convert column index to column name ( 1 -> 'A' )

// require:
const {columnName} = require('./columnName');
const {A1Notation} = require('./A1Notation');

;[
    // column name
    columnName(4),                // "D"
    columnName(27),               // "AA"
    columnName(26*26 + 26),       // "ZZ"
    columnName(26**2 + 2*26 + 3), // "ABC"
    
    // A1 notation
    A1Notation(2, 4),             // "D2"
    A1Notation(3, 27),            // "AA3"
    A1Notation(3, 26*26 + 26),    // "ZZ3"

].forEach(x => console.log(x));

main.js

/**
 *  In main file
 *  let {columnName} = require('./columnName');
 *  console.log(columnName(12));
 */
 
 /**
 * convert column index to column name (A1 Notation).
 *
 * @param {number} col - column number of cell. (A = 1)
 * @returns {string} 
 *
 * @example
 *
 *   columnName(4)           // "D"
 *   columnName(27)          // "AA"
 *   columnName(26*26 + 26)  // "ZZ"
 *
 */
function columnName(col) {

    // integer division
    function intDiv(a, b){
        return [ Math.floor(a / b), a % b ];    // quotient, remaider
    }
    
    // 1 -> A, 2 -> B, ... 26 -> Z
    function alphabet(i){
        if ( i === 0 ) i = 26;    // ⭐️ 0 -> Z
        return String.fromCharCode(i - 1 + 'A'.charCodeAt()) 
    }

    const Z = 26;                     

    let name = ``;

    // example 1: n = ABC = A*Z² + B*Z + C
    //   n % Z = C 
    //   n / Z = A*Z + B = AB
    //
    // example 2: n = ZZ = Z*Z + Z
    //   n % Z = 0 (⭐️ which means the last digit is 'Z')
    //   n / Z = Z + 1 (⭐️ this extra 1 must be removed)
    let n = col;

    while (n >= 1) {
        const [q, r] = intDiv(n, Z);
        name = alphabet(r) + name;
        n = q - (r === 0 ? 1 : 0);    // ⭐️ 
    }

    return name;
}




module.exports = { columnName };

columnName.js

/**
 *  In main file
 *  let {A1Notation} = require('./A1Notation');
 *  console.log(A1Notation(1, 2));
 */
 
 const {columnName} = require('./columnName');
 
 /**
 * convert (row, col) to A1 Notation (String)
 *
 * @param {number} row - row number of cell.
 * @param {number} col - column number of cell.
 * @returns {string} 
 *
 * @example
 *
 *   A1Notation(2, 4)        // "D2"
 *   A1Notation(3, 27)       // "AA3"
 */
function A1Notation(row, col) {
    return `${columnName(col)}${row}`
}

// exports
module.exports = { A1Notation };