Sys is undefined when registering script via RegisterClientScriptBlock

Writing web applications using ASP.NET, most often than not, requires you to implement JavaScript code. If you are using AJAX and embed the script code as part of your page rendering you may run into the vague Sys is undefined error.

I have found that the error Sys is undefined is often caused when registering script using RegisterClientScriptBlock. The reason is that the AJAX Script Manager has yet to initialize. When using RegisterClientScriptBlock the script is placed immediately after the start of the form tag. In contrast, RegisterStartupScript places the script just before the closing form tag.

Therefore, the solution for this issue is to use RegisterStartupScript instead of RegisterClientScriptBlock.