Javascript Listbox Filtering to rearrange the order of values

js-listbox-filtering.js

function listbox_move(listID, direction) {
 
    var listbox = document.getElementById(listID);
    var selIndex = listbox.selectedIndex;
 
    if(-1 == selIndex) {
        alert("Please select an option to move.");
        return;
    }
 
    var increment = -1;
    if(direction == 'up')
        increment = -1;
    else
        increment = 1;
 
    if((selIndex + increment) < 0 ||
        (selIndex + increment) > (listbox.options.length-1)) {
        return;
    }
 
    var selValue = listbox.options[selIndex].value;
    var selText = listbox.options[selIndex].text;
    listbox.options[selIndex].value = listbox.options[selIndex + increment].value
    listbox.options[selIndex].text = listbox.options[selIndex + increment].text
 
    listbox.options[selIndex + increment].value = selValue;
    listbox.options[selIndex + increment].text = selText;
 
    listbox.selectedIndex = selIndex + increment;
}