class Reservation {
  constructor(id, partySize) {
  this.id = id;
  this.party_of = partySize;
}

  update(callback) {
    var reservation = { reservation: this };
    $.ajax({
        method: 'patch',
        url: 'reservations/' + this.id + '.json',
        data: reservation
      }).done(function(){
        callback();
      })
    };
  };

class Reservation {
  constructor(id, partySize) {
  this.id = id;
  this.party_of = partySize;
}

  update() {
    var reservation = { reservation: this };
    $.ajax({
        method: 'patch',
        url: 'reservations/' + this.id + '.json',
        data: reservation
      }).done(function(){

      })
    };
  };

--- 
+++ 
@@ -4,14 +4,14 @@
   this.party_of = partySize;
 }
 
-  update() {
+  update(callback) {
     var reservation = { reservation: this };
     $.ajax({
         method: 'patch',
         url: 'reservations/' + this.id + '.json',
         data: reservation
       }).done(function(){
-
+        callback();
       })
     };
   };
Add callback parameter to update function.
