Global Variables in JavaScript?

I'm new to Javascript and the following code is confusing
the hell out of me. By my understanding, you can assign values to global variables by
dropping the var in front of them, and you don't need to define any initial value when
you declare the global in the first place. However, why is the following code not
working? (By not working I mean that the call to document.write within main() spits out
"undefined" for "ulat").

var
            ulat;

function write(latitude) {
 ulat =
            latitude;
 document.write("
 This works:" + ulat +
            "
");
}


function getloc()
            {

 if (navigator.geolocation) {

            navigator.geolocation.getCurrentPosition(
 function(position) {

            write(position.coords.latitude);
 },
 function (error) {

            switch(error.code) {

 case error.TIMEOUT:
 alert
            ('Timeout');
 break;
 case error.POSITION_UNAVAILABLE:

            alert ('Position unavailable');
 break;
 case
            error.PERMISSION_DENIED:
 alert ('Permission denied');

            break;
 case error.UNKNOWN_ERROR:

 alert ('Unknown
            error');
 break;
 }
 });


            }
 else {
 alert("Geolocation is not supported by your web
            browser."); 
 }
            
}



function main() {

            getloc();
 document.write("
 This doesn't work: " + ulat +
            "
");
}

Any
help? Thanks!