If you have service reference (e.g. to an AJAX Enabled WCF Service) in your ScriptManager (or ToolkitScriptManager):

<asp:ToolkitScriptManager ID="ToolkitScriptManager1" runat="server">
    <Services>
        <asp:ServiceReference Path="~/MotoService.svc" />
    </Services>
</asp:ToolkitScriptManager>

and your are using an external JavaScript file you can enable JavaScript intellisense by adding the following 2 lines to the top of your .js (replacing ~/MotoService.svc with the path to your own service):

/// <reference name="MicrosoftAjax.js" />
/// <reference path="~/MotoService.svc" />

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