Iterator for Combination LeetCode solution in JavaScript

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var CombinationIterator = function (characters, combinationLength) {
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  this.stack = Array.from({ length: combinationLength }, (_, i) => i);
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  this.combinationLength = combinationLength;
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  this.characters = characters;
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};
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CombinationIterator.prototype.next = function () {
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  const word = this.stack.map((i) => this.characters[i]).join("");
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  for (
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    let lastIndex = this.characters.length - 1;
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    this.stack.at(-1) == lastIndex;
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  )
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    lastIndex = this.stack.pop() - 1;
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  if (this.stack.length > 0)
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    for (
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      let i = this.stack.pop() + 1;
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      this.stack.length < this.combinationLength;
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      i++
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    )
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      this.stack.push(i);
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  return word;
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};
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CombinationIterator.prototype.hasNext = function () {
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  return this.stack.length > 0;
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};