fix: revalidate with initialData when changing the key

[koba04]

This fixes #947.
SWR seems not to revalidate when having initialData.

Another strange thing: when state contains empty string refetching happens. But if you change state to any string it returns initial data.

This PR doesn't fix the above that is also mentioned on #947.
I'll investigate it as another issue.

[codesandbox-ci]

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Latest deployment of this branch, based on commit 655fec4:

Sandbox	Source
SWR-Basic	Configuration
SWR-States	Configuration
SWR-Infinite	Configuration
crimson-sunset-pdgv2	Issue #947

[koba04]

The test has been passed in this PR, but I think this shouldn't be passed.
What should SWR return as data with initialData when being revalidating?
I expect that it returns undefined, but SWR in this PR seems to return initialData even it's not an initial mount, so I have to fix the behavior.

  it('should revalidate even if initialData is provided', async () => {
    const fetcher = async (key) => {
      sleep(100);
      return key
    }

    function Page() {
      const [key, setKey] = useState("initial-data-returns-empty-string")
      const { data } = useSWR(key, fetcher, {
        initialData: 'Initial'
      })
      return (
        <div onClick={() => setKey('initial-data-returns-empty-string-update')}>
          hello, {data}
        </div>
      )
    }

    const { container } = render(<Page />)

    // render with the initial data
    await screen.findByText('hello, Initial')

    await act(() => sleep(1))
    fireEvent.focus(window)

    await screen.findByText('hello, initial-data-returns-empty-string')

    // change the key
    await act(() => sleep(1))
    fireEvent.click(container.firstElementChild)

    await act(() => sleep(10))
    // THIS SHOULD BE FAILED, BUT IT'S PASSED
    expect(container.firstChild.textContent).toMatchInlineSnapshot(`"hello, Initial"`)

    // render with data the fetcher returns
    await screen.findByText('hello, initial-data-returns-empty-string-update')
  })

You can also see the behavior on https://codesandbox.io/s/swr-basic-forked-qjnnp?file=/src/App.js.

[koba04]

I'm not sure what the expected value is.
But I expected SWR returns undefined as the return value of data when revalidating. But SWR returns the initialData Is it an expected behavior?

[shuding]

Yeah initialData works like a fallback value currently. I think we need to better name for it, maybe just fallback. And for the real initialData, we need a completely new API, which works like a prefill. (also cc @huozhi)

[koba04]

That seems to be great 👍 fallback would be more intuitive than initialData for the behavior.

[shuding]

The fix looks good, thanks!

[koba04]

Thank you!