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<!DOCTYPE html>
<html>
<head>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width">
  <title>JS Bin</title>
</head>
<body>
​
</body>
</html>
 
const countdown = {
  [Symbol.iterator]() {
    const iterator = {
      value: 10,
      done: false,
      next() {
        this.done = this.done || this.value < 0;
​
        if (this.done) {
          return { done: true };
        } else {
          return { done: false, value: this.value-- };
        }
      },
      return(value) {
        console.log('Return to Forever')
        this.done = true;
        if (arguments.length === 1) {
          return { done: true, value };
        } else {
          return { done: true };
        }
      }
    };
​
    return iterator;
  }
};
​
function zipWith (zipper, ...iterables) {
  return {
    [Symbol.iterator]() {
      return {
        done: false,
        iterators: iterables.map(i => i[Symbol.iterator]()),
        zipper,
        next() {
          const pairs = this.iterators.map(j => j.next()),
                dones = pairs.map(p => p.done),
                values = pairs.map(p => p.value);
​
          if (dones.indexOf(true) >= 0) {
            return this.return();
          } else {
            return { done: false, value: this.zipper(...values) };
          }
        },
        return(optionalValue) {
          if (!this.done) {
            this.done = true;
​
            for (const iterable of this.iterators) {
              if (typeof iterable.return === 'function') {
                iterable.return();
              }
            }
          }
​
          if (arguments.length === 1) {
            return { done: true, value:optionalValue };
          } else {
            return { done: true };
          }
        }
      };
    }
  };
}
const fewWords = ['alper', 'bethe', 'gamow'];
​
​
const [[firstCount, firstWord]] = zipWith((l, r) => [l, r], countdown, fewWords);
  //=> Return to Forever
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