Understanding act function#

When writing RNTL tests one of the things that confuses developers the most are cryptic act() function errors logged into console. In this article I will try to build an understanding of the purpose and behaviour of act() so you can build your tests with more confidence.

act warnings#

Let’s start with typical act() warnings logged to console. There are two kinds of these issues, let’s call the first one the "sync act()" warning:

The second one relates to async usage of act so let’s call it the "async act" error:

Synchronous act#

Responsibility#

This function is intended only for using in automated tests and works only in development mode. Attempting to use it in production build will throw an error.

The responsibility for act function is to make React renders and updates work in tests in a similar way they work in real application by grouping and executing related units of interaction (e.g. renders, effects, etc) together.

To showcase that behaviour let make a small experiment. First we define a function component that uses useEffect hook in a trivial way.

function TestComponent() {
  const [count, setCount] = React.useState(0);
  React.useEffect(() => {
    setCount((c) => c + 1);
  }, []);

  return <Text>Count {count}</Text>;
}

In the following tests we will directly use ReactTestRenderer instead of RNTL render function to render our component for tests. In order to expose familiar queries like getByText we will use within function from RNTL.

test('render without act', () => {
  const renderer = TestRenderer.create(<TestComponent />);

  // Bind RNTL queries for root element.
  const view = within(renderer.root);
  expect(view.getByText('Count 0')).toBeOnTheScreen();
});

When testing without act call wrapping rendering call, we see that the assertion runs just after the rendering but before useEffecthooks effects are applied. Which is not what we expected in our tests.

test('render with act', () => {
  let renderer: ReactTestRenderer;
  act(() => {
    renderer = TestRenderer.create(<TestComponent />);
  });

  // Bind RNTL queries for root element.
  const view = within(renderer!.root);
  expect(view.getByText('Count 1')).toBeOnTheScreen();
});

When wrapping rendering call with act we see that the changes caused by useEffect hook have been applied as we would expect.

When to use act#

The name act comes from Arrange-Act-Assert unit testing pattern. Which means it’s related to part of the test when we execute some actions on the component tree.

So far we learned that act function allows tests to wait for all pending React interactions to be applied before we make our assertions. When using act we get guarantee that any state updates will be executed as well as any enqueued effects will be executed.

Therefore, we should use act whenever there is some action that causes element tree to render, particularly:

• initial render call - ReactTestRenderer.create call
• re-rendering of component -renderer.update call
• triggering any event handlers that cause component tree render

Thankfully, for these basic cases RNTL has got you covered as our render, update and fireEvent methods already wrap their calls in sync act so that you do not have to do it explicitly.

Note that act calls can be safely nested and internally form a stack of calls. However, overlapping act calls, which can be achieved using async version of act, are not supported.

Implementation#

As of React version of 18.1.0, the act implementation is defined in the ReactAct.js source file inside React repository. This implementation has been fairly stable since React 17.0.

RNTL exports act for convenience of the users as defined in the act.ts source file. That file refers to ReactTestRenderer.js source file from React Test Renderer package, which finally leads to React act implementation in ReactAct.js (already mentioned above).

Asynchronous act#

So far we have seen synchronous version of act which runs its callback immediately. This can deal with things like synchronous effects or mocks using already resolved promises. However, not all component code is synchronous. Frequently our components or mocks contain some asynchronous behaviours like setTimeout calls or network calls. Starting from React 16.9, act can also be called in asynchronous mode. In such case act implementation checks that the passed callback returns object resembling promise.

Asynchronous code#

Asynchronous version of act also is executed immediately, but the callback is not yet completed because of some asynchronous operations inside.

Lets look at a simple example with component using setTimeout call to simulate asynchronous behaviour:

function TestAsyncComponent() {
  const [count, setCount] = React.useState(0);
  React.useEffect(() => {
    setTimeout(() => {
      setCount((c) => c + 1);
    }, 50);
  }, []);

  return <Text>Count {count}</Text>;
}

import { render, screen } from '@testing-library/react-native';

test('render async natively', () => {
  render(<TestAsyncComponent />);
  expect(screen.getByText('Count 0')).toBeOnTheScreen();
});

If we test our component in a native way without handling its asynchronous behaviour we will end up with sync act warning:

Note that this is not yet the infamous async act warning. It only asks us to wrap our event code with act calls. However, this time our immediate state change does not originate from externally triggered events but rather forms an internal part of the component. So how can we apply act in such scenario?

Solution with fake timers#

First solution is to use Jest's fake timers inside out tests:

test('render with fake timers', () => {
  jest.useFakeTimers();
  render(<TestAsyncComponent />);

  act(() => {
    jest.runAllTimers();
  });
  expect(screen.getByText('Count 1')).toBeOnTheScreen();
});

That way we can wrap jest.runAllTimers() call which triggers the setTimeout updates inside an act call, hence resolving the act warning. Note that this whole code is synchronous thanks to usage of Jest fake timers.

Solution with real timers#

If we wanted to stick with real timers then things get a bit more complex. Let’s start by applying a crude solution of opening async act() call for the expected duration of components updates:

test('render with real timers - sleep', async () => {
  render(<TestAsyncComponent />);
  await act(async () => {
    await sleep(100); // Wait a bit longer than setTimeout in `TestAsyncComponent`
  });

  expect(screen.getByText('Count 1')).toBeOnTheScreen();
});

This works correctly as we use an explicit async act() call that resolves the console error. However, it relies on our knowledge of exact implementation details which is a bad practice.

Let’s try more elegant solution using waitFor that will wait for our desired state:

test('render with real timers - waitFor', async () => {
  render(<TestAsyncComponent />);

  await waitFor(() => screen.getByText('Count 1'));
  expect(screen.getByText('Count 1')).toBeOnTheScreen();
});

This also works correctly, because waitFor call executes async act() call internally.

The above code can be simplified using findBy query:

test('render with real timers - findBy', async () => {
  render(<TestAsyncComponent />);

  expect(await screen.findByText('Count 1')).toBeOnTheScreen();
});

This also works since findByText internally calls waitFor which uses async act().

Note that all of the above examples are async tests using & awaiting async act() function call.

Async act warning#

If we modify any of the above async tests and remove await keyword, then we will trigger the notorious async act()warning:

Warning: You called act(async () => ...) without await. This could lead to unexpected
testing behaviour, interleaving multiple act calls and mixing their scopes. You should
- await act(async () => ...);

React decides to show this error whenever it detects that async act()call has not been awaited.

The exact reasons why you might see async act() warnings vary, but finally it means that act() has been called with callback that returns Promise-like object, but it has not been waited on.

References#

• React act implementation source
• React testing recipes: act()