Convert numbers to letters beyond the 26 character alphabet

I think you're looking for something like this

    function colName(n) {
        var ordA = 'a'.charCodeAt(0);
        var ordZ = 'z'.charCodeAt(0);
        var len = ordZ - ordA + 1;
      
        var s = "";
        while(n >= 0) {
            s = String.fromCharCode(n % len + ordA) + s;
            n = Math.floor(n / len) - 1;
        }
        return s;
    }

// Example:

    for(n = 0; n < 125; n++)
            document.write(n + ":" + colName(n) + "<br>");

This is a very easy way:

function numberToLetters(num) {
    let letters = ''
    while (num >= 0) {
        letters = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'[num % 26] + letters
        num = Math.floor(num / 26) - 1
    }
    return letters
}

function getColumnDescription(i) {
  const m = i % 26;
  const c = String.fromCharCode(65 + m);
  const r = i - m;
  return r > 0
    ? `${getColumnDescription((r - 1) / 26)}${c}`
    : `Column ${c}`
}

Usage:

getColumnDescription(15)
"Column P"

getColumnDescription(26)
"Column AA"

getColumnDescription(4460)
"Column FOO"

If you have your data in a two-dimensional array, e.g.

var data = [
  ['Day', 'score],
  ['Monday', 99],
];

you can map the rows/columns to spreadsheet cell numbers as follows (building on the code examples above):

function getSpreadSheetCellNumber(row, column) {
  let result = '';

  // Get spreadsheet column letter
  let n = column;
  while (n >= 0) {
    result = String.fromCharCode(n % 26 + 65) + result;
    n = Math.floor(n / 26) - 1;
  }

  // Get spreadsheet row number
  result += `${row + 1}`;

  return result;
};

E.g. the 'Day' value from data[0][0] would go in spreadsheet cell A1.

> getSpreadSheetCellNumber(0, 0)
> "A1"

This also works when you have 26+ columns:

> getSpreadSheetCellNumber(0, 26)
> "AA1"