Longest Happy Prefix LeetCode solution in JavaScript

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var longestPrefix = function (s) {
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  const len = s.length;
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  const z = new Array(len).fill(0);
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  let l = 0,
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    r = 0;
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  for (let i = 1; i < len; i++) {
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    if (i <= r) z[i] = Math.min(r - i + 1, z[i - l]);
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    while (i + z[i] < len && s[z[i]] == s[i + z[i]]) {
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      z[i]++;
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    }
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    if (i + z[i] - 1 > r) {
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      r = i + z[i] - 1;
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      l = i;
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    }
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  }
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  let idx = -1;
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  for (let i = 1; i < len; i++) {
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    if (i + z[i] == len) {
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      idx = i;
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      break;
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    }
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  }
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  if (idx == -1) return "";
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  return s.slice(idx);
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};