#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 the expectation callback to pass. waitFor may run the callback multiple times until the timeout is reached, as specified by the timeout and interval options. The callback must throw an error when the expectation isn't met. Returning any value, including a falsy one, is treated as meeting the expectation, and the callback result is returned to the caller of waitFor.

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

waitFor executes the expectation callback every interval (default: every 50 ms) until timeout (default: 1000 ms) is reached. The repeated execution stops as soon as it doesn't throw an error, and the value returned by the callback is returned to the waitFor caller. Otherwise, when it reaches the timeout, the final error thrown by expectation is 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 the expectation callback multiple times, it's highly recommended 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's also recommended to have a single assertion per each waitFor for more consistency and faster failing tests. If you want to make several assertions, put them in separate waitFor calls. In many cases you won't need to wrap the second assertion in waitFor since the first one will do the waiting required for the asynchronous change to happen.

waitFor checks whether Jest fake timers are enabled and adapts its behavior accordingly. 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 will be much faster and more reliable. 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 until the 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 the element to be initially present in the render tree and then removed from it. If the element isn't present when you call this method, it throws an error.

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