handler(event)

Handlers registered on can.keys events will be called back as follows.

import {DefineMap} from "can";

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

person.on("can.keys", (event) => {
  console.log(event.target.serialize()); //-> {name: "Justin", age: 33}
});
person.set("age", 33);

Use canReflect.onPatches() to know which property changed.