mutate with optimisticData clears data on revalidation

[cbejensen]

Bug report

When using mutate with optimisticData and revalidate: true, the data is cleared and isLoading is set to true during revalidation.

Expected Behavior

I would expect data to be preserved while revalidating. Also, I would expect isValidating to be true, but isLoading to be false.

Code Example

https://codesandbox.io/s/swr-basic-forked-6srjsk?file=/src/App.js

[koba04]

@cbejensen
In your api.js, addTodo returns nothing, and populateCache is true (the default value of mutate), which means that SWR updates its cache data with undefined, so useSWR returns undefined when the next revalidation.

export async function addTodo(todo) {
  await delay();
  todos = [...todos, todo];
}

https://swr.vercel.app/docs/mutation#parameters

To fix that, you can disable the populateCache option at mutate or return the updated data from addTodo.

[cbejensen]

Ah, just my misunderstanding then - thank you!

[koba04]

Thank you for your feedback! 👍