ajax异步回调函数中给外部变量赋值的问题探讨

!doctype html>
head>
meta charset="utf-8">
/head>
body>
script src="http://localhost/UIBMSPHPProj/public/js/jquery-1.8.2.min.js">/script>
script>
$(function(){
var str = "aaa";
$.post("http://localhost/UIBMSPHPProj/index.php/Index/abc",{},function(data){
//data = "bbb";
console.log(str);//aaa
str = data;
console.log(str);//"bbb"
});
console.log(str);//"aaa"
});
/script>
/body>
/html>