How to retrieve all data of a kendo ui dropdown list?

Can you try :

var data = $("#dropDownList").data("kendoDropDownList");

Try this it will retrive all values from Kendo dropdown.

var values = [];
    var grid = $("#SampleDropdown").data("kendoDropDownList");

    var ds = grid.dataSource;
    var len = ds._data.length;
    if (len > 0) {
        var i;

        for (i = 0; i < len; i++) {
            var val = ds._data[i].Value;
            values.push(val);
        }
      }

If you want the actual DataItems from the DDL you can get them by Viewing the DataSource:

$("#dropDownList").data("kendoDropDownList").dataSource.view()

You can then also find individual items easily by id if you need to:

$("#dropDownList").data("kendoDropDownList").dataSource.view().find(x=>x.Value === 'ID')