I see what you mean. Pasting "myinvalidvar === undefined" into Firebug's console does indeed throw and error. If you use "use strict", you shouldn't be able to use a variable that hasn't been defined.
http://msdn.microsoft.com/en-us/library/ie/br230269(v=vs.94).aspx#rest
M.
http://msdn.microsoft.com/en-us/library/ie/br230269(v=vs.94).aspx#rest
M.