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"/>



![I think this image tells the whole story][1]


  [1]: https://i.sstatic.net/STg5Z.png