index.js

const maxLetterInSegmentNumber = 26;

module.exports.getColumnNumber = function getColumnNumber(columnName) {
  const letters = columnName.toUpperCase().split("");

  const columnNumber = letters.reduce((columnNumber, letter, index) => {
    const reverseIndex = letters.length - 1 - index;

    const letterNumber = getLetterNumber(letter);

    if (reverseIndex === 0) {
      return columnNumber + letterNumber;
    }

    return (
      columnNumber +
      (letterNumber + 1) * Math.pow(maxLetterInSegmentNumber, reverseIndex)
    );
  }, 0);

  return columnNumber;
};

function getLetterNumber(letter) {
  const differenceBetweenAsciiAndAlphabetStart = 65;
  return letter.charCodeAt(0) - differenceBetweenAsciiAndAlphabetStart;
}