RazorJavaScriptSupport.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Compilation;
using System.Web.Mvc;
using System.Web.Razor;
using System.Web.WebPages.Razor;

[assembly: PreApplicationStartMethod(typeof(AshMind.Web.Mvc.Razor.RazorJavaScriptSupport), "SetupAll")]

namespace AshMind.Web.Mvc.Razor {
    public static class RazorJavaScriptSupport {
        private static bool setupPerformed;

        public static void SetupAll() {
            if (setupPerformed)
                return;

            var razor = ViewEngines.Engines.OfType<RazorViewEngine>().SingleOrDefault();
            if (razor != null) {
                razor.FileExtensions = AppendJavaScriptVariants(razor.FileExtensions);
                razor.MasterLocationFormats = AppendJavaScriptVariants(razor.MasterLocationFormats);
                razor.PartialViewLocationFormats = AppendJavaScriptVariants(razor.PartialViewLocationFormats);
                razor.ViewLocationFormats = AppendJavaScriptVariants(razor.ViewLocationFormats);
            }
             
            BuildProvider.RegisterBuildProvider(".csjs", typeof(RazorBuildProvider));
            BuildProvider.RegisterBuildProvider(".vbjs", typeof(RazorBuildProvider));

            RazorCodeLanguage.Languages.Add("csjs", new CSharpRazorCodeLanguage());
            RazorCodeLanguage.Languages.Add("vbjs", new VBRazorCodeLanguage());

            setupPerformed = true;
        }

        private static string[] AppendJavaScriptVariants(string[] razorValues) {
            return razorValues.Concat(
                razorValues.Select(v => v.Replace("html", "js"))
            ).Distinct().ToArray();
        }
    }
}

Hey can this be used???

Why not? Though I haven't tested it for a long time now and also Intellisense does not understand it.