// var global; // note it is commented out
isset(global);

.. will result in a ‘global is not defined’ JS exception thrown. Instead, you need to remember to do either one of these:

isset(window.global); // these are equivalent
isset(window['global']); // these are equivalent

I don’t think there is any way to code for this in the isset() routine, since the exception is thrown before isset() is called. JS doesn’t seem to mind passing an undefined reference, as long as the highest-level piece (i.e. ‘window’ in this case) does exist. BTW, this will work the same if trying to test for the existence of a local variable … like

function func()
{
isset(loc); // will also throw an exception
}

since (i think) you can’t specifically reference local variables without targeting globals (though you can specifically reference globals via window.variable_name).

This is pretty basic stuff. Do we really have to go to these lengths?

var a = new Array();
isset(a['pop']); // returns true – as a.pop() exists

your routine will work for these cases (there are many) with this change:

function isset(varname)
{
var t=typeof(varname);
return(t != ‘undefined’ && t != ‘function’);
}