why computed property can not return anything?

i am using computed property like below to fetch data from webapi, and it tested with good json result. but when i always got , even i tried jQuery.parseJSON() to transfer data first, but don't know how can i pass json to data property.

computed: {

    fetchBookData:function () {
     var item;
     var bookJson=$.getJSON('/api/book/getJson',function (data) {

      item = data;

      $.each(data,function (key,val) {
         // item[key]= key;
         // item[val]= val;

      })
    });
      return item;
    }, 

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