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class IterableBackwards {
  constructor(xs) {
    this._xs = xs;
    this._i = xs.length;
  }

  [Symbol.iterator]() { return this; }

  next() {
    return (this._i <= 0)
      ? { done: true, value: undefined }
      : { done: false, value: this._xs[--this._i] };
  }
}

const xs = ['A1', 'A2', 'A3'];

for (const x of new IterableBackwards(xs)) {
  console.info(x);
}