Promise combinators

combinators.js

Promise.all = iterable => {
  return new this((resolve, reject) => {
    const values = [];
    let countdown = 0;
    let index = 0;
    
    for (let nextValue of iterable) {
      const nextPromise = this.resolve(nextValue);
      const countdownFunction = makeCountdownFunction(index);
      
      nextPromise.then(countdownFunction, reject);
      
      ++index;
      ++countdown;
    }
    
    function makeCountdownFunction(index) {
      return value => {
        values[index] = value;
        if (--countdown === 0) {
          resolve(values);
        }
      };
    }
  });
};

Promise.race = iterable => {
  return new this((resolve, reject) => {
    for (let nextValue of iterable) {
      let nextPromise = this.resolve(nextValue);
      nextPromise.then(resolve, reject);
    }
  });
};

I doubt this inside arrow functions is the correct implementation here.