getColumnName = function (columnNumber){
    let dividend = columnNumber+1;
    let columnName = "";
    let modulo;

    while (dividend > 0) {
        modulo = (dividend - 1) % 26;
        columnName = String.fromCharCode(65 + modulo).toString() + columnName;
        dividend = parseInt((dividend - modulo) / 26);
    }

    return  columnName;
}
Get letter based on a number

@param {number} columnNumber
@return string letter