(message);
    this.name = 'AggregateError';
    this.errors = errors;
  }
}

function promiseAny<T>(values: Array<PromiseLike<T>>): Promise<Awaited<T>> {
  const identity = <T>(x: T) => x;
  const errors: Array<Error> = [];
  return Promise.all(
    values.map((promise) =>
      promise.then(
        (result) => {
          throw result;
        },
        (error) => {
          errors.push(error);
          return error;
        }
      )
    )
  ).then(() => {
    /**
     * Promise.any implementation throws AggregateError:
     * https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/AggregateError
     */
    const message = 'All promises were rejected';
    throw new AggregateErrorFallback(message, errors);
  }, identity);
}