Using variable outside of ajax callback function

Programmer Question

What is the best to use global variables outside of a callback function?

    var icon; 
    $(function(){

      $.get('data.xml', function(xml){

           icon = xml.documentElement.getElementsByTagName("icon");
           //this outputs a value
           console.log(icon);
       });
       //this is null
       //How can this maintain the value set above?
       console.log(icon);
    });

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