Integer to English Words LeetCode solution in JavaScript

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let numberMap = {
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  0: "Zero",
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  100: "Hundred",
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  1: "One",
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  2: "Two",
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  3: "Three",
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  4: "Four",
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  5: "Five",
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  6: "Six",
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  7: "Seven",
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  8: "Eight",
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  9: "Nine",
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  10: "Ten",
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  11: "Eleven",
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  12: "Twelve",
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  13: "Thirteen",
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  14: "Fourteen",
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  15: "Fifteen",
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  16: "Sixteen",
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  17: "Seventeen",
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  18: "Eighteen",
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  19: "Nineteen",
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  20: "Twenty",
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  30: "Thirty",
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  40: "Forty",
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  50: "Fifty",
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  60: "Sixty",
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  70: "Seventy",
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  80: "Eighty",
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  90: "Ninety",
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};
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let bigNumberMap = {
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  1000000000: "Billion",
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  1000000: "Million",
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  1000: "Thousand",
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  1: "One",
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};
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let bases = [];
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for (let base in numberMap) {
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  bases.push(parseInt(base, 10));
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}
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bases = bases.sort((a, b) => b - a);
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let bigBases = [];
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for (let base in bigNumberMap) {
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  bigBases.push(parseInt(base, 10));
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}
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bigBases = bigBases.sort((a, b) => b - a);
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var numberToWords = function (num) {
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  let res = "";
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  if (num === 0) return numberMap[num];
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  bigBases.forEach((base) => {
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    base = parseInt(base, 10);
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    let baseAsString = base + "";
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    let nums = (num / base) << 0;
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    let remainder = num - nums * base;
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    if (nums >= 1) {
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      [res, nums] = getForHundredBase(res, nums);
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      res += baseAsString.length > 1 ? " " + bigNumberMap[base + ""] : "";
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      num = remainder;
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    }
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  });
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  return res;
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};
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function getForHundredBase(res, num) {
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  bases.forEach((base) => {
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    base = parseInt(base, 10);
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    [num, res] = getNums(num, base, res);
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  });
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  return [res, num];
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}
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function getNums(num, base, res) {
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  let nums = (num / base) << 0;
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  let baseAsString = base + "";
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  if (nums > 1 || (nums == 1 && baseAsString.length > 2)) {
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    res += res === "" ? res : " ";
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    res += numberMap[nums] + " " + numberMap[baseAsString];
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  } else if (nums == 1) {
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    res += res === "" ? res : " ";
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    res += numberMap[baseAsString];
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  }
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  return [num - nums * base, res];
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}