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Hooks

Loading "Intro to Hooks"
In React, we have a primitive that's even lower level than a component: a hook.
A hook is a function that uses other hooks. That's it. That's the definition of a hook. But what makes it special regarding testing is if you try to call a hook outside of a React component, React doesn't know what to do with it and will error out on you.
But sometimes it can be really useful to test a hook in isolation for all the same reasons we want to write lower-level tests. Primarily, if there's some complex logic going on in the hook, it can be nice to test it directly rather than indirectly through a component.
So @testing-library/react has a special function called renderHook that allows you to test a hook in isolation. Here's an example:
import { useState } from 'react'

function useCounter(initialCount) {
	const [count, setCount] = useState(initialCount)

	const increment = () => setCount(c => c + 1)
	const decrement = () => setCount(c => c - 1)

	return { count, increment, decrement }
}
/**
 * @vitest-environment jsdom
 */
import { renderHook, act } from '@testing-library/react'
import { expect, test } from 'vitest'
import { useCounter } from './use-counter'

test('increments and decrements', async () => {
	const { result } = await renderHook(() => useCounter(0))

	expect(result.current.count).toBe(0)
	await act(() => result.current.increment())
	expect(result.current.count).toBe(1)
	await act(() => result.current.decrement())
	expect(result.current.count).toBe(0)
})
Check out the renderHook docs if you want more info on the docs

result.current

Due to the nature of JavaScript, it's very important you always reference all values off of result.current rather than doing what may seem more natural and destructuring values off of result.current and using those. This is because result.current is a reference to the current value of the hook. If you destructure values off of it, then you're creating a new reference to the values. This means that if you call a function that mutates the values, then your destructured values will be out of date.
This is a common mistake that people make when testing hooks with this utility.
So be sure to always reference values off of result.current.

Act

What's this act thing? To be short, it's a way to tell React that you're performing an action that will cause a state change. This way, when you're done with your action, React can flush all the state changes and re-render the component. This is important because React is asynchronous, so if you don't tell React that you're done with your action, it won't know when to flush the state changes and re-render the component. If it doesn't re-render, then your next line of code will be testing the old state, not the new state.
If you're interested in a deeper dive on act, read Fix the "not wrapped in act(...)" warning.

Test Components

An alternative to using renderHook is to write your own TestComponent. Here's how we would do that with the example above:
/**
 * @vitest-environment jsdom
 */
import { renderHook, screen } from '@testing-library/react'
import { userEvent } from '@testing-library/user-event'
import { useState } from 'react'
import { expect, test } from 'vitest'
import { useCounter } from './use-counter'

function TestComponent() {
	const { count, increment, decrement } = useCounter(0)
	return (
		<div>
			<output>{count}</output>
			<button onClick={increment}>+</button>
			<button onClick={decrement}>-</button>
		</div>
	)
}

test('increments and decrements', async () => {
	const user = userEvent.setup()

	await render(<TestComponent />)

	expect(screen.getByRole('status').textContent).toBe(0)
	await user.click(screen.getByRole('button', { name: '+' }))
	expect(screen.getByRole('status').textContent).toBe(1)
	await user.click(screen.getByRole('button', { name: '-' }))
	expect(result.current.count).toBe(0)
})
Interestingly, the TestComponent is actually what renderHook does, though it's more generic and doesn't render any React elements. Making your own TestComponent comes with the benefit of you being able to help use the component in the way it's intended to be used by consumers of the hook. In some cases this makes the test easier to understand.

User Event

You may have noticed in our test component example that we're using userEvent to trigger the increment and decrement. And we don't have to wrap those calls in act. This is because userEvent will fire browser events in a way that simulates a user interaction. This is a great way to test your components because it ensures that your components are working as expected when a user interacts with them.
The reason we don't need to wrap these in act is because userEvent actually just calls fireEvent, (a lower-level utility you rarely need) which in a React context is automatically wrapped in act for you. So you never need to worry about wrapping fireEvent or userEvent in act yourself. Incidentally, render is also wrapped in act. So in fact you very rarely need to use act directly.