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';
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);
});
Usage with mount()
There have been a few issues reporting problems when using the library on the tests that rely on mount()
.
You don't need this library to trigger useEffect()
and useLayoutEffect()
when doing full rendering with mount()
. However, you may have a mix of tests that rely both rely on shallow()
and mount()
. In those cases, you may run into issues with the tests that call mount()
.
Version 1.3.0
of this library provides a solution for that.
If you have a test that uses mount()
simply call disableHooks()
before mounting a component:
import { disableHooks } from 'jest-react-hooks-shallow';
test('Full rendering with `mount()`', () => {
disableHooks();
const component = mount(<App />);
});
Now, that will disable hooks for the test in the same suite which rely on shallow()
. So you need to re-enable them by calling reenableHooks()
.
import { disableHooks, reenableHooks } from 'jest-react-hooks-shallow';
test('Full rendering with `mount()`', () => {
disableHooks();
const component = mount(<App />);
});
test('Back to shallow rendering with', () => {
reenableHooks();
const component = shallow(<App />);
});
Examples
Please, see use-effect-component.test.jsx for the examples of that. Namely, have a look at the test called effects mockable when used with mount() and disableHooks()
and the one that comes after it.
How does that work?
You enable useEffect()
/useLayoutEffect()
by calling enableHooks()
in a file specified by setupFilesAfterEnv
in the Jest configuration. The code in that code will be called before each test suite.
So if in our test suite you call disableHooks()
it will not affect the other ones. But it will disable hooks in shallow rendering for the tests that come after the one with disableHooks()
. So if any the tests defined after need shallow rendering and hooks, just call reenableHooks()
.
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](https://github.com/facebook/react/pull/16168) 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.