How to mock/spy useState hook in jest?

问题

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

回答1:

You need to use React.useState instead of the single import useState.

I think is about how the code gets transpiled, as you can see in the babel repl the useState from the single import ends up being different from the one of the module import

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

So you spy on _react.default.useState but your component uses _react.useState. It seems impossible to spyOn a single import since you need the function to belong to an object, here is a very extensive guide that explains the ways of mocking/spying modules https://github.com/HugoDF/mock-spy-module-import

And as @Alex Mackay mentioned, you probably want to change your mindset about testing react components, moving to react-testing-library is recommended, but if you really need to stick to enzyme you don't need to go that far as to mock react library itself

回答2:

Annoyingly Codesandbox is currently having trouble with its testing module so I can't post a working example but I will try to explain why mocking useState is generally a bad thing to do.

The user doesn't care if useState has been called, they care about when I click increment the count should increase by one therefore that is what you should be testing for.

// 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', () => {});
});

Definitely check out @testing-library if you are interested in this style of testing. I switched from enzyme about 2 years ago and haven't touched it since.

来源：https://stackoverflow.com/questions/64165138/how-to-mock-spy-usestate-hook-in-jest

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