[Erolast]

Так?

var pos = 0;
var arr_A = ['A','B','C'];
var arr_B = ['Z','Y','X'];
  
function find_with_mask(mask)
{
  var result = "";
	for (var i in mask)
	{
      	var char = mask[i];
		result += window["arr_"+char][pos];
	}
  pos++;
  return result;
}

alert(find_with_mask('AABA'));
alert(find_with_mask('AABA'));
alert(find_with_mask('AABA'));

http://learn.javascript.ru/play/Zguugc