How to mock/spy useState hook in jest?

Question

I am trying to spy on useState React hook but i always get the test failed

This is my React component:

const Counter= () => {
    const[counter, setCounter] = useState(0);

    const handleClick=() => {
        setCounter(counter + 1);
    }

    return (
        <div>
            <h2>{counter}</h2>
            <button onClick={handleClick} id="button">increment</button>
        </div>
    )
}

counter.test.js:

it('increment counter correctlry', () => {
    let wrapper = shallow(<Counter/>);
    const setState = jest.fn();
    const useStateSpy = jest.spyOn(React, 'useState');

    useStateSpy.mockImplementation((init) => [init, setState]);
     const button = wrapper.find("button")
     button.simulate('click');
     expect(setState).toHaveBeenCalledWith(1);
})

Unfortunately this doesn't work and i get the test failed with that message:

expected 1
Number of calls: 0

Answer 1

dieu的回答引导了我正确的方向，我想出了这个解决方案：

1. 模拟使用反应中的状态以返回 jest.fn() 作为 useState：
   1.1 还立即导入 useState - 现在将是 e jest 模拟（从 jest.fn() 调用返回）

jest.mock('react', ()=>({
  ...jest.requireActual('react'),
  useState: jest.fn()
}))
import { useState } from 'react';

2. 稍后在 beforeEach 中，将其设置为原始 useState，以适应您需要不被模拟的所有情况

describe("Test", ()=>{
  beforeEach(()=>{
    useState.mockImplementation(jest.requireActual('react').useState);
    //other preperations
  })
  //tests
})

3. 在测试本身中根据需要模拟它：

it("Actual test", ()=>{
  useState.mockImplementation(()=>["someMockedValue", someMockOrSpySetter])
})

临别说明：虽然从概念上来说，在“黑匣子”中亲自动手进行单元测试可能有些错误，但有时这样做确实非常有用。

Answer 2

您需要使用React.useState而不是单个 import useState。

我认为是关于代码如何转换的，正如您在 babel repl 中看到的那样，useState从单个导入最终与模块导入之一不同

_react.useState // useState
_react.default.useState // React.useState;

所以你监视_react.default.useState但你的组件使用_react.useState. 监视单个导入似乎是不可能的，因为您需要该函数属于一个对象，这里有一个非常广泛的指南，解释了模拟/监视模块的方法https://github.com/HugoDF/mock-spy-module-进口

正如@Alex Mackay 提到的，你可能想改变你对测试 React 组件的心态，推荐使用 react-testing-library，但如果你真的需要坚持使用酶，你不需要走那么远去模拟反应库本身

Answer 3

只需要在测试文件中导入 React 即可，例如：

import * as React from 'react';

之后就可以使用mock函数了。

import * as React from 'react';

:
:
it('increment counter correctlry', () => {
    let wrapper = shallow(<Counter/>);
    const setState = jest.fn();
    const useStateSpy = jest.spyOn(React, 'useState');

    useStateSpy.mockImplementation((init) => [init, setState]);
     const button = wrapper.find("button")
     button.simulate('click');
     expect(setState).toHaveBeenCalledWith(1);
})

Answer 4

令人烦恼的是，Codesandbox 目前在其测试模块上遇到了问题，因此我无法发布工作示例，但我会尝试解释为什么模拟useState通常是一件坏事。

用户不关心是否useState已被调用，他们关心当我单击增量时计数应该增加一，因此这就是您应该测试的内容。

// App
import React, { useState } from "react";
export default function App() {
  const [count, setCount] = useState(0);
  return (
    <div>
      <h1>Count: {count}</h1>
      <button onClick={() => setCount((prev) => prev + 1)}>Increment</button>
    </div>
  );
}
// Tests
import React from "react";
import App from "./App";
import { screen, render } from "@testing-library/react";
import userEvent from "@testing-library/user-event";

describe("App should", () => {
  it('increment count value when "Increment" btn clicked', () => {
    // Render the App
    render(<App />);
    // Get the count in the same way the user would, by looking for 'Count'
    let count = screen.getByText(/count:/);
    // As long as the h1 element contains a '0' this test will pass
    expect(count).toContain(0);
    // Once again get the button in the same the user would, by the 'Increment'
    const button = screen.getByText(/increment/);
    // Simulate the click event
    userEvent.click(button);
    // Refetch the count
    count = screen.getByText(/count:/);
    // The 'Count' should no longer contain a '0'
    expect(count).not.toContain(0);
    // The 'Count' should contain a '1'
    expect(count).toContain(1);
  });
  // And so on...
  it('reset count value when "Reset" btn is clicked', () => {});
  it('decrement count value when "Decrement" btn is clicked', () => {});
});

一定要检查一下@testing-library您是否对这种测试方式感兴趣。我enzyme大约两年前就换了，从那以后就再也没有碰过它。