When set to true, renders a sticky "Select all" header at the top of the dropdown list. Clicking it selects all currently rendered items. When checkboxes is also enabled, the header shows a checkbox that reflects the aggregate selection state: unchecked (none selected), indeterminate (some selected), or checked (all selected).

The built-in click behavior can be overridden by handling the preventable selectAllChange event.

Example - enable the selectAll header

<select id="multiselect" multiple="multiple">
    <option>Item1</option>
    <option>Item2</option>
    <option>Item3</option>
</select>
<script>
$("#multiselect").kendoMultiSelect({
    selectAll: true,
    checkboxes: true
});
</script>