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.

 1// App.js
 2import React, { useEffect, useState } from 'react'
 3import ReactDOM from 'react-dom'
 4
 5const App = ({ root }) => {
 6  const [container] = useState(document.createElement('div'))
 7
 8  useEffect(() => {
 9    root.appendChild(container)
10
11    return () => {
12      root.removeChild(container)
13    }
14  }, [container, root])
15
16  return ReactDOM.createPortal(<div>Portal content</div>, container)
17}
18
19export 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.

 1// App.test.js
 2import { render, within } from '@testing-library/react'
 3import React from 'react'
 4import App from './App'
 5import '@testing-library/jest-dom/extend-expect'
 6
 7test('appends the element when the component is mounted', () => {
 8  const root = document.createElement('div')
 9
10  render(<App root={root} />)
11
12  const { getByText } = within(root)
13
14  expect(root).toContainElement(getByText(/portal content/i))
15})

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.

1// Our example
2expect(root).toContainElement(getByText(/portal content/i))
3
4// How I would normally test it
5expect(screen.getByText(/portal content/i)).toBeInTheDocument()