selectAllButton

defines whether the Select All button should be shown

selectAllButton?: boolean;

Example

const combo = new dhx.Combobox("combo_container", {
    // enables multiple selection of options
    multiselection:true,
    // shows the Select All button
    selectAllButton: true
});

Related sample: Combobox. Multiselection, add new options (free text), select all button

note

To be able to select several options you need to enable the multiselection option.