Why has Object.observe() been deprecated

You can achieve this with getters and setters.

var obj = {
  get foo() {
    console.log({ name: 'foo', object: obj, type: 'get' });
    return obj._foo;
  },
  set bar(val) {
    console.log({ name: 'bar', object: obj, type: 'set', oldValue: obj._bar });
    return obj._bar = val;
  }
};

obj.bar = 2;
// {name: 'bar', object: <obj>, type: 'set', oldValue: undefined}

obj.foo;
// {name: 'foo', object: <obj>, type: 'get'}

Alternatively, in a browser with support for Proxies, you can write a more generic solution.

var obj = {
  foo: 1,
  bar: 2
};

var proxied = new Proxy(obj, {
  get: function(target, prop) {
    console.log({ type: 'get', target, prop });
    return Reflect.get(target, prop);
  },
  set: function(target, prop, value) {
    console.log({ type: 'set', target, prop, value });
    return Reflect.set(target, prop, value);
  }
});

proxied.bar = 2;
// {type: 'set', target: <obj>, prop: 'bar', value: 2}

proxied.foo;
// {type: 'get', target: <obj>, prop: 'bar'}