How to find control id from user control

See more:

ASP.NET

user-controls

I have a web user control which contains 'AssociatedLableId' property and a label.

<uc1:UC id="uc1" run="Ser" AssociatedLableId="lblLable" />

and added a Label and a button in aspx page and registered User Control .
Now when I click the button then the text of label control of aspx page assigned to label control of web user control using JS.
I tried following but not right.

<asp:button id ="btn" run="Ser" text="Click" onClientClick="return assignValue();" />
function assignValue()
{
     document.getElementById('UserControlsAssocitedControlId').value = document.getElementById('aspxPageLableId').value;
}

AssociatedLableId property is used to get the Id of aspx page's Label control.
I can find the Id of UserControlsAssocitedControlId but how can I find the aspxPageLableId which I assigned in the AssociatedLableId property in JS.