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May 29, 2021
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Simplify Writing Tests recipe with hooks #3876

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90 changes: 7 additions & 83 deletions docs/recipes/WritingTests.md
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Original file line number Diff line number Diff line change
Expand Up @@ -256,9 +256,7 @@ describe('todos reducer', () => {

### Components

A nice thing about React components is that they are usually small and only rely on their props. That makes them easy to test.

First, we will install [React Testing Library](https://testing-library.com/docs/react-testing-library/intro). React Testing Library is a simple and complete React DOM testing utilities that encourage good testing practices. It uses react-dom's `render` function and `act` from react-dom/tests-utils.
First, we will install [React Testing Library](https://testing-library.com/docs/react-testing-library/intro). React Testing Library is a simple and complete React DOM testing utility that encourage good testing practices. It uses react-dom's `render` function and `act` from react-dom/tests-utils.

```sh
npm install --save-dev @testing-library/react
Expand All @@ -270,92 +268,18 @@ If you are using jest as recommended above, we also recommend installing [jest-d
npm install --save-dev @testing-library/jest-dom
```

To test the components, we `render` them into the DOM and pass stubbed callbacks as props, then we assert whether the callbacks were called when expected.

#### Example

```js
import React from 'react'
import PropTypes from 'prop-types'
import TodoTextInput from './TodoTextInput'

const Header = ({ addTodo }) => {
const handleSave = text => {
if (text.length !== 0) {
addTodo(text)
}
}

return (
<header className="header">
<h1>todos</h1>
<TodoTextInput
newTodo={true}
onSave={handleSave}
placeholder="What needs to be done?"
/>
</header>
)
}

Header.propTypes = {
addTodo: PropTypes.func.isRequired
}

export default Header
```

can be tested like:

```js
import React from 'react'
import { render, fireEvent, screen } from '@testing-library/react'
import Header from '../../components/Header'

it('should not call addTodo if length of text is 0', () => {
const mockAddTodo = jest.fn()
render(<Header addTodo={mockAddTodo} />)

fireEvent.change(screen.getByPlaceholderText(/what needs to be done/i), {
target: { value: '' }
})

expect(mockAddTodo).toHaveBeenCalledTimes(0)
})

it('should call addTodo if length of text is greater than 0', () => {
const mockAddTodo = jest.fn()
render(<Header addTodo={mockAddTodo} />)

fireEvent.change(screen.getByPlaceholderText(/what needs to be done/i), {
target: { value: 'Use Redux' }
})

expect(mockAddTodo).toHaveBeenCalledTimes(1)
})
```

### Connected Components

If you use a library like [React Redux](https://github.com/reduxjs/react-redux), you might be using [higher-order components](https://medium.com/@dan_abramov/mixins-are-dead-long-live-higher-order-components-94a0d2f9e750) like [`connect()`](https://react-redux.js.org/api/connect). This lets you inject Redux state into a regular React component.

Consider the following `App` component:

```js
import { connect } from 'react-redux'

const App = props => {
return <div>{props.user}</div>
}
import { useSelector } from 'react-redux'

const mapStateToProps = state => {
return state
export default function App() {
const user = useSelector(state => state.user)
return <div>{user}</div>
}

export default connect(mapStateToProps)(App)
```

To test it, we can use the `wrapper` option in React Testing Library's `render` function and export our own `render` function as explained in React Testing Library's [setup docs](https://testing-library.com/docs/react-testing-library/setup).
To test the component, we `render` it into the DOM and pass stubbed callbacks as props, then we assert whether the callbacks were called when expected. We can use the `wrapper` option in the `render` function and export our own `render` function as explained in React Testing Library's [setup docs](https://testing-library.com/docs/react-testing-library/setup).

Our `render` function can look like this:

Expand Down Expand Up @@ -398,7 +322,7 @@ import React from 'react'
import { render, fireEvent, screen } from '../../test-utils'
import App from '../../containers/App'

it('Renders the connected app with initialState', () => {
it('Renders the app with initialState', () => {
render(<App />, { initialState: { user: 'Redux User' } })

expect(screen.getByText(/redux user/i)).toBeInTheDocument()
Expand Down