b.js

const unbound = Symbol('unbound')

const bindMethod = (obj, fn) => {
  const value = fn[unbound] || fn
  return Object.assign(value.bind(obj), { [unbound]: value })
}

const bindObjMethods = obj => {
  const proto = Object.getPrototypeOf(obj)
  for (const name of Object.getOwnPropertyNames(proto)) {
    if (name === 'constructor')
      continue
    const descriptor = Object.getOwnPropertyDescriptor(proto, name)
    if (('value' in descriptor) && typeof descriptor.value === 'function') {
      descriptor.value = bindMethod(obj, descriptor.value)
      Object.defineProperty(obj, name, descriptor)
    }
    if (typeof descriptor.get === 'function' || typeof descriptor.set === 'function') {
      const { get, set } = descriptor
      if (typeof get === 'function') {
        descriptor.get = () => {
          const value = get.call(obj)
          return typeof value !== 'function' ? value
            : bindMethod(obj, value)
        }
      }
      if (typeof set === 'function') {
        descriptor.set = value => {
          value = typeof value !== 'function' ? value
            : bindMethod(obj, value)
          set.call(obj, value)
        }
      }
    }
    Object.defineProperty(obj, name, descriptor)
  }
}

class A {
  method () {
    return this
  }
}

class B {
  _method () {
    return this
  }
  get method () {
    return this._method
  }
  set method (value) {
    this._method = value
  }
}

const a = Object.assign(new A(), { a: 1 })
bindObjMethods(a)
const { method: aMethod } = a
const b = Object.assign(new B(), { b: 2 })
bindObjMethods(b)
const { method: bMethod } = b

console.log('a.method()', a.method())
console.log('aMethod()', aMethod())
console.log('b.method()', b.method())
console.log('bMethod()', bMethod())
const c = { c: 3 }
b.method = function () { return [this, c] }
const { method: changedBMethod } = b
console.log('changed b.method()', b.method())
console.log('changedBMethod()', changedBMethod())

const bChild = Object.assign(Object.create(b), {bChild: 4})
bindObjMethods(bChild)
const { method: bChildMethod } = bChild
console.log('bChild.method()', bChild.method())
console.log('bChildMethod()', bChildMethod())

/*
a.method() A { a: 1 }
aMethod() A { a: 1 }
b.method() B { b: 2 }
bMethod() B { b: 2 }
changed b.method() [ B { b: 2 }, { c: 3 } ]
changedBMethod() [ B { b: 2 }, { c: 3 } ]
bChild.method() [ B { bChild: 4, b: 2 }, { c: 3 } ]
bChildMethod() [ B { bChild: 4, b: 2 }, { c: 3 } ]
*/