Enable Disable Javascript for tables

function enabledisabletable(item){
var str=item.id;
var formula_text = 'f07_' + str.substr(4,4);
var qty_text = 'f05_' + str.substr(4,4);
disableItems('document.getElementById("' + str + '").value==\'N\'',formula_text);
disableItems('document.getElementById("' + str + '").value==\'Y\'',qty_text);
}