Merge async iterators

merge.js

(async function() {
  console.clear();

  class Deferred {
    constructor() {
      this.promise = new Promise((resolve, reject) => {
        this.resolve = resolve;
        this.reject = reject;
      });
    }
  }

  function wait(time) {
    return new Promise(resolve => setTimeout(resolve, time));
  }

  // ---

  async function* asyncCounter(time = 1000, maxIterations = 3) {
    let count = 0;
    
    while (count < maxIterations) {
      await wait(time);
      console.log('counter', count);
      yield count++;
    }
  }

  // ---

  const merge = (collection) => {
    let nextValue = new Deferred();
    const it = collection[Symbol.asyncIterator]();

    return {
      [Symbol.asyncIterator]() {
        return this;
      },
      // Synchronously proxy the iterator
      async next() {
        console.log('before');
        
        it.next().then(val => {
          const currentValue = nextValue;
          nextValue = new Deferred();
          currentValue.resolve(val)
        });

        return nextValue.promise;
      }
    }
  }; 

  for await (const a of merge(asyncCounter(0))) {
    console.log('MAIN', a);
  }

})().then(()=> console.log('async complete'));