til/testing React createPortal with Testing Library

I have a component that uses ReactDOM.createPortal and appends it to a DOM node that is passed as a prop. However, I couldn't find a good example of testing it using Testing Library.

I've created a CodeSandbox with some extended tests if you want to follow along using an interactive example.

// App.js
import React, { useEffect, useState } from 'react'
import ReactDOM from 'react-dom'

const App = ({ root }) => {
  const [container] = useState(document.createElement('div'))

  useEffect(() => {
    root.appendChild(container)

    return () => {
      root.removeChild(container)
    }
  }, [container, root])

  return ReactDOM.createPortal(<div>Portal content</div>, container)
}

export default App

The component receives a DOM node, root, through props. The portal component is then appended to root inside useEffect.

At first, I thought that I could use screen.getByText to get the text "Portal content", but since th content is mounted to root I can't use the screen queries.

// App.test.js
import { render, within } from '@testing-library/react'
import React from 'react'
import App from './App'
import '@testing-library/jest-dom/extend-expect'

test('appends the element when the component is mounted', () => {
  const root = document.createElement('div')

  render(<App root={root} />)

  const { getByText } = within(root)

  expect(root).toContainElement(getByText(/portal content/i))
})

After some searching, I found within – also called getQueriesForElement – in the Testing Library docs which seemed to fit this case perfectly. Passing root to within gives me all the queries that I'm used to from screen.

Using toContainElement from jest-dom/extend-expect I can then write an assertion that is similar to how I would normally write it.

// Our example
expect(root).toContainElement(getByText(/portal content/i))

// How I would normally test it
expect(screen.getByText(/portal content/i)).toBeInTheDocument()