incrementally convert from numbers to letters or from letters to number

number-to-letter-to-number.js

/*
These javascript functions allow for converting from numbers to letters or from letters to number

I am using these functions to increment channels in a firebase realtime db using numbers as letters
going beyond 26 letters in the alphabet

For example 0 => A or 26 => AA similar to spreadsheet naming

numberToLetters(123) => "DT"
lettersToNumber('DT') => 123
*/

/**
 * Converts a number to letters
 * 
 * @param { Number } num - some number
 * @returns { string } - a string representing the number. for example 123 will return "DT"
 */
function numberToLetters(num) {
  const alphabet = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ';
  const base = alphabet.length;
  let letters = '';
  while (num >= 0) {
    letters = alphabet[num % base] + letters;
    num = Math.floor(num / base) - 1;
  }
  return letters;
}

/**
 * Convert letters to a number
 * 
 * @param { string } letters - some letters
 * @returns { Number } - a number representing the letters. for example "DT" will return 123
 */
function lettersToNumber(letters) {
  const alphabet = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ';
  const base = alphabet.length;
  let num = 0;

  letters.split('').reverse().forEach((letter, index) => {
    const letterIndex = alphabet.indexOf(letter);
    if (index === 0) {
      num += letterIndex;
    } else {
      num += Math.pow(base, index) * (letterIndex + 1);
    }
  });
  return num;
}