en.javascript.info/1-js/99-js-misc/01-proxy/03-observable/solution.md

The solution consists of two parts:

1. Whenever .observe(handler) is called, we need to remember the handler somewhere, to be able to call it later. We can store handlers right in the object, using our symbol as the property key.
2. We need a proxy with set trap to call handlers in case of any change.

let handlers = Symbol('handlers');

function makeObservable(target) {
  // 1. Initialize handlers store
  target[handlers] = [];

  // Store the handler function in array for future calls
  target.observe = function(handler) {
    this[handlers].push(handler);
  };

  // 2. Create a proxy to handle changes
  return new Proxy(target, {
    set(target, property, value, receiver) {
      let success = Reflect.set(...arguments); // forward the operation to object
      if (success) { // if there were no error while setting the property
        // call all handlers
        target[handlers].forEach(handler => handler(property, value));
      }
      return success;
    }
  });
}

let user = {};

user = makeObservable(user);

user.observe((key, value) => {
  alert(`SET ${key}=${value}`);
});

user.name = "John";