I am using autocomplete in jquery but getting error autocomplete is not a function please help me out in this...

function doSearch(term) { 
alert("search here");
  return  $.ajax({
                    url: "URL",
                    data: {
                        q: term,
                    },
                    dataType: "json",
                    contentType: "text/plain",
                    type: "GET"
                  //,
                  //  success: function (data) {
                    //       searchResponse = data;
                      //  }
                });         
}


$(document).ready(function () {
alert("Hi");
       $("#txtSearchBox").autocomplete({
	   
	   source : function (request, response) {
                   doSearch(request.term);   
            }
	   })
	   alert(data);
	   });