A observable function use Proxy and Reflect

observable.js

function observable (obj, onchange) {
  return new Proxy(obj, {
    set (target, key, value) {
      Reflect.set(target, key, value)
      onchange(key, value)
    },
    delete (target, key, value) {
      Reflect.delete(key)
      onchange(key, undefined)
    }
  })
}

let a = {}
let pA = observable(a, (k, v) => console.log('Observalbe:', k, v))
pA.foo = 'bar'