Brace Expansion II LeetCode solution in JavaScript

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var braceExpansionII = function (expression) {
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  // , mutiplier = ['']
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  const char = ("{" + expression + "}").split("").values();
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  const result = [...rc(char)];
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  result.sort((a, b) => a.localeCompare(b));
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  return result;
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};
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function rc(char) {
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  const result = new Set();
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  let resolved = [""];
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  let chars = "";
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  let currentChar = char.next().value;
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  while (currentChar !== "}" && currentChar) {
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    if (currentChar === "{") {
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      resolved = mix(mix(resolved, [chars]), rc(char));
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      chars = "";
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    } else if (currentChar === ",") {
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      for (const v of mix(resolved, [chars])) {
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        result.add(v);
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      }
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      chars = "";
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      resolved = [""];
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    } else {
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      chars += currentChar;
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    }
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    currentChar = char.next().value;
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  }
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  for (const v of mix(resolved, [chars])) {
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    result.add(v);
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  }
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  return result; // everything
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}
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function mix(a, b) {
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  const result = [];
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  for (const ca of a) {
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    for (const cb of b) {
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      result.push(ca + cb);
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    }
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  }
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  return result;
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}