Array of column Letters to Array of Column Numbers not working

This is more or less my first attempt at writing a Javascript function and I want to convert an array of column numbers to an array of column letters

If I run testFunction I get undefined

function testFunction() {
  var ui = SpreadsheetApp.getUi();

   aryCLTCN(["A","C","D"])

   ui.alert(aryCLTCN[3]);
}

function aryCLTCN(array) {
  var columnLet = array
  var output = [];

  for (var i = 0, length = columnLet.length; i < length; i++) {
    output[i] = [];
    output[i] = CLTCN(columnLet[(i)]);
    }
}

function CLTCN(letter)
{
  var column = 0, length = letter.length;
  for (var i = 0; i < length; i++)
  {
    column += (letter.charCodeAt(i) - 64) * Math.pow(26, length - i - 1);
  }
  retu column;
}