First Unique Character in a String LeetCode solution in JavaScript

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/**
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 * @param {string} s
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 * @return {number}
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 */
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var firstUniqChar = function (s) {
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  let dec = {};
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  for (let i = 0; i < s.length; i++) {
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    if (dec[s[i]]) {
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      dec[s[i]]["counter"]++;
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    } else {
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      (dec[s[i]] = {}), (dec[s[i]]["counter"] = 1);
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      dec[s[i]]["index"] = i;
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    }
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  }
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  for (let i = 0; i < s.length; i++) {
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    if (dec[s[i]]["counter"] == 1) {
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      return dec[s[i]]["index"];
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      break;
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    }
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  }
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  return -1;
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};