我试图在我的LayoutDummy.js上获取所有快照以完成我的单元测试覆盖

当我运行测试时出现错误，它说 dummyFunc 不是如下所示的函数

我已经在功能组件和测试文件中编写了我的实际代码，如下所示

LayoutDummy.js

import React, { useEffect, useState } from 'react';

const LayoutDummy = () => {
  const [caption, setCaption] = useState('loading...');
  useEffect(() => {
    dummyFunc();
  }, []);

  const dummyFunc = () => {
    setCaption('dummyFunc is running');
  };
  return (
    <div>
      <p>LayoutDummy</p>
      <p>{caption}</p>
    </div>
  );
};

export default LayoutDummy;

测试/LayoutDummy.js

import React, { useEffect } from 'react';
import ShallowRenderer from 'react-test-renderer/shallow';
import LayoutDummy from '../LayoutDummy';


const dummyFunc = jest.fn();

useEffect.mockImplementationOnce(fn => fn());

describe('mock func in useEffct', () => {

  test('render', () => {
    const shallow = new ShallowRenderer();
    const tree = shallow.render(<LayoutDummy />);
    dummyFunc.mockReturnValueOnce('dummyFunc is running');
    expect(tree).toMatchSnapshot();
  });
});

为什么该函数不能在我的单元测试中模拟，如果有任何建议通过所有快照进行测试。