@rnattochdag Posts What I Use

Testing React createPortal with Testing Library

2020-10-22 • 2 min read • Original post

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();