JavaScript

//selectBox is the selectbox ID, selectall is a boolean whether to select all elements,
//ensureOneSelected applies when false is specified for selectall
//it then ensures that the first element is always selected.

<script>

function selectAll(selectBox, selectAll, ensureOneSelected) {
    // have we been passed an ID
    var selectBoxElement = null;
    var selectBoxOption = null;

    var boolSelect = selectAll;

    if (typeof selectBox == "string") {
        selectBoxElement = document.getElementById(selectBox);
        selectBoxOption = selectBoxElement.options;
    }
    // is the select box a multiple select box?
    if (selectBoxElement.type == "select-multiple") {
        var max = (selectBoxOption.length);

        var modMaxUnrollFactor = (max % 4);

        max -= modMaxUnrollFactor ;

        for(var i = 0; i < max; i+=4){
            selectBoxOption[i].selected = boolSelect;
            selectBoxOption[i + 1].selected = boolSelect;
            selectBoxOption[i + 2].selected = boolSelect;
            selectBoxOption[i + 3].selected = boolSelect;

        }
        for (var i = max; i < selectBoxOption.length; i++) {
            selectBoxOption[i].selected = boolSelect;
        }

        if (typeof ensureOneSelected == "boolean") {
            if(selectBoxOption[0].selected == false)
            {
                selectBoxOption[0].selected = ensureOneSelected;
            }
        }
    }
}
</script>