How to Validation Client Side in Combobox using javascript?

Question

How to Validation Client Side in Combobox using javascript?

Example:

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<asp:ComboBox ID="ddlPickupPlace" runat="server" AutoCompleteMode="SuggestAppend"

                                                CaseSensitive="false" DropDownStyle="DropDownList" ItemInsertLocation="Append"

                                                ListItemHoverCssClass="ComboBoxListItemHover" RenderMode="Inline" Width="180px"

                                                AutoPostBack="true" OnSelectedIndexChanged="ddlPickupPlace_SelectedIndexChanged"

                                                TabIndex="1">

</asp:ComboBox>