Cannot get ID of modalpopup in javascript

I have an update panel in my page. But modal popup is outside of that update panel. When i try to get modal popup from javascript either using document.getElementById or $g开发者_StackOverflow社区et or $find , it returns null, i.e. it cannot find that modal popup.

rough structural position of update panel and modal popup

<update panel>
</update panel>

<modal popup>
</modal popup>

<update panel>
</update panel>

<java script>
</java script>

How to resolve this issue.

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This work well for me.

 var ModalAction = '<%= ActionModalPopup.ClientID %>';
 $find(ModalAction).hide();