Seleciona todos os valores do ListBox

function selectAllListBox(formName, filterId) {
var check = document.getElementById("filter_" + filterId + "_all");
var select = document.getElementById("filter_" + filterId);
var selected = true;
var options = select.options;
var i = 0;
while (i < options.length && selected) {
var option = options.item(i);
selected = option.selected;
i++;
}
check.checked = selected;
}