父方法未使用实例化的子方法调用

假设:

class Parent {
  foo = (message) => 'parent and ' + message
}

class Child extends Parent {
  foo = (message = 'child foo') => super.foo(message)
}

为什么后续测试不通过?

describe('Child', () => {
  it('should call parent foo and return \'parent and child foo\'', () => {
    const child = new Child()
    const expectedResponse = 'parent and child foo'

    const response = child.foo()

    expect(response).toBe(expectedResponse)
  })
})

引发错误: TypeError: (intermediate value).foo is not a function

不应该 Parent S FO存在并被调用?