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Add FAQ section about how flushPromises work (closes #11) #15

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Add FAQ section about how flushPromises work
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Some subtle changes
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22 changes: 22 additions & 0 deletions README.md
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Expand Up @@ -345,6 +345,27 @@ That's not to say that there's never a use case for doing those things, so they
should be possible to accomplish, just not the default and natural way to test
react components.

**How does `flushPromises` work and why would I need it?**

As mentioned [before](#flushpromises), `flushPromises` uses
[`setImmediate`][set-immediate] to schedule resolving a promise after any pending
tasks in
[the message queue](https://developer.mozilla.org/en-US/docs/Web/JavaScript/EventLoop)
are processed. This includes any promises fired before in your test.

If there are promise callbacks already in JavaScript's message queue pending to be
processed at the time `flushPromises` is called, then these will be processed before
the promise returned by `flushPromises` is resolved. So when you
`await flushPromises()` the code immediately after it is guaranteed to occur after
all the side effects of your async requests have ocurred. This includes any data
your test components might have requested.

This is useful for instance, if your components perform any data requests and update
their state with the results when the request is resolved. It's important to note
that this is only effective if you've mocked out your async requests to resolve
immediately (like the `axios` mock we have in the examples). It will not `await`
for promises that are not already resolved by the time you attempt to flush them.

## Other Solutions

In preparing this project,
Expand Down Expand Up @@ -421,3 +442,4 @@ MIT
[twitter-badge]: https://img.shields.io/twitter/url/https/github.com/kentcdodds/react-testing-library.svg?style=social
[emojis]: https://github.com/kentcdodds/all-contributors#emoji-key
[all-contributors]: https://github.com/kentcdodds/all-contributors
[set-immediate]: https://developer.mozilla.org/en-US/docs/Web/API/Window/setImmediate