handler(event, newValue, oldValue)

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

import {DefineMap} from "can";

const person = new DefineMap({name: "Justin"});

person.on("name", (event, newVal, oldVal) => {
  console.log( newVal ); //-> "Brian"
  console.log( oldVal ); //-> "Justin"
});
person.set("name", "Brian");