Async utilities#

findBy* queries#

The findBy* queries are used to find elements that are not instantly available but will be added as a result of some asynchronous action. Learn more details here.

waitFor#

function waitFor<T>(
  expectation: () => T,
  options?: { timeout: number; interval: number }
): Promise<T>;

Waits for a period of time for the expectation callback to pass. waitFor may run the callback a number of times until timeout is reached, as specified by the timeout and interval options. The callback must throw an error when the expectation is not met. Returning any value, including a falsy one, will be treated as meeting the expectation, and the callback result will be returned to the caller of waitFor function.

await waitFor(() => expect(mockFunction).toHaveBeenCalledWith());

waitFor function will be executing expectation callback every interval (default: every 50 ms) until timeout (default: 1000 ms) is reached. The repeated execution of callback is stopped as soon as it does not throw an error, in such case the value returned by the callback is returned to waitFor caller. Otherwise, when it reaches the timeout, the final error thrown by expectation will be re-thrown by waitFor to the calling code.

// ❌ `waitFor` will return immediately because callback does not throw
await waitFor(() => false);

waitFor is an async function so you need to await the result to pause test execution.

// ❌ missing `await`: `waitFor` will just return Promise that will be rejected when the timeout is reached
waitFor(() => expect(1).toBe(2));

Since waitFor is likely to run expectation callback multiple times, it is highly recommended for it not to perform any side effects in waitFor.

await waitFor(() => {
  // ❌ button will be pressed on each waitFor iteration
  fireEvent.press(screen.getByText('press me'));
  expect(mockOnPress).toHaveBeenCalled();
});

It is also recommended to have a single assertion per each waitFor for more consistency and faster failing tests. If you want to make several assertions, then they should be in seperate waitFor calls. In many cases you won't actually need to wrap the second assertion in waitFor since the first one will do the waiting required for asynchronous change to happen.

waitFor checks whether Jest fake timers are enabled and adapts its behavior in such case. The following snippet is a simplified version of how it behaves when fake timers are enabled:

let fakeTimeRemaining = timeout;
let lastError;

while (fakeTimeRemaining > 0) {
  fakeTimeRemaining = fakeTimeRemaining - interval;
  jest.advanceTimersByTime(interval);
  try {
    // resolve
    return expectation();
  } catch (error) {
    lastError = error;
  }
}

// reject
throw lastError;

In the following example we test that a function is called after 10 seconds using fake timers. Since we're using fake timers, the test won't depend on real time passing and thus be much faster and more reliable. Also we don't have to advance fake timers through Jest fake timers API because waitFor already does this for us.

// in component
setTimeout(() => {
  someFunction();
}, 10000);

// in test
jest.useFakeTimers();

await waitFor(() => {
  expect(someFunction).toHaveBeenCalledWith();
}, 10000);

waitForElementToBeRemoved#

function waitForElementToBeRemoved<T>(
  expectation: () => T,
  options?: { timeout: number; interval: number }
): Promise<T>;

Waits for non-deterministic periods of time until queried element is removed or times out. waitForElementToBeRemoved periodically calls expectation every interval milliseconds to determine whether the element has been removed or not.

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

test('waiting for an Banana to be removed', async () => {
  render(<Banana />);

  await waitForElementToBeRemoved(() => screen.getByText('Banana ready'));
});

This method expects that the element is initially present in the render tree and then is removed from it. If the element is not present when you call this method it throws an error.

You can use any of getBy, getAllBy, queryBy and queryAllBy queries for expectation parameter.