The aspnet_client directory contains JavaScript files that are used by the ASP.NET runtime for client-side validation functionality using the ASP.NET server validation controls. This directory is created automatically when ASP.NET is enabled on your site. This directory is not required for ASP.NET applications to run. It is only required if you are using client-side validation with ASP.NET server validation controls. However, we recommend you leave this directory intact.
If you are using ASP.NET validation controls with client-side validation enabled and your aspnet_client directory is missing, your users may encounter errors like the following when browsing your site:
Note: The above example shows the Internet Explorer Error. The error may appear differently in other browsers.