you can place two ways to access javascript file
In master page
<head runat="server">
<script src="Registration.js" type="text/javascript"></script>
</head>
In Content Page
<asp:content id="Content1" contentplaceholderid="head" runat="server" xmlns:asp="#unknown">
<script src="Registration.js" type="text/javascript"></script>
</asp:content>
<asp:content id="Content2" contentplaceholderid="head" runat="server" xmlns:asp="#unknown">
</asp:content>