Finally I got answer from this one http://sharepointkids.blogspot.in/2014/04/two-level-cascading-drop-down-in.html
I have just added my js cdn to the seattle.master
if we want to use our own js file mean we have two options, one is *.js should inserted to custom library and then write the script link on seattle.master or else we have copy to this location C:\Program Files\Common Files\microsoft shared\Web Server Extensions\15\TEMPLATE\LAYOUTS\1033 then add
<SharePoint:ScriptLink language="javascript" name="(name only not full path)*.js" runat="server"/>
