var data;

$.ajax({
                  type: "POST",
                  url: "it_lecture_add_step2.jsp",
                  async:false,
                  data: params,
                  dataType: "json",
                  success: function (returnData) {
                      data = "zzzz";
                  }
              });

document.write(data); // zzzz

ajax에 async옵션을 false으로 줘야 전역변수로 담아진다.

출처: http://karaway00.tistory.com/7