This library allows you to make render-per-render assertions on your React components and hooks. This is usually not necessary, but can be highly beneficial when testing hot code paths.
This library is intended to test libraries or library-like code. It requires you to write additional components so you can test how your components interact with other components in specific scenarios.
As such, it is not intended to be used for end-to-end testing of your application.
This library originally was part of the Apollo Client test suite and is maintained by the Apollo Client team.
If used with snapshotDOM, RSTL will create a snapshot of your DOM after every
render, and you can iterate through all the intermediate states of your DOM at
your own pace, independenly of how fast these renders actually happened.
test('iterate through renders with DOM snapshots', async () => {
const {takeRender, render} = createRenderStream({
snapshotDOM: true,
})
const utils = render(<Counter />)
const incrementButton = utils.getByText('Increment')
await userEvent.click(incrementButton)
await userEvent.click(incrementButton)
{
const {withinDOM} = await takeRender()
const input = withinDOM().getByLabelText('Value')
expect(input.value).toBe('0')
}
{
const {withinDOM} = await takeRender()
const input = withinDOM().getByLabelText('Value')
expect(input.value).toBe('1')
}
{
const {withinDOM} = await takeRender()
const input = withinDOM().getByLabelText('Value')
expect(input.value).toBe('2')
}
})In every place you would call
const renderStream = createRenderStream(options)
const utils = renderStream.render(<Component />, options)you can also call
const renderStream = renderToRenderStream(<Component />, combinedOptions)
// if required
const utils = await renderStream.renderResultPromiseThis might be shorter (especially in cases where you don't need to access
utils), but keep in mind that the render is executed asynchronously after
calling renderToRenderStream, and that you need to await renderResultPromise
if you need access to utils as returned by render.
Usage is very similar to RTL's renderHook, but you get a snapshotStream
object back that you can iterate with takeSnapshot calls.
test('`useQuery` with `skip`', async () => {
const {takeSnapshot, rerender} = renderHookToSnapshotStream(
({skip}) => useQuery(query, {skip}),
{
wrapper: ({children}) => <Provider client={client}>{children}</Provider>,
},
)
{
const result = await takeSnapshot()
expect(result.loading).toBe(true)
expect(result.data).toBe(undefined)
}
{
const result = await takeSnapshot()
expect(result.loading).toBe(false)
expect(result.data).toEqual({hello: 'world 1'})
}
rerender({skip: true})
{
const snapshot = await takeSnapshot()
expect(snapshot.loading).toBe(false)
expect(snapshot.data).toEqual(undefined)
}
})You can track if a component was rerendered during a specific render by calling
useTrackRenders within it.
test('`useTrackRenders` with suspense', async () => {
function ErrorComponent() {
useTrackRenders()
// return ...
}
function DataComponent() {
useTrackRenders()
const data = useSuspenseQuery(someQuery)
// return ...
}
function LoadingComponent() {
useTrackRenders()
// return ...
}
function App() {
useTrackRenders()
return (
<ErrorBoundary FallbackComponent={ErrorComponent}>
<React.Suspense fallback={<LoadingComponent />}>
<DataComponent />
</React.Suspense>
</ErrorBoundary>
)
}
const {takeRender, render} = createRenderStream()
render(<App />)
{
const {renderedComponents} = await takeRender()
expect(renderedComponents).toEqual([App, LoadingComponent])
}
{
const {renderedComponents} = await takeRender()
expect(renderedComponents).toEqual([DataComponent])
}
})Note
The order of components in renderedComponents is the order of execution of
useLayoutEffect. Keep in mind that this might not be the order you would
expect.
If you need to, you can also take custom snapshots of data in each render.
test('custom snapshots with `replaceSnapshot`', async () => {
function Counter() {
const [value, setValue] = React.useState(0)
replaceSnapshot({value})
// return ...
}
const {takeRender, replaceSnapshot, render} = createRenderStream<{
value: number
}>()
const utils = render(<Counter />)
const incrementButton = utils.getByText('Increment')
await userEvent.click(incrementButton)
{
const {snapshot} = await takeRender()
expect(snapshot).toEqual({value: 0})
}
{
const {snapshot} = await takeRender()
expect(snapshot).toEqual({value: 1})
}
})Tip
replaceSnapshot can also be called with a callback that gives you access to
the last snapshot value.
Tip
You can also use mergeSnapshot, which shallowly merges the last snapshot
with the new one instead of replacing it.
test('assertions in `onRender`', async () => {
function Counter() {
const [value, setValue] = React.useState(0)
replaceSnapshot({value})
return (
<CounterForm value={value} onIncrement={() => setValue(v => v + 1)} />
)
}
const {takeRender, replaceSnapshot, renderResultPromise} =
renderToRenderStream<{
value: number
}>({
onRender(info) {
// you can use `expect` here
expect(info.count).toBe(info.snapshot.value + 1)
},
})
const utils = await renderResultPromise
const incrementButton = utils.getByText('Increment')
await userEvent.click(incrementButton)
await userEvent.click(incrementButton)
await takeRender()
await takeRender()
await takeRender()
})Note
info contains the
base profiling information
passed into onRender of React's Profiler component, as well as snapshot,
replaceSnapshot and mergeSnapshot
This library adds to matchers to expect that can be used like
test('basic functionality', async () => {
const {takeRender} = renderToRenderStream(<RerenderingComponent />)
await expect(takeRender).toRerender()
await takeRender()
// trigger a rerender somehow
await expect(takeRender).toRerender()
await takeRender()
// ensure at the end of a test that no more renders will happen
await expect(takeRender).not.toRerender()
await expect(takeRender).toRenderExactlyTimes(2)
})These matchers can be used on multiple different objects:
await expect(takeRender).toRerender()
await expect(renderStream).toRerender()
await expect(takeSnapshot).toRerender()
await expect(snapshotStream).toRerender()Note
By default, .toRerender and toRenderExactlyTimes will wait 100ms for
renders or to ensure no more renders happens.
You can modify that with the timeout option:
await expect(takeRender).not.toRerender({timeout: 300})Tip
If you don't want these matchers not to be automatically installed, you can
import from @testing-library/react-render-stream instead.
You might want to avoid using this library with act, as act
can end up batching multiple renders
into one in a way that would not happen in a production application.
While that is convenient in a normal test suite, it defeats the purpose of this library.
Keep in mind that tools like userEvent.click use act internally. Many of
those calls would only trigger one render anyways, so it can be okay to use
them, but avoid this for longer-running actions inside of act calls.