Promise2-polyfill.js

const SYMBOL_RESOLVER = Symbol('RESOLVER');
const SYMBOL_REJECTOR = Symbol('REJECTOR');
const SYMBOL_STATUS = Symbol('STATUS');
const SYMBOL_VALUE = Symbol('VALUE');
const SYMBOL_SHADOW_RESOLVER = Symbol('SHADOW_RESOLVER');

class Promise2 {
  static all(promises) {
    const promisesData = [];
    let resolvedCount = 0;

    return new Promise2(resolve => {
      promises.forEach((promise, index) => {
        promise.then(value => {
          promisesData[index] = value;
          resolvedCount++;

          if (resolvedCount === promises.length)
            resolve(promisesData);
        })
      })
    })
  }

  constructor(executor) {
    this[SYMBOL_STATUS] = 'pending';
    this[SYMBOL_RESOLVER] = null;
    this[SYMBOL_REJECTOR] = null;
    this[SYMBOL_VALUE] = null;
    this[SYMBOL_SHADOW_RESOLVER] = null;

    executor(data => { // resolve
      if (this[SYMBOL_STATUS] !== 'pending') return;

      this[SYMBOL_VALUE] = data;
      this[SYMBOL_STATUS] = 'resolved';

      if (this[SYMBOL_RESOLVER]) {
        const returnedValue = this[SYMBOL_RESOLVER](this[SYMBOL_VALUE]);

        if (this[SYMBOL_SHADOW_RESOLVER] && returnedValue && returnedValue instanceof this.constructor)
          returnedValue.then(data => this[SYMBOL_SHADOW_RESOLVER](data));
      }
    }, data => { // reject
      if (this[SYMBOL_STATUS] !== 'pending') return;

      this[SYMBOL_VALUE] = data;
      this[SYMBOL_STATUS] = 'rejected';

      if (this[SYMBOL_REJECTOR]) {
        const returnedValue = this[SYMBOL_REJECTOR](this[SYMBOL_VALUE]);

        if (this[SYMBOL_SHADOW_RESOLVER] && returnedValue && returnedValue instanceof this.constructor)
          returnedValue.then(data => this[SYMBOL_SHADOW_RESOLVER](data));
      }
    });
  }

  then(callback) {
    if (this[SYMBOL_STATUS] === 'resolved')
      return callback(this[SYMBOL_VALUE]);
    else {
      this[SYMBOL_RESOLVER] = callback;

      return new this.constructor(resolve => {
        this[SYMBOL_SHADOW_RESOLVER] = resolve;
      });
    }
  }

  catch(callback) {
    if (this[SYMBOL_STATUS] === 'rejected')
      return callback(this[SYMBOL_VALUE]);
    else {
      this[SYMBOL_REJECTOR] = callback;

      return new this.constructor(resolve => {
        this[SYMBOL_SHADOW_RESOLVER] = resolve;
      });
    }
  }
}