I've asked a question on forum:
AJAX ScriptManager Class has a IsDebuggingEnabled property, that is available in server code. I want in my client Javascript function to determine, do I running debug or release mode.
MS Online Community Support suggested :
You can create a hidden field to the page, then, check the IsDebuggingEnabled property of ScriptManager , if it is true, set the value of the hidden field to "true", else, false.
It will work, but I expected the method available in some AJAX client library class.