Track object changes with a JavaScript proxy

proxy.js

const pick = (keys, object) =>
  Object.entries(object).reduce(
    (acc, [key, value]) => (keys.includes(key) ? {...acc, [key]: value} : acc),
    {},
  );

const parse = x => JSON.parse(JSON.stringify(x));

export default function proxy(initialAttrs = {}) {
  let current = parse(initialAttrs);
  let original = parse(initialAttrs);

  const utils = {
    isChanged: () =>
      [...Object.keys(current), Object.keys(original)]
        .filter((value, index, self) => self.indexOf(value) === index)
        .some(key => current[key] !== original[key]),

    applyChanged: newOriginal =>
      Object.assign(original, parse(pick(Object.keys(original), newOriginal))),
  };

  return new Proxy(current, {
    get: (obj, prop) => utils[prop] || obj[prop],
    set: (obj, prop, value) => prop in obj && (obj[prop] = value),
  });
}

proxy.test.js

const attrs = {foo: 'bar', bar: 'baz'};
let subject;

beforeEach(() => {
  subject = proxy(attrs);
});

test('to return Object', () => {
  expect(subject.constructor).toEqual(Object);
});

test('to #get value for existing key', () => {
  expect(subject.foo).toEqual('bar');
  expect(subject.baz).toBe(undefined);
});

test('to #set value for existing key', () => {
  subject.foo = 'baz';
  expect(subject.foo).toEqual('baz');
});

test('to not #set value for missing key', () => {
  subject.baz = 'foo';
  expect(subject.baz).toBe(undefined);
});

test('to track changed state', () => {
  expect(subject.isChanged()).toBe(false);

  subject.foo = 'bar';
  expect(subject.isChanged()).toBe(false);

  subject.foo = 'baz';
  expect(subject.isChanged()).toBe(true);
});

test('to applyChanged for original state', () => {
  subject.applyChanged({foo: 'bar'});
  expect(subject.foo).toEqual('bar');
  expect(subject.isChanged()).toBe(false);

  subject.applyChanged({foo: 'xyz'});
  expect(subject.foo).toEqual('bar');
  expect(subject.isChanged()).toBe(true);
});

test('to not applyChanged for newly added keys', () => {
  subject.applyChanged({baz: 'foo'});

  expect(subject.foo).toEqual('bar');
  expect(subject.bar).toBe('baz');
  expect(subject.baz).toBe(undefined);
});