We currently inline IIFEs by creating a temporary and a labeled block w
the original code. The original return statements turn into an
assignment to the temporary and break out of the label. However, many
cases of IIFEs are due to inlining of manual `useMemo()`, and these
cases often have only a single return statement. Here, the output is
cleaner if we avoid the temporary and label - so that's what we do in
this PR.
Note that the most complex part of the change is actually around
ValidatePreserveExistingMemo - we have some logic to track the IIFE
temporary reassignmetns which needs to be updated to handle the simpler
version of inlining.
---
[//]: # (BEGIN SAPLING FOOTER)
Stack created with [Sapling](https://sapling-scm.com). Best reviewed
with [ReviewStack](https://reviewstack.dev/facebook/react/pull/33726).
* __->__ #33726
* #33725
DiffTrain build for [956d770adf](https://github.com/facebook/react/commit/956d770adf59e1f8a00a7b7c52b5727ef9e353e7)
We unnecessarily render the preamble in a task. This updates the
implementation to perform this render inline.
Testing this is tricky because one of the only ways you could assert
this was even happening is based on how things error if you abort while
rendering the root.
While adding a test for this I discovered that not all abortable tasks
report errors when aborted during a normal render. I've asserted the
current behavior and will address the other issue at another time and
updated the assertion later as necessary
DiffTrain build for [befc1246b0](https://github.com/facebook/react/commit/befc1246b07a04b401bc6e914b7f336a442dca1a)
Stacked on #33658 and #33659.
If we detect that a component is receiving only deeply equal objects,
then we highlight it as potentially problematic and worth looking into.
<img width="1055" alt="Screenshot 2025-06-27 at 4 15 28 PM"
src="https://github.com/user-attachments/assets/e96c6a05-7fff-4fd7-b59a-36ed79f8e609"
/>
It's fairly conservative and can bail out for a number of reasons:
- We only log it on the first parent that triggered this case since
other children could be indirect causes.
- If children has changed then we bail out since this component will
rerender anyway. This means that it won't warn for a lot of cases that
receive plain DOM children since the DOM children won't themselves get
logged.
- If the component's total render time including children is 100ms or
less then we skip warning because rerendering might not be a big deal.
- We don't warn if you have shallow equality but could memoize the JSX
element itself since we don't typically recommend that and React
Compiler doesn't do that. It only warns if you have nested objects too.
- If the depth of the objects is deeper than like the 3 levels that we
print diffs for then we wouldn't warn since we don't know if they were
equal (although we might still warn on a child).
- If the component had any updates scheduled on itself (e.g. setState)
then we don't warn since it would rerender anyway. This should really
consider Context updates too but we don't do that atm. Technically you
should still memoize the incoming props even if you also had unrelated
updates since it could apply to deeper bailouts.
DiffTrain build for [0b78161d7d](https://github.com/facebook/react/commit/0b78161d7d76b7fb9786f25dd222010b9e417191)
View Transitions has this annoying quirk where it adds `width` and
`height` to keyframes automatically when generating keyframes even when
it's not needed. This causes them to deopt from running on the
compositor thread in both Chrome and Safari. @bramus has a [good article
on
it](https://www.bram.us/2025/02/07/view-transitions-applied-more-performant-view-transition-group-animations/).
In React we can automatically rewrite the keyframes when we're starting
a View Transition to drop the `width` and `height` from the keyframes
when they have the same value and the same value as the pseudo element.
To compare it against the pseudo element we first apply the new
keyframes without the width/height and then read it back to see if it
has changed. For gestures, we have already cancelled the previous
animation so we can just read out from that.
DiffTrain build for [c0d151ce7e](https://github.com/facebook/react/commit/c0d151ce7ed9ebdcbcb7ea69c31d68b03b852d37)
`react-stack-bottom-frame` -> `react_stack_bottom_frame`.
This survives `@babel/plugin-transform-function-name`, but now frames
will be displayed as `at Object.react_stack_bottom_frame (...)` in V8.
Checks that were relying on exact function name match were updated to
use either `.indexOf()` or `.includes()`
For backwards compatibility, both React DevTools and Flight Client will
look for both options. I am not so sure about the latter and if React
version is locked.
DiffTrain build for [91d097b2c5](https://github.com/facebook/react/commit/91d097b2c588a0977a7a10ed12512dc8a34e3a5b)
If an aborted task is not rendering, then this is an async abort.
Conceptually it's as if the abort happened inside the async gap. The
abort reason's stack frame won't have that on the stack so instead we
use the owner stack and debug task of any halted async debug info.
One thing that's a bit awkward is that if you do have a sync abort and
you use that error as the "reason" then that thing still has a sync
stack in a different component. In another approach I was exploring
having different error objects for each component but I don't think
that's worth it.
DiffTrain build for [cee7939b00](https://github.com/facebook/react/commit/cee7939b0017ff58230e19663c22393bfd9025ef)
When we abort a render we don't really have much information about the
task that was aborted. Because before a Promise resolves there's no
indication about would have resolved it. In particular we don't know
which I/O would've ultimately called resolve().
However, we can at least emit any information we do have at the point
where we emit it. At the least the stack of the top most Promise.
Currently we synchronously flush at the end of an `abort()` but we
should ideally schedule the flush in a macrotask and emit this debug
information right before that. That way we would give an opportunity for
any `cacheSignal()` abort to trigger rejections all the way up and those
rejections informs the awaited stack.
---------
Co-authored-by: Hendrik Liebau <mail@hendrik-liebau.de>
DiffTrain build for [e67b4fe22e](https://github.com/facebook/react/commit/e67b4fe22e0c3c267303ee6737aec1db48055022)
Substantially improves the last major known issue with the new inference
model's implementation: inferring effects of function expressions. I
knowingly used a really simple (dumb) approach in
InferFunctionExpressionAliasingEffects but it worked surprisingly well
on a ton of code. However, investigating during the sync I saw that we
the algorithm was literally running out of memory, or crashing from
arrays that exceeded the maximum capacity. We were accumluating data
flow in a way that could lead to lists of data flow captures compounding
on themselves and growing very large very quickly. Plus, we were
incorrectly recording some data flow, leading to cases where we reported
false positive "can't mutate frozen value" for example.
So I went back to the drawing board. InferMutationAliasingRanges already
builds up a data flow graph which it uses to figure out what values
would be affected by mutations of other values, and update mutable
ranges. Well, the key question that we really want to answer for
inferring a function expression's aliasing effects is which values
alias/capture where. Per the docs I wrote up, we only have to record
such aliasing _if they are observable via mutations_. So, lightbulb:
simulate mutations of the params, free variables, and return of the
function expression and see which params/free-vars would be affected!
That's what we do now, giving us precise information about which such
values alias/capture where. When the "into" is a param/context-var we
use Capture, iwhen the destination is the return we use Alias to be
conservative.
---
[//]: # (BEGIN SAPLING FOOTER)
Stack created with [Sapling](https://sapling-scm.com). Best reviewed
with [ReviewStack](https://reviewstack.dev/facebook/react/pull/33584).
* #33626
* #33625
* #33624
* __->__ #33584
DiffTrain build for [94cf60bede](https://github.com/facebook/react/commit/94cf60bede7cd6685e07a4374d1e3aa90445130b)
This adds better support for serializing class instances as Debug
values.
It adds a new marker on the object `{ "": "$P...", ... }` which
indicates which constructor's prototype to use for this object's
prototype. It doesn't encode arbitrary prototypes and it doesn't encode
any of the properties on the prototype. It might get some of the
properties from the prototype by virtue of `toString` on a `class`
constructor will include the whole class's body.
This will ensure that the instance gets the right name in logs.
Additionally, this now also invokes getters if they're enumerable on the
prototype. This lets us reify values that can only be read from native
classes.
---------
Co-authored-by: Hendrik Liebau <mail@hendrik-liebau.de>
DiffTrain build for [18ee505e77](https://github.com/facebook/react/commit/18ee505e7791f2bb55f0e520667c51588df7ba48)
This was really meant to be there from the beginning. A `cache()`:ed
entry has a life time. On the server this ends when the render finishes.
On the client this ends when the cache of that scope gets refreshed.
When a cache is no longer needed, it should be possible to abort any
outstanding network requests or other resources. That's what
`cacheSignal()` gives you. It returns an `AbortSignal` which aborts when
the cache lifetime is done based on the same execution scope as a
`cache()`ed function - i.e. `AsyncLocalStorage` on the server or the
render scope on the client.
```js
import {cacheSignal} from 'react';
async function Component() {
await fetch(url, { signal: cacheSignal() });
}
```
For `fetch` in particular, a patch should really just do this
automatically for you. But it's useful for other resources like database
connections.
Another reason it's useful to have a `cacheSignal()` is to ignore any
errors that might have triggered from the act of being aborted. This is
just a general useful JavaScript pattern if you have access to a signal:
```js
async function getData(id, signal) {
try {
await queryDatabase(id, { signal });
} catch (x) {
if (!signal.aborted) {
logError(x); // only log if it's a real error and not due to cancellation
}
return null;
}
}
```
This just gets you a convenient way to get to it without drilling
through so a more idiomatic code in React might look something like.
```js
import {cacheSignal} from "react";
async function getData(id) {
try {
await queryDatabase(id);
} catch (x) {
if (!cacheSignal()?.aborted) {
logError(x);
}
return null;
}
}
```
If it's called outside of a React render, we normally treat any cached
functions as uncached. They're not an error call. They can still load
data. It's just not cached. This is not like an aborted signal because
then you couldn't issue any requests. It's also not like an infinite
abort signal because it's not actually cached forever. Therefore,
`cacheSignal()` returns `null` when called outside of a React render
scope.
Notably the `signal` option passed to `renderToReadableStream` in both
SSR (Fizz) and RSC (Flight Server) is not the same instance that comes
out of `cacheSignal()`. If you abort the `signal` passed in, then the
`cacheSignal()` is also aborted with the same reason. However, the
`cacheSignal()` can also get aborted if the render completes
successfully or fatally errors during render - allowing any outstanding
work that wasn't used to clean up. In the future we might also expand on
this to give different
[`TaskSignal`](https://developer.mozilla.org/en-US/docs/Web/API/TaskSignal)
to different scopes to pass different render or network priorities.
On the client version of `"react"` this exposes a noop (both for
Fiber/Fizz) due to `disableClientCache` flag but it's exposed so that
you can write shared code.
DiffTrain build for [e1dc03492e](https://github.com/facebook/react/commit/e1dc03492eedaec517e14a6e32b8fda571d00767)
Includes #31412.
The issue is that `pushTreeFork` stores some global state when reconcile
children. This gets popped by `popTreeContext` in `completeWork`.
Normally `completeWork` returns its own `Fiber` again if it wants to do
a second pass which will call `pushTreeFork` again in the next pass.
However, `SuspenseList` doesn't return itself, it returns the next child
to work on.
The fix is to keep track of the count and push it again it when we
return the next child to attempt.
There are still some outstanding issues with hydration. Like the
backwards test still has the wrong behavior in it because it hydrates
backwards and so it picks up the DOM nodes in reverse order.
`tail="hidden"` also doesn't work correctly.
There's also another issue with `useId` and `AsyncIterable` in
SuspenseList when there's an unknown number of children. We don't
support those showing one at a time yet though so it's not an issue yet.
To fix it we need to add variable total count to the `useId` algorithm.
E.g. by falling back to varint encoding.
---------
Co-authored-by: Rick Hanlon <rickhanlonii@fb.com>
Co-authored-by: Ricky <rickhanlonii@gmail.com>
DiffTrain build for [c38e268978](https://github.com/facebook/react/commit/c38e26897848374c34ac6b651fce4a9088ed4dd0)
When deeply nested Suspense boundaries inside a fallback of another
boundary resolve it is possible to encounter situations where you either
attempt to flush an aborted Segment or you have a boundary without any
root segment. We intended for both of these conditions to be impossible
to arrive at legitimately however it turns out in this situation you
can. The fix is two-fold
1. allow flushing aborted segments by simply skipping them. This does
remove some protection against future misconfiguraiton of React because
it is no longer an invariant that you hsould never attempt to flush an
aborted segment but there are legitimate cases where this can come up
and simply omitting the segment is fine b/c we know that the user will
never observe this. A semantically better solution would be to avoid
flushing boudaries inside an unneeded fallback but to do this we would
need to track all boundaries inside a fallback or create back pointers
which add to memory overhead and possibly make GC harder to do
efficiently. By flushing extra we're maintaining status quo and only
suffer in performance not with broken semantics.
2. when queuing completed segments allow for queueing aborted segments
and if we are eliding the enqueued segment allow for child segments that
are errored to be enqueued too. This will mean that we can maintain the
invariant that a boundary must have a root segment the first time we
flush it, it just might be aborted (see point 1 above).
This change has two seemingly similar test cases to exercise this fix.
The reason we need both is that when you have empty segments you hit
different code paths within Fizz and so each one (without this fix)
triggers a different error pathway.
This change also includes a fix to our tests where we were not
appropriately setting CSPnonce back to null at the start of each test so
in some contexts scripts would not run for some tests
DiffTrain build for [142aa0744d](https://github.com/facebook/react/commit/142aa0744d0e73dc5390bc19d4d41dd8aeda2b19)
Adding throttling or delaying on images, can obviously impact metrics.
However, it's all in the name of better actual user experience overall.
(Note that it's not strictly worse even for metric. Often it's actually
strictly better due to less work being done overall thanks to batching.)
Metrics can impact things like search ranking but I believe this is on a
curve. If you're already pretty good, then a slight delay won't suddenly
make you rank in a completely different category. Similarly, if you're
already pretty bad then a slight delay won't make it suddenly way worse.
It's still in the same realm. It's just one weight of many. I don't
think this will make a meaningful practical impact and if it does,
that's probably a bug in the weights that will get fixed.
However, because there's a race to try to "make everything green" in
terms of web vitals, if you go from green to yellow only because of some
throttling or suspensey images, it can feel bad. Therefore this
implements a heuristic where if the only reason we'd miss a specific
target is because of throttling or suspensey images, then we shorten the
timeout to hit the metric. This is a worse user experience because it
can lead to extra flashing but feeling good about "green" matters too.
If you then have another reveal that happens to be the largest
contentful paint after that, then that's throttled again so that it
doesn't become flashy after that. If you've already missed the deadline
then you're not going to hit your metric target anyway. It can affect
average but not median.
This is mainly about LCP. It doesn't affect FCP since that doesn't have
a throttle. If your LCP is the same as your FCP then it also doesn't
matter.
We assume that `performance.now()`'s zero point starts at the "start of
the navigation" which makes this simple. Even if we used the
`PerformanceNavigationTiming` API it would just tell us the same thing.
This only implements for Fizz since these metrics tend to currently only
by tracked for initial loads, but with soft navs tracking we could
consider implementing the same for Fiber throttles.
DiffTrain build for [6ccf328499](https://github.com/facebook/react/commit/6ccf328499f06c140ffe96a096744c22319394cc)
We want to make sure that we can block the reveal of a well designed
complete shell reliably. In the Suspense model, client transitions don't
have any way to implicitly resolve. This means you need to use Suspense
or SuspenseList to explicitly split the document. Relying on implicit
would mean you can't add a Suspense boundary later where needed. So we
highly encourage the use of them around large content.
However, if you have constructed a too large shell (e.g. by not adding
any Suspense boundaries at all) then that might take too long to render
on the client. We shouldn't punish users (or overzealous metrics
tracking tools like search engines) in that scenario.
This opts out of render blocking if the shell ends up too large to be
intentional and too slow to load. Instead it deopts to showing the
content split up in arbitrary ways (browser default). It only does this
for SSR, and not client navs so it's not reliable.
In fact, we issue an error to `onError`. This error is recoverable in
that the document is still produced. It's up to your framework to decide
if this errors the build or just surface it for action later.
What should be the limit though? There's a trade off here. If this limit
is too low then you can't fit a reasonably well built UI within it
without getting errors. If it's too high then things that accidentally
fall below it might take too long to load.
I came up with 512kB of uncompressed shell HTML. See the comment in code
for the rationale for this number. TL;DR: Data and theory indicates that
having this much content inside `rel="expect"` doesn't meaningfully
change metrics. Research of above-the-fold content on various websites
indicate that this can comfortable fit all of them which should be
enough for any intentional initial paint.
DiffTrain build for [d177272802](https://github.com/facebook/react/commit/d177272802b7f86a847312c23b7e60a6f56434de)
Block the view transition on suspensey images Up to 500ms just like the
client.
We can't use `decode()` because a bug in Chrome where those are blocked
on `startViewTransition` finishing we instead rely on sync decoding but
also that the image is live when it's animating in and we assume it
doesn't start visible.
However, we can block the View Transition from starting on the `"load"`
or `"error"` events.
The nice thing about blocking inside `startViewTransition` is that we
have already done the layout so we can only wait on images that are
within the viewport at this point. We might want to do that in Fiber
too. If many image doesn't have fixed size but need to load first, they
can all end up in the viewport. We might consider only doing this for
images that have a fixed size or only a max number that doesn't have a
fixed size.
DiffTrain build for [22b929156c](https://github.com/facebook/react/commit/22b929156c325eaf52c375f0c62801831951814a)
We want to change the defaults for `revealOrder` and `tail` on
SuspenseList. This is an intermediate step to allow experimental users
to upgrade.
To explicitly specify these options I added `revealOrder="independent"`
and `tail="visible"`.
I then added warnings if `undefined` or `null` is passed. You must now
always explicitly specify them. However, semantics are still preserved
for now until the next step.
We also want to change the rendering order of the `children` prop for
`revealOrder="backwards"`. As an intermediate step I first added
`revealOrder="unstable_legacy-backwards"` option. This will only be
temporary until all users can switch to the new `"backwards"` semantics
once we flip it in the next step.
I also clarified the types that the directional props requires iterable
children but not iterable inside of those. Rows with multiple items can
be modeled as explicit fragments.
DiffTrain build for [d742611ce4](https://github.com/facebook/react/commit/d742611ce40545127032f4e221c78bf9f70eb437)
This is the same technique we do for the client except we don't check
whether this is newly created font loading to keep code small.
Unfortunately, we can't use this technique for Suspensey images. They'll
need to block before we call `startViewTransition` in a separate
refactor. This is due to a bug in Chrome where `img.decode()` doesn't
resolve until `startViewTransition` does.
DiffTrain build for [c0464aedb1](https://github.com/facebook/react/commit/c0464aedb16b1c970d717651bba8d1c66c578729)
josephsavona
gnoff
sebmarkbage
rickhanlonii
kassens
hoxyq
eps1lon
jbrown215
yungsters
mofeiZ