value of the DropDownList which is in Usercontrol from the Master page using Javascript
Once the whole page is constructed, there is no master page-content page-user control in HTML format. All are clubbed together and a single page in HTML is formed. Thus, accessing a control of usercontrol from master page would be same as accessing a control on master page.
Try:
var dllcontrol = document.getElementById("<%=ddlCity.ClientID %>");