Number of Atoms LeetCode solution in JavaScript

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var countOfAtoms = function (formula) {
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  function getElementCount(flatFormula) {
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    const reElementCount = /([A-Z][a-z]*)(\d*)/g;
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    let matches = flatFormula.matchAll(reElementCount);
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    let output = new Map();
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    for (let [match, element, count] of matches) {
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      count = count === "" ? 1 : +count;
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      if (output.has(element)) {
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        output.set(element, output.get(element) + count);
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      } else {
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        output.set(element, count);
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      }
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    }
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    return output;
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  }
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  function flattenElementCount(elCountMap) {
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    let arr = [...elCountMap.entries()].sort();
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    return arr.reduce((str, item) => {
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      let num = item[1] === 1 ? "" : item[1];
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      return str + item[0] + num;
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    }, "");
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  }
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  function unnest(fullMatch, innerFormula, multiplier) {
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    let elementCount = getElementCount(innerFormula);
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    multiplier = multiplier === "" ? 1 : +multiplier;
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    for (let [element, count] of elementCount) {
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      elementCount.set(element, elementCount.get(element) * multiplier);
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    }
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    return flattenElementCount(elementCount);
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  }
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  function flattenFormula(formula) {
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    const reNested = /\((\w+)\)(\d*)/g;
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    while (reNested.test(formula)) {
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      formula = formula.replaceAll(reNested, unnest);
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    }
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    return formula;
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  }
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  let flatFormula = flattenFormula(formula);
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  return flattenElementCount(getElementCount(flatFormula));
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};