Jquery set HTML via ajax

Jquery set HTML via ajax

I have the following span:
<span class='username'> </span>
to populate this i have to get a value from PHP therefor i use Ajax:
$('.username').html(getUsername());
function getUsername(){
$.ajax({
type: 'POST',
url: myBaseUrl + 'Profiles/ajax_getUsername',
dataType: 'json',
data: {
},
success: function(data){
document.write(data);
}
})
}
Now when i debug i see that the returned data (data) is the correct value
but the html between the span tags stay the same.
What am i doing wrong?
Little update
I have tried the following:
function getUsername(){
$.ajax({
type: 'POST',
url: myBaseUrl + 'Profiles/ajax_getUsername',
dataType: 'json',
data: {
},
success: function(data){
$('.username').html('RRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRr');
}
})
}
getUsername();
Still there is no html between the tags (no text) but when i look at the
console the method is completed and has been executed.