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When migrating some internal tests I found it annoying that I couldn't
return anything from the `act` scope. You would have to declare the
variable on the outside then assign to it. But this doesn't play well
with type systems — when you use the variable, you have to check
the type.
Before:
```js
let renderer;
act(() => {
renderer = ReactTestRenderer.create(<App />);
})
// Type system can't tell that renderer is never undefined
renderer?.root.findByType(Component);
```
After:
```js
const renderer = await act(() => {
return ReactTestRenderer.create(<App />);
})
renderer.root.findByType(Component);
```
react-test-renderer
This package provides an experimental React renderer that can be used to render React components to pure JavaScript objects, without depending on the DOM or a native mobile environment.
Essentially, this package makes it easy to grab a snapshot of the "DOM tree" rendered by a React DOM or React Native component without using a browser or jsdom.
Documentation:
https://reactjs.org/docs/test-renderer.html
Usage:
const ReactTestRenderer = require('react-test-renderer');
const renderer = ReactTestRenderer.create(
<Link page="https://www.facebook.com/">Facebook</Link>
);
console.log(renderer.toJSON());
// { type: 'a',
// props: { href: 'https://www.facebook.com/' },
// children: [ 'Facebook' ] }
You can also use Jest's snapshot testing feature to automatically save a copy of the JSON tree to a file and check in your tests that it hasn't changed: https://jestjs.io/blog/2016/07/27/jest-14.html.