jest-react-hooks-shallow

jest-react-hooks-shallow

Short Story

This package makes React Hooks (namely, useEffect() and useLayoutEffect()) work with shallow rendering. In other words, you can use enzyme. Yay!

Installation

Just install this package with

npm install --save-dev jest-react-hooks-shallow
# or
yarn add --dev jest-react-hooks-shallow

and add these lines to your Jest setup file (specified by setupFilesAfterEnv):

import enableHooks from 'jest-react-hooks-shallow';

// pass an instance of jest to `enableHooks()`
enableHooks(jest);

And voilà - useEffect() and useLayoutEffect() will work with shallow rendering. From this moment on your test don't need to know anything about useEffect(). After all, it's a mere implementation detail.

Testing

So if you have a component like this:

const ComponentWithHooks = () => {
  const [text, setText] = useState<>();
  const [buttonClicked, setButtonClicked] = useState<boolean>(false);

  useEffect(() => setText(
    `Button clicked: ${buttonClicked.toString()}`), 
    [buttonClicked]
  );

  return (
    <div>
      <div>{text}</div>
      <button onClick={() => setButtonClicked(true)}>Click me</button>
    </div>
  );
};

You can easily test it with code like this:

test('Renders default message and updates it on clicking a button', () => {
  const component = shallow(<App />);

  expect(component.text()).toContain('Button clicked: false');

  component.find('button').simulate('click');

  expect(component.text()).toContain('Button clicked: true');
});

Please note, that those tests didn't have to import anything else. They simply don't know that a component calls useEffect(). Yet, it's being called when you invoke shallow().

That said, often you want to test that a specific function has been called on some event. For example, you're calling a Redux action creator or a Mobx action. If you're using React Hooks, chances are you'll pass that function as a callback to useEffect().

No problems! You can easily test it with simple Jest mocks.

Say, we have a component like this:

import someAction from './some-action';

const ComponentWithHooks = () => {
  const [text, setText] = useState<>();
  const [buttonClicked, setButtonClicked] = useState<boolean>(false);

  useEffect(someAction, [buttonClicked]);

  return (
    <div>
      <div>{text}</div>
      <button onClick={() => setButtonClicked(true)}>Click me</button>
    </div>
  );
};

test('Calls `myAction()` on the first render and on clicking the button`', () => {
  const component = shallow(<App />);
  expect(callback).toHaveBeenCalledTimes(1);

  component.find('button').simulate('click');
  expect(callback).toHaveBeenCalledTimes(2);
});

Dependencies

This library expects that you use Jest as a testing library.

Frankly speaking, you don't have to use enzyme, as it's not the only library which providers shallow rendering. In fact, it doesn't even implement shadow rendering. See the Long Story for details.

Hooks Support Status

Hook	Support
useEffect	✅
useLayoutEffect	✅
useImperativeHandle	Coming soon
useDebugValue	No support plans

All other hooks (e.g. useState(), useReducer()) already work with shallow rendering.

FAQ

Q: Does it call cleanup functions?

A: Yes, it does, but only before calling the same effect again. It won't call cleanup functions when component gets unmounted. That's because, unfortunately, this library doesn't have access to the component life cycle.

Long Story

Context

In case, you wonder why I have created this package instead of extending enzyme, here's a slightly longer story.

Actually, it's not enzyme's per se fault that useEffect() doesn't work in shallow rendering. It relies on react-test-renderer for some aspects of shallow rendering. And it is react-test-renderer that implements certain hooks, like useState() and does not implement the other ones (e.g. useEffect()).

Now, react-test-renderer is part of the React library. And there is a PR that brings useEffect() to shallow rendering. However, that PR has been closed by Facebook.

According to the comments on the same PR, there are plans to spin off react-test-renderer as a separate package. And indeed, the master branch of React does have it as an NPM dependency. However, all currently released versions of React have react-test-renderer built-in.

Once it's a standalone package, I believe it'll be easier to merge the said PR and the need for this library will go away.

How this Library is Implemented

If someone wonders how this library is implemented, then we just provide a naïve implementation of useEffect(). After all, it's just a function that takes two arguments and executes the first one, if the values of the second argument change (or the function is called for the first time with them or the second argument is undefined).

Okay, the actual implementation of React does have knowledge on whether it's the first render or the second.

We don't, so we have to rely on mapping callback functions to a list of dependencies (the second argument). If useEffect() is called with the same function, but with different dependency values, we will execute that function.