Replace All ?'s to Avoid Consecutive Repeating Characters LeetCode solution in JavaScript

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var modifyString = function (s) {
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  let t = "";
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  for (let i = 0; i < s.length; i++) {
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    if (s[i] == "?") {
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      let j = "a";
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      while (t[i - 1] == j || s[i + 1] == j) {
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        j = String.fromCharCode(j.charCodeAt() + 1);
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      }
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      t += j;
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    } else {
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      t += s[i];
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    }
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  }
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  return t;
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};