Finally i got a solution how can i call java-script function on dropdownextender
closing.
We can call javascript function by using this.
<ScriptAction Script="alert('Dropdown is colsing');" />
Complete code
<ajaxToolkit:DropDownExtender runat="server" ID="DDE" TargetControlID="txtDisplay"
DropDownControlID="DropPanel" HighlightBorderColor="Transparent" OnClientPopup="OnClientPopup" >
<Animations>
<OnShow>
<Sequence>
<HideAction Visible="true" />
<FadeIn Duration=".5" Fps="10" />
</Sequence>
</OnShow>
<OnHide>
<Sequence>
<FadeOut Duration=".5" Fps="10" />
<HideAction Visible="false" />
<StyleAction Attribute="display" Value="none"/>
<ScriptAction Script="alert('Dropdown is colsing');" />
</Sequence>
</OnHide>
</Animations>
</ajaxToolkit:DropDownExtender>
Now my problem is solved.