ASP.NET MVC ySlow Add Expires Headers for dynamically served script

I am serving Javascript vai a controller method that looks like this:

  [OutputCache(Duration = 120)]
  [Compress]
  public ActionResult JavascriptFile(String scriptName) {
     string content;
     if (HttpContext.IsDebuggingEnabled) {
        content = ReadApplicationScript(string.Format("~/scripts/{0}", scriptName));
        return Content(content, "application/javascript");
     }
     else {
        content = ReadApplicationScript(string.Format("~/scripts/Built/{0}", scriptName));
        return Content(content, "application/javascript");
     }
  }

The Compress attribute is from here.

When I run ySlow I get an F grade on "Add Expires headers". What can I do to add these?

Add the following inside system.webServer section

<staticContent>
      <clientCache cacheControlMode="UseExpires"
         httpExpires="Tue, 19 Jan 2038 03:14:07 GMT" />
    </staticContent>

Here the httpExpires value will be the expiration date.

Edit

You can also try adding the content to cache like this:

var cacheName = "someName";
var value = HttpRuntime.Cache.Get(cacheName) as ContentResult;

            if (value == null)
            {                
                var contentResult = ReadApplicationScript(string.Format("~/scripts/{0}",scriptName));
                System.Web.HttpContext.Current.Cache.Insert(cacheName, contentResult );
                return contentResult;
            }

            return value;