promise to solve the problem of asynchronous nest

   function a(){
      return new Promise(function(res,rej){
        $.ajax({
          url:"a",
          type: "GET",
          async:true,
          dataType:"json",
          success:function(data){
            console.log(data,"a");
            res(data);
          }
        })
      });
    }
    function b(data){
      console.log(data,"data");
      return new Promise(function(res,rej){
        $.ajax({
            url:"b",
            type: "POST",
            async:true,
            data:JSON.stringify(data),
            dataType:"json",
            success:function(data){
              console.log(data,"b");
              res();
            }
          })
      });
    }
    $("#btn").click(function(){
      a().then(function (data){
        b(data);
      }).then(function(){
      })
    })

b request depends on a result of data