Mini Parser LeetCode solution in JavaScript

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var deserialize = function (s) {
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  if (s[0] !== "[") return new NestedInteger(s);
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  const stack = [];
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  let start = 0;
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  const process = {
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    "["(index) {
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      stack.push(new NestedInteger());
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      start = index + 1;
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    },
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    ","(index) {
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      if (index > start) {
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        const value = s.slice(start, index);
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        stack[stack.length - 1].add(new NestedInteger(value));
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      }
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      start = index + 1;
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    },
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    "]"(index) {
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      this[","](index);
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      if (stack.length < 2) return;
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      const last = stack.pop();
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      stack[stack.length - 1].add(last);
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    },
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  };
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  [...s].forEach((current, index) => process[current]?.(index));
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  return stack[0];
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};