handler(event, newValue, oldValue)

Handlers registered on propertyName events will be called back as follows.

import {DefineList} from "can";

const list = new DefineList();
list.set("totalCount", 500);

list.on("totalCount", (event, newVal, oldVal) => {
  console.log(newVal); //-> 5
  console.log(oldVal); //-> 500
});
list.set("totalCount", 5);