Flatten Nested List Iterator LeetCode solution in JavaScript

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/**
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 * // This is the interface that allows for creating nested lists.
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 * // You should not implement it, or speculate about its implementation
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 * function NestedInteger() {
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 *
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 *     Return true if this NestedInteger holds a single integer, rather than a nested list.
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 *     @return {boolean}
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 *     this.isInteger = function() {
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 *         ...
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 *     };
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 *
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 *     Return the single integer that this NestedInteger holds, if it holds a single integer
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 *     Return null if this NestedInteger holds a nested list
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 *     @return {integer}
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 *     this.getInteger = function() {
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 *         ...
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 *     };
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 *
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 *     Return the nested list that this NestedInteger holds, if it holds a nested list
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 *     Return null if this NestedInteger holds a single integer
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 *     @return {NestedInteger[]}
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 *     this.getList = function() {
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 *         ...
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 *     };
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 * };
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 */
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/**
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 * @constructor
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 * @param {NestedInteger[]} nestedList
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 */
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var NestedIterator = function (nestedList) {
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  this.nestedList = [];
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  this.ptr = 0;
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  const dfs = (elem) => {
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    if (!elem.isInteger()) {
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      let list = elem.getList();
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      for (let i = 0; i < list.length; i++) {
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        dfs(list[i]);
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      }
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    } else {
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      this.nestedList.push(elem.getInteger());
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    }
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  };
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  for (let i = 0; i < nestedList.length; i++) {
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    dfs(nestedList[i]);
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  }
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};
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/**
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 * @this NestedIterator
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 * @returns {boolean}
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 */
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NestedIterator.prototype.hasNext = function () {
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  if (this.ptr < this.nestedList.length) return true;
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  else return false;
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};
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/**
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 * @this NestedIterator
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 * @returns {integer}
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 */
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NestedIterator.prototype.next = function () {
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  return this.nestedList[this.ptr++];
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};
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/**
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 * Your NestedIterator will be called like this:
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 * var i = new NestedIterator(nestedList), a = [];
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 * while (i.hasNext()) a.push(i.next());
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 */