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Stefano Magni
Stefano Magni

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From unreadable React Component Tests to simple, stupid ones

Here are the rationales, the mental process, and the patterns at the base of the refactor of some old React Component Tests of mine.

Photo by Mauro Gigli on Unsplash.


The problem

Last Spring, I added a new section to my "Front-end Testing" workshop, dedicated to unit-testing some React components. No need to reinvent the wheel. I already wrote some articles ("Testing a Virtual List component with Cypress and Storybook" and "Unit Testing React components with Cypress") on this topic last year. I just needed to bring them into the course.

Suddenly, I felt terrible while realizing that my tests were almost unreadable. Many abstractions prevented me from carefully understanding what the test did in a while, resulting in a long time reading them. This is unacceptable friction.

How to improve tests readability?

Kent Beck, the father of TDD, said

Test code is NOT production code. It must be 1000x times simpler and smaller.

but how? What adds such friction to my tests? What worsens readability?

I will to analyze a bunch of my tests, reasoning about them, and propose a more straightforward approach. The component under test is a VirtualList mentioned in the previous articles.

Test 1, low complexity

The next test is the simplest one: checking that the VirtualList renders nothing when nothing is passed. Following is the original test

it('When there are no items, then nothing is showed', () => {
  const itemsAmount = 0
  const itemHeight = 30
  const listHeight = 300
  const items = getStoryItems({ amount: itemsAmount })

  mount(
    <VirtualList
      items={items}
      getItemHeights={() => itemHeight}
      RenderItem={createRenderItem({ height: itemHeight })}
      listHeight={listHeight}
    />,
  )

  cy.findByTestId('VirtualList')
    .then($el => $el.text())
    .should('be.empty')
})
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Considering the simplicity of the test, is speaking about readability here bikeshedding? No, I've already some questions about it, such as:

  • What does getStoryItems do?
  • What does createRenderItem do?

I don't report getStoryItems's code here, but it's just a function that generates an array containing X items in the form of [{ id: 1, name: 'Item 1' }, { id: 2, name: 'Item 2' }, /* etc. */]. Its purpose is to generate thousands of items easily, but I wrote it to ease writing Storybook's stories without tests in mind! Then, I reused it for the tests, but stories and tests have completely different needs and purposes!

At the same time, createRenderItem is a factory that generates the list items (some React components). Again, I made it such dynamic for Storybook, and tests weren't originally in my mind.

Both the functions are easy to read, but why am I forcing the reader to follow these abstractions? Are they needed? The answer is no.

Following is the simplified code of the test, following the differences.

it('When there are no items, then nothing is showed', () => {
  // ------------------------------------------
  // Arrange
  const items = [];

  mount(
    <VirtualList
      items={items}
      listHeight={90}
      getItemHeights={() => 30}
      RenderItem={RenderItem}
    />
  );

  // ------------------------------------------
  // Assert
  cy.findByTestId('VirtualList')
    .then(($el) => $el.text())
    .should('be.empty');
});
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-const itemsAmount = 0
-const itemHeight = 30
-const listHeight = 300
-const items = getStoryItems({ amount: itemsAmount })
+const items = [];

mount(
  <VirtualList
    items={items}
    listHeight={listHeight}
-   getItemHeights={() => itemHeight}
+   getItemHeights={() => 30}
-   RenderItem={createRenderItem({ height: itemHeight })}
+   RenderItem={RenderItem}
  />,
)
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In the simplified test, the most significant changes are:

  1. I made items explicit instead of generating them through getStoryItems. The goal is immediately to clarify to the reader which are the items the VirtualList renders.
  2. I removed the "unnecessary" constants. How tall are the items and the list don't make a difference if no items are rendered.
  3. I removed the need for the createRenderItem factory. Generating components with height pre-set is useless here.

Test 2, medium complexity

The next test is not complex. But the way I implemented it adds complexity when not needed. The goal is testing the base functionality of a VirtualList: rendering the visible items and not rendering the invisible ones.

it('When the list receives 10000 items, then only the minimum number of them are rendered', () => {
  const itemsAmount = 10000
  const itemHeight = 30
  const listHeight = 300
  const items = getStoryItems({ amount: itemsAmount })
  const visibleItemsAmount = listHeight / itemHeight

  mount(
    <VirtualList
      items={items}
      getItemHeights={() => itemHeight}
      RenderItem={createRenderItem({ height: itemHeight })}
      listHeight={listHeight}
    />,
  )

  const visibleItems = items.slice(0, visibleItemsAmount - 1)
  itemsShouldBeVisible(visibleItems)

  // first not-rendered item check
  cy.findByText(getItemText(items[visibleItemsAmount])).should('not.exist')
})
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Here things are getting weirder. Again, there are some simple abstractions, but the reader must:

  • Comprehend the value of visibleItemsAmount.
  • Comprehend what visibleItems contains, by reading items.slice. Again, one more complexity layer.
  • Guess about what itemsShouldBeVisible does, or reading its code.
  • Guess what items[visibleItemsAmount] contains.

Yeah, there are some comments. Yeah, I tried to create meaningful names. But I could simplify it a lot.

One more thing. Imagine this situation: the test is failing. It doesn't matter what you have done. This test is failing. You start debugging it, but you know that debugging a highly dynamic test (you don't know what items, visibleItemsAmount, visibleItems, items[visibleItemsAmount] contains upfront) is hard. You need to console.log/cy.log/debugger/breakpoint the test code to have an overall idea of the contents managed by the test, and then you can start debugging the VirtualList. Could I avoid future debugger these problems? Do I need the test to render 10.000 items?

Following is how I improved the test.

it('When the list is longer than the available space, then only the minimum number of items are rendered', () => {
  // ------------------------------------------
  // Arrange

  // creating the data
  const items = [
    // visible ones
    { id: 1, name: 'Item 1' },
    { id: 2, name: 'Item 2' },
    { id: 3, name: 'Item 3' },
    // invisible one
    { id: 3, name: 'Item 4' },
  ];

  // only 3 items are visible
  const itemHeight = 30;
  const listHeight = 90;

  // mounting the component
  mount(
    <VirtualList
      items={items}
      getItemHeights={() => itemHeight}
      RenderItem={RenderItem}
      listHeight={listHeight}
    />
  );

  // ------------------------------------------
  // Act

  // ------------------------------------------
  // Assert
  cy.findByText('Item 1').should('be.visible');
  cy.findByText('Item 2').should('be.visible');
  cy.findByText('Item 3').should('be.visible');
  cy.findByText('Item 4').should('not.exist');
});
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Also, here, the sole and relevant change is easing the future reader/debugger. You don't have to guess/calculate/log the value of the constants, nor what the abstractions do. The code is under your eyes, following the KISS principle (Keep It Simple, Stupid).

-const itemsAmount = 10000
-const itemHeight = 30
-const listHeight = 300
-const items = getStoryItems({ amount: itemsAmount })
-const visibleItemsAmount = listHeight / itemHeight
+// creating the data
+const items = [
+ // visible ones
+ { id: 1, name: 'Item 1' },
+ { id: 2, name: 'Item 2' },
+ { id: 3, name: 'Item 3' },
+ // invisible one
+ { id: 3, name: 'Item 4' },
+];

+// only 3 items are visible
+const itemHeight = 30;
+const listHeight = 90;

/* ... */

-const visibleItems = items.slice(0, visibleItemsAmount - 1)
-itemsShouldBeVisible(visibleItems)
-cy.findByText(getItemText(items[visibleItemsAmount])).should('not.exist')
+cy.findByText('Item 1').should('be.visible');
+cy.findByText('Item 2').should('be.visible');
+cy.findByText('Item 3').should('be.visible');
+cy.findByText('Item 4').should('not.exist');
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Test 3, medium complexity

In the next test, the purpose is to test the VirtualList's buffer property that allows rendering some items even if they aren't visible yet.

it('When some items buffered, then they exist in the page', () => {
  const itemsAmount = 1000
  const itemHeight = 30
  const listHeight = 300
  const items = getStoryItems({ amount: itemsAmount })
  const visibleItemsAmount = listHeight / itemHeight
  const bufferedItemsAmount = 5

  mount(
    <VirtualList
      items={items}
      getItemHeights={() => itemHeight}
      RenderItem={createRenderItem({ height: itemHeight })}
      listHeight={listHeight}
      buffer={bufferedItemsAmount}
    />,
  )

  fastScrollVirtualList().then(() => {
    const firstRenderedItemIndex = getFirstRenderedItemIndex(items, getItemText)
    const firstVisibleItemIndex = firstRenderedItemIndex + bufferedItemsAmount
    const lastVisibleItemIndex = firstVisibleItemIndex + visibleItemsAmount + 1
    const lastRenderedItemIndex = lastVisibleItemIndex + bufferedItemsAmount

    items.slice(firstRenderedItemIndex, firstVisibleItemIndex).forEach(item => {
      cy.findByText(getItemText(item)).should('not.be.visible')
    })

    items.slice(firstVisibleItemIndex, lastVisibleItemIndex).forEach(item => {
      cy.findByText(getItemText(item)).should('be.visible')
    })

    items.slice(lastVisibleItemIndex, lastRenderedItemIndex).forEach(item => {
      cy.findByText(getItemText(item)).should('not.be.visible')
    })
  })
})
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The complexity comes from

  1. Scrolling the list to test that the buffer acts on both sides.
  2. When scrolling stops, I don't know in advance which items are visible and which not, that's why the content of fastScrollVirtualList().then(() => { /* ... */ }) is dynamic.

I return on #2 later, but for this test, I cut the snake's head by removing #1: why do I need to test the entire buffer behavior here through a Cypress Component Test? I have a lot of unit tests that check that the internal VirtualList functions do their job. I don't need to re-test the same behavior again. Once buffer works, it works on both sides. The test benefit from this choice; now it's way simpler, containing a lot of comments that help the reader!

it('Should render only the visible items and the buffered ones when an item is partially visible', () => {
  // ------------------------------------------
  // Arrange

  // creating the data
  const items = [
    // visible ones
    { id: 1, name: 'Item 1' },
    { id: 2, name: 'Item 2' },
    { id: 3, name: 'Item 3' },
    // visible ones
    { id: 4, name: 'Item 4' },
    // buffered ones
    { id: 5, name: 'Item 5' },
    { id: 6, name: 'Item 6' },
    // non-rendered one
    { id: 7, name: 'Item 7' },
  ];

  const itemHeight = 30;
  // 3 items are fully visible, 1 is partially visible
  const listHeight = 100;
  // 2 items are buffered
  const buffer = 2;

  // mounting the component
  mount(
    <VirtualList
      items={items}
      getItemHeights={() => itemHeight}
      RenderItem={RenderItem}
      listHeight={listHeight}
      buffer={buffer}
    />
  );

  // ------------------------------------------
  // Act

  // ------------------------------------------
  // Assert
  cy.findByText('Item 1').should('be.visible');
  cy.findByText('Item 2').should('be.visible');
  cy.findByText('Item 3').should('be.visible');
  cy.findByText('Item 4').should('be.visible');
  cy.findByText('Item 5').should('not.be.visible');
  cy.findByText('Item 6').should('not.be.visible');
  cy.findByText('Item 7').should('not.exist');
});
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Test 4, high complexity

Testing the selection is the hardest part of the VirtualList, because:

  1. The VirtualList manages keyboard modifiers, allowing simple, additive, subtractive, and range selection.
  2. The VirtualList is stateless: we need to wrap it with a stateful wrapper that stores the previous selection to test the additive, subtractive, and range selection.

My old test is very hard or read, because:

  1. The stateful wrapper is declared in the body of the test itself, using the test's scope.
  2. All the selections are checked out in a single long flow.

Step by step, let's simplify it.

Moving the abstraction away

The first part of the old test is the following

it('When the items are clicked, then they are selected', () => {
  const itemHeight = 30
  const listHeight = 300
  let testItems

  const WithSelectionManagement: React.FC<{
    testHandleSelect: (newSelectedIds: ItemId[]) => {}
  }> = props => {
    const { testHandleSelect } = props
    const items = getStoryItems({ amount: 10000 })

    const [selectedItems, setSelectedItems] = React.useState<(string | number)[]>([])

    const handleSelect = React.useCallback<(params: OnSelectCallbackParams<StoryItem>) => void>(
      ({ newSelectedIds }) => {
        setSelectedItems(newSelectedIds)
        testHandleSelect(newSelectedIds)
      },
      [setSelectedItems, testHandleSelect],
    )

    React.useEffect(() => {
      testItems = items
    }, [items])

    return (
      <VirtualList
        items={items}
        getItemHeights={() => itemHeight}
        RenderItem={createSelectableRenderItem({ height: itemHeight })}
        listHeight={listHeight}
        updateScrollModeOnDataChange={{
          addedAtTop: true,
          removedFromTop: true,
          addedAtBottom: true,
          removedFromBottom: true,
        }}
        selectedItemIds={selectedItems}
        onSelect={handleSelect}
      />
    )
  }
  WithSelectionManagement.displayName = 'WithSelectionManagement'

  mount(<WithSelectionManagement testHandleSelect={cy.stub().as('handleSelect')} />)

  /* ... rest of the test ... */
})
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It's pretty hard reading a test that starts with such a boilerplate. You get lost in a while, losing the critical parts of the test itself. Let's move it away from the test.

// wrap the VirtualList to internally manage the selection, passing outside only the new selection
function SelectableList(props) {
  const { onSelect, ...virtualListProps } = props;

  // store the selection in an internal state
  const [selectedItems, setSelectedItems] = React.useState([]);
  const handleSelect = React.useCallback(
    ({ newSelectedIds }) => {
      setSelectedItems(newSelectedIds);
      // call the passed spy to notify the test about the new selected ids
      onSelect({ newSelectedIds });
    },
    [setSelectedItems, onSelect]
  );

  // Transparently renders the VirtualList, apart from:
  // - storing the selection
  // - passing the new selection back to the test
  return (
    <VirtualList
      selectedItemIds={selectedItems}
      onSelect={handleSelect}
      // VirtualList props passed from the test
      {...virtualListProps}
    />
  );
}

it('When two items are clicked pressing the meta button, then they are both selected', () => {
    // ------------------------------------------
    // Arrange

    // creating the data
    const itemHeight = 30;
    const listHeight = 90;
    const items = [
      { id: 1, name: 'Item 1' },
      { id: 2, name: 'Item 2' },
      { id: 3, name: 'Item 3' },
    ];

    // mounting the component
    mount(
      <SelectableList
        // test-specific props
        onSelect={cy.spy().as('onSelect')}
        // VirtualList props
        items={items}
        getItemHeights={() => itemHeight}
        RenderItem={RenderItem}
        listHeight={listHeight}
      />
    );

  /* ... rest of the test ... */
})
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The background noise is still high, but you don't encounter it when you read the test. Then, do you notice that consuming the wrapper moved from

mount(<WithSelectionManagement testHandleSelect={cy.stub().as('handleSelect')} />)
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to

mount(
  <SelectableList
    // test-specific props
    onSelect={cy.spy().as('onSelect')}
    // VirtualList props
    items={items}
    getItemHeights={() => itemHeight}
    RenderItem={RenderItem}
    listHeight={listHeight}
  />
);
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? The purpose is getting the code similar as much as possible to the previous, simpler tests. SelectableList is now more transparent, and its code is simpler because it does just one thing: storing the previous selection.

Splitting the long test

Fasten the seatbelt: apart from the wrapper, the old test is the following

it('When the items are clicked, then they are selected', () => {
  /* ... the code of the wrapper ... */

  cy.then(() => expect(testItems).to.have.length.greaterThan(0))
  cy.wrap(testItems).then(() => {
    cy.findByText(getItemText(testItems[0])).click()
    cy.get('@handleSelect').should(stub => {
      expect(stub).to.have.been.calledOnce
      expect(stub).to.have.been.calledWith([testItems[0].id])
    })

    cy.findByText(getItemText(testItems[1])).click().window()
    cy.get('@handleSelect').should(stub => {
      expect(stub).to.have.been.calledTwice
      expect(stub).to.have.been.calledWith([testItems[1].id])
    })

    cy.get('body')
      .type('{meta}', { release: false })
      .findByText(getItemText(testItems[2]))
      .click()
      .get('@handleSelect')
      .should(stub => {
        expect(stub).to.have.been.calledThrice
        expect(stub).to.have.been.calledWith([testItems[1].id, testItems[2].id])
      })
      .get('body')
      .type('{meta}', { release: true })

    cy.get('body')
      .type('{shift}', { release: false })
      .findByText(getItemText(testItems[0]))
      .click()
      .get('@handleSelect')
      .should(stub => {
        expect(stub).to.have.been.callCount(4)
        expect(stub).to.have.been.calledWith([testItems[2].id, testItems[1].id, testItems[0].id])
      })
      .get('body')
      .type('{shift}', { release: true })

    cy.get('body')
      .type('{alt}', { release: false })
      .findByText(getItemText(testItems[1]))
      .click()
      .get('@handleSelect')
      .should(stub => {
        expect(stub).to.have.been.callCount(5)
        expect(stub).to.have.been.calledWith([testItems[2].id, testItems[0].id])
      })
      .get('body')
      .type('{alt}', { release: true })

    fastScrollVirtualList().then(() => {
      const firstRenderedItemIndex = getFirstRenderedItemIndex(testItems, getItemText)
      const firstRenderedItem = testItems[firstRenderedItemIndex]
      const expectedSelectedItemIds = testItems
        .slice(0, firstRenderedItemIndex + 1)
        .map(item => item.id)

      cy.get('body')
        .type('{shift}', { release: false })
        .findByText(getItemText(firstRenderedItem))
        .click()
        .get('@handleSelect')
        .should(stub => {
          expect(stub).to.have.been.callCount(6)
          expect(stub).to.have.been.calledWith(expectedSelectedItemIds)
        })
        .get('body')
        .type('{shift}', { release: true })
    })
  })
})
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Looking at it now, the weird things are:

  • cy.then(() => expect(testItems).to.have.length.greaterThan(0)) since the items were generated dynamically, I bailed out in case of problems with the external function.
  • cy.findByText(getItemText(testItems[0])).click() is too much dynamic, what testItems[0] contains?
  • The fastScrollVirtualList().then(() => { /* ... */ } content 😩
  • The length of the test itself.
  • It's not clear where testing a type of selection ends and where testing the next one begins.

Let's start by removing the need for such an extended test splitting a four-selection test into four ones.

Single selection

Guess what? Testing the simple selection doesn't need the wrapper at all 😊

it('When an item is clicked, then it is selected', () => {
  // ------------------------------------------
  // Arrange

  // creating the spy
  // a spy is needed to intercept the call the VirtualList does
  const onSelectSpy = cy.spy().as('onSelect');

  // creating the data
  const items = [
    { id: 1, name: 'Item 1' },
    { id: 2, name: 'Item 2' },
    { id: 3, name: 'Item 3' },
  ];

  // mounting the component
  mount(
    <VirtualList
      items={items}
      getItemHeights={() => 30}
      RenderItem={RenderItem}
      listHeight={90}
      onSelect={onSelectSpy}
    />
  );

  // ------------------------------------------
  // Act
  cy.findByText('Item 1').click();

  // ------------------------------------------
  // Assert
  cy.get('@onSelect').should((spy) => {
    expect(spy).to.have.been.calledOnce;

    // Sinon matchers allow to assert about partials of the params
    // see
    // https://sinonjs.org/releases/latest/assertions/
    // https://sinonjs.org/releases/latest/matchers/
    expect(spy).to.have.been.calledWith(
      Cypress.sinon.match({ newSelectedIds: [1] })
    );
    expect(spy).to.have.been.calledWith(
      Cypress.sinon.match({ item: { id: 1, name: 'Item 1' } })
    );
  });
});
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The most relevant changes, compared to the first part of the old test:

  • No need for the wrapper.
  • No more dynamic values.
  • More expressive assertions.
  • It's a single-purpose test.

Additive and subtractive selection

Here we need to leverage the wrapper.

it('When two items are clicked pressing the meta button, then they are both selected', () => {
    // ------------------------------------------
    // Arrange

    // creating the data
    const itemHeight = 30;
    const listHeight = 90;
    const items = [
      { id: 1, name: 'Item 1' },
      { id: 2, name: 'Item 2' },
      { id: 3, name: 'Item 3' },
    ];

    // mounting the component
    mount(
      // mount `SelectableList`instead of `VirtualList`
      <SelectableList
        // test-specific props
        onSelect={cy.spy().as('onSelect')}
        // VirtualList props
        items={items}
        getItemHeights={() => itemHeight}
        RenderItem={RenderItem}
        listHeight={listHeight}
      />
    );

    // ------------------------------------------
    // click on the first item
    // Act
    cy.findByText('Item 1')
      .click()
      // Assert
      .get('@onSelect')
      .should((spy) => {
        expect(spy).to.have.been.calledOnce;
        expect(spy).to.have.been.calledWith({ newSelectedIds: [1] });
      });

    // ------------------------------------------
    // click on the second item
    // Act
    // keep the meta button  pressed
    cy.get('body').type('{meta}', { release: false });

    cy.findByText('Item 2')
      .click()
      // Assert
      .get('@onSelect')
      .should((spy) => {
        expect(spy).to.have.been.calledTwice;
        expect(spy).to.have.been.calledWith({ newSelectedIds: [1, 2] });
      });
  });
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The huge difference here is avoiding scrolling because it doesn't affect the additive selection. Again explicit values, clarity, and brevity allow having a more straightforward and more expressive test.

Testing the subtractive selection follows the same pattern. I don't report it here.

Range selection

Compared to the previous selections, I think that the range selection could be affected by scrolling the list because some items that will be selected aren't rendered anymore. How could I make something that's dynamic by definition—scrolling the list—more static? Through manual tests and some comments.

it('When the list is scrolled amd some items are clicked pressing the shift button, then the range selection works', () => {
  // ------------------------------------------
  // Arrange

  // creating the data
  const items = [
    { id: 1, name: 'Item 1' },
    { id: 2, name: 'Item 2' },
    { id: 3, name: 'Item 3' },
    { id: 4, name: 'Item 4' },
    { id: 5, name: 'Item 5' },
    { id: 6, name: 'Item 6' },
    { id: 7, name: 'Item 7' },
    { id: 8, name: 'Item 8' },
    { id: 9, name: 'Item 9' },
    { id: 10, name: 'Item 10' },
    { id: 11, name: 'Item 11' },
    { id: 12, name: 'Item 12' },
    { id: 13, name: 'Item 13' },
    { id: 14, name: 'Item 14' },
    { id: 15, name: 'Item 15' },
    { id: 16, name: 'Item 16' },
    { id: 17, name: 'Item 17' },
    { id: 18, name: 'Item 18' },
    { id: 19, name: 'Item 19' },
    { id: 20, name: 'Item 20' },
  ];
  // only 3 items are visible
  const itemHeight = 30;
  const listHeight = 90;

  // mounting the component
  mount(
    <SelectableList
      // test-specific props
      onSelect={cy.spy().as('onSelect')}
      // VirtualList props
      items={items}
      getItemHeights={() => itemHeight}
      RenderItem={RenderItem}
      listHeight={listHeight}
    />
  );

  // Act

  // click on the first item
  cy.findByText('Item 1').click();

  // scroll the list
  cy.findAllByTestId('VirtualList').first().trigger('wheel', {
    deltaX: 0,
    deltaY: 200,
  });

  cy.get('body').type('{shift}', { release: false });

  // Item 7, 8, 9, and 10 are going to be visible after the scroll
  // click on the 10th item. It's going to be clicked as soon as it's in the DOM and clickable
  cy.findByText('Item 10')
    .click()
    // Assert
    .get('@onSelect')
    .should((spy) => {
      expect(spy).to.have.been.calledTwice;
      expect(spy).to.have.been.calledWith({ newSelectedIds: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10] });
    });
});
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The worst thing that could happen here is that the scrolling logics of the VirtualList scroll less or more when triggering the wheel event

cy.findAllByTestId('VirtualList').first().trigger('wheel', {
  deltaX: 0,
  deltaY: 200,
});
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but if it happens, the reader has a clear idea of what went wrong because of the comments 😊

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