Inject JavaScript into Umbraco Admin pages

InjectJavascriptIntoUmbracoAdminPage.cs

public class Global : HttpApplication
{
  protected void Application_Start(object sender, EventArgs e)
  {
    umbracoPage.Load += InjectJavascript;
  }
  
  private void InjectJavascript(object sender, EventArgs eventArgs)
  {  
    var page = sender as umbracoPage;
    if (page != null && page.IsEditorPage()) {
      page.Script('/path/to/your.js');
      page.Css('/path/to/your.css');
    }
  }
}

UmbracoPageExtensions.cs

    public static class UmbracoPageExtensions
    {
        public static bool IsEditorPage(this umbracoPage page)
        {
            return page.Request.Path == "/umbraco/editContent.aspx";
        }
        
        public static void Script(this umbracoPage page, string path)
        {
            page.Controls.Add(new LiteralControl(string.Format(@"<script src=""{0}""></script>", path)));
        }


        public static void Css(this umbracoPage page, string path)
        {
            page.Controls.Add(new LiteralControl(string.Format(@"<link rel=""stylesheet"" href=""{0}"">", path)));
        }
    }