jQuery Ajax Post with data

function cart(id1)
{
    var id=id1;
    alert("enterd "+id);
    document.getElementById("disp").innerHTML ="hi";
        $.ajax({
        url:"/add.php ",
        method:"POST", //First change type to method here

        data:{
          item_id: id,
        },
        success:function(response) {
         document.getElementById("disp").innerHTML =response;
       },
       error:function(){
        alert("error");
       }

      });

}

 type:"POST" ->  method: "POST"

$.ajax({
  url: "add.php",
  method: "POST",
  data: { item_id: id},
}).done(function(response) {
  ......
}).fail(function( jqXHR, textStatus ) {
  ......
});