JQuery :: How To Write Global Variables In Functions

I cannot findout how I can write to global variables in a a JQUERY function.
<script type="text/javascript">
function checkusername (username ) {
$.post("[URL]", {
action: 5, username: username },
function(xml) {
result = $("apot", xml).text();
//<-- this variable is not global... why ??
message = $("message", xml).text();
//<-- this variable is not global... why ??
alert(message); //<--here variables are shown ok
alert(result);
});
alert(message);
//<--here variable is NULL !!
alert(result);
//<--here variable is NULL !!
}
</sctipt>