parallel async iteration via generator

00-condition.js

(async function() {
  async function f1() {
    return new Promise(res => {
      setTimeout(() => {
        res('1.1');
      }, 1000);
    });
  }

  async function f2() {
    return new Promise(res => {
      setTimeout(() => {
        res('2.1');
      }, 100);
    });
  }

  function runParallel(promises) {
    // implementation here
  }

  for await (const results of runParallel([f1(), f2()])) {
    console.log(results);
  }

  /**
   * should output 2 lines:
   *
   * [undefined, "2.1"] // (after 100 ms)
   * ["1.1", "2.1"] // (after 1000 ms)
   */
})();

01-async-generator.js

async function* runParallel(promises) {
  const result = Array(promises.length).fill(void 0);
  const resolved = new Set();

  promises.forEach((promise, index) => {
    promise.then(value => {
      result[index] = value;
      resolved.add(promise);
    });
  });

  while (promises.length !== resolved.size) {
    await Promise.race(promises.filter(promise => !resolved.has(promise)));
    yield result;
  }
}

02-async-iterator.js

function runParallel(promises) {
  const result = Array(promises.length).fill(void 0);
  const resolved = new Set();

  promises.forEach((promise, index) => {
    promise.then(value => {
      result[index] = value;
      resolved.add(promise);
    });
  });

  const iterator = {
    next: async () => {
      const done = resolved.size === promises.length;

      if (done) {
        return { done: true };
      }

      await Promise.race(promises.filter(promise => !resolved.has(promise)));

      return {
        value: result,
        done: false
      };
    }
  };

  return { [Symbol.asyncIterator]: () => iterator };
}

Neat!