Jest react testing: Check state after delay

I'm really confused trying to create test with the help of Jest documentation https://facebook.github.io/jest/docs/timer-mocks.html#content

I'm trying to check a state when container mounts and then few seconds later, after I have manually set values in the state (using setTimeout()).

I have a function inside Main's componentDidMount like this:

componentDidMount() {
    this.setStateAfterDelay();
}

And what the function does is:

setStateAfterDelay = () => {
    setTimeout(() => {
        this.setState({ fruits: ['banana', 'apple', 'orange', 'vodka', 'kiwi'] });
    }, 1500);
}

I achieved the first part with:

const component = mount(<Main />);
expect(component.state().fruits).toEqual(null);

But I have no clue how to check the state again after, lets say 2000ms?

Any help is appreciated :)

Solution 1:

while jest can run async code easily, you can use promise and setTimeout combination to wait a bit. For example this code will wait for 2 seconds:

await new Promise((r) => setTimeout(r, 2000));

Full sample test. Don't forget to add async flag before the callback function:

test('some test title', async () => {
  const foo = true;
  await new Promise((r) => setTimeout(r, 2000));
  expect(foo).toBeDefined();
});

Also, keep in mind that default "timeout" is 5 seconds (5000ms). If your test may run longer, you can add jest.setTimeout(30000); above the test(). 30000 will make sure to not timeout for 30 seconds. You can add any number you need. Full example with setTimeout:

jest.setTimeout(30000);

test('some test title', async () => {
  const foo = true;
  await new Promise((r) => setTimeout(r, 2000));
  expect(foo).toBeDefined();
});

Solution 2:

I haven't really tested this code. But, something similar to this should work I think.

const fruits = ['banana', 'apple', 'orange', 'vodka', 'kiwi'];

it('mock setTimeout test', () => {
 jest.useFakeTimers();
 setTimeout(() => {
   expect(component.state().fruits).toEqual(fruits);
 }, 1500);
 jest.runAllTimers();
});