Magical String LeetCode solution in JavaScript

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var magicalString = function (n) {
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  const stack = ["1", "2", "2"];
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  let magic = 2;
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  while (stack.length < n) {
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    const count = stack[magic++];
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    const last = stack[stack.length - 1];
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    const addStr = last === "1" ? "2" : "1";
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    for (let n = 1; n <= count; n++) stack.push(addStr);
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  }
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  return stack.slice(0, n).filter((str) => str === "1").length;
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};