toAlphaNumber() - Converts a positive integer into an alphabetic representation, similar to ordered list lettering and Excel column naming.

toAlphaNumber.js

/**
 * Converts a positive integer into an alphabetic representation, similar to
 * ordered list lettering and Excel column naming.
 *
 * @param {number} int
 *   The positive integer to convert. Non-integers will be floored.
 * @param {boolean} [returnLowerCase=false]
 *   If `true`, returns lowercase letters (e.g., 'a', 'b', ...).
 *   If `false`, returns uppercase letters (e.g., 'A', 'B', ...).
 * @returns {string}
 *   The alphabetic representation of the integer (e.g., 1 -> 'A', 28 -> 'AB').
 * 
 * @throws {Error}
 *   Throws an error if the input integer is less than 1.
 *
 * @example
 * toAlphaNumber(1); // Returns "A"
 * toAlphaNumber(28); // Returns "AB"
 * toAlphaNumber(28, true); // Returns "ab"
 * toAlphaNumber(0); // Throws Error
 */
function toAlphaNumber(int, returnLowerCase=false) {
  if (!(int > 0 && int === Math.floor(int))) {
    throw new Error('Input must be a positive integer greater than 0.');
  }
  const ASCII_OFFSET = returnLowerCase ? 97 : 65;  // 97 for 'a', 65 for 'A'
  let result = '';
  while (int > 0) {
    result = String.fromCharCode(ASCII_OFFSET + (int - 1) % 26) + result;
    int = Math.floor((int - 1) / 26); // Adjust for 1-indexing
  }
  return result;
}