Excel Sheet Column Number LeetCode solution in JavaScript

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/**
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 * @param {string} columnTitle
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 * @return {number}
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 */
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var titleToNumber = function (columnTitle) {
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  /*
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        one letter: result between 1-26.
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        two letter: result between 26^1 + 1 -> 26^2 + digit. 27 - 702. All the combinations of A-Z and A-Z. 
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    */
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  let sum = 0;
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  for (let letter of columnTitle) {
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    let d = letter.charCodeAt(0) - "A".charCodeAt(0) + 1;
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    sum = sum * 26 + d;
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  }
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  return sum;
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};