The previous PR only updated simple assignment expressions (where the lvalue is
an identifier), this PR extends the same idea to all assignment variants. Note
that there is one case that doesn't work yet, which is complex destructuring
assignment as a value:
```javascript
let x = makeObject();
x.foo(([[x]] = makeObject()));
```
What happens here is that we lower the destructuring to a series of steps:
```
tmp1: Destructure Const [ tmp0 ] = makeObject();
tmp2: Destructure Reassign [ x ] = tmp0;
PropertyCall x, 'foo', [ tmp1 ]
```
Thankfully we can detect this case: if we have a const/let declaration with an
lvalue, that's invalid. See the new error test case which shows we correctly
detect & reject this case for now.
This PR subtly changes how we represent assignment expressions in order to
accurately model them _as expressions_. Specifically, the result of lowering an
assignment is now the temporary created for the assignment's lvalue. This allows
us to restore the assignment as a value (expression) during codegen. Note how
this fixes a bug and cleans up some output.
Updates ConstantPropagation so that each instruction is responsible for whether
to replace its `.value` with the resolved constant value (if found).
Specifically, for `StoreLocal` we don't want to replace the value — we want to
keep the assignment — but we do want to propagate the _result_ of the assignment
downstream. This more accurately models the semantics of assignment expressions,
and helps with subsequent PRs.
Today if something suspends, React will continue rendering the siblings
of that component.
Our original rationale for prerendering the siblings of a suspended
component was to initiate any lazy fetches that they might contain. This
was when we were more bullish about lazy fetching being a good idea some
of the time (when combined with prefetching), as opposed to our latest
thinking, which is that it's almost always a bad idea.
Another rationale for the original behavior was that the render was I/O
bound, anyway, so we might as do some extra work in the meantime. But
this was before we had the concept of instant loading states: when
navigating to a new screen, it's better to show a loading state as soon
as you can (often a skeleton UI), rather than delay the transition.
(There are still cases where we block the render, when a suitable
loading state is not available; it's just not _all_ cases where
something suspends.) So the biggest issue with our existing
implementation is that the prerendering of the siblings happens within
the same render pass as the one that suspended — _before_ the loading
state appears.
What we should do instead is immediately unwind the stack as soon as
something suspends, to unblock the loading state.
If we want to preserve the ability to prerender the siblings, what we
could do is schedule special render pass immediately after the fallback
is displayed. This is likely what we'll do in the future. However, in
the new implementation of `use`, there's another reason we don't
prerender siblings: so we can preserve the state of the stack when
something suspends, and resume where we left of when the promise
resolves without replaying the parents. The only way to do this
currently is to suspend the entire work loop. Fiber does not currently
support rendering multiple siblings in "parallel". Once you move onto
the next sibling, the stack of the previous sibling is discarded and
cannot be restored. We do plan to implement this feature, but it will
require a not-insignificant refactor.
Given that lazy data fetching is already bad for performance, the best
trade off for now seems to be to disable prerendering of siblings. This
gives us the best performance characteristics when you're following best
practices (i.e. hoist data fetches to Server Components or route
loaders), at the expense of making an already bad pattern a bit worse.
Later, when we implement resumable context stacks, we can reenable
sibling prerendering. Though even then the use case will mostly be to
prerender the CPU-bound work, not lazy fetches.
(This was reviewed and approved as part of #26380; I'm extracting it
into its own PR so that it can bisected later if it causes an issue.)
I noticed while working on a PR that when an error happens during
hydration, and we revert to client rendering, React actually does _two_
additional render passes instead of just one. We didn't notice it
earlier because none of our tests happened to assert on how many renders
it took to recover, only on the final output.
It's possible this extra render pass had other consequences that I'm not
aware of, like messing with some assumption in the recoverable errors
logic.
This adds a test to demonstrate the issue. (One problem is that we don't
have much test coverage of this scenario in the first place, which
likely would have caught this earlier.)
This is in line with the refactor I already did on Fizz earlier and
brings Fiber up to a similar structure.
We end up with a lot of extra checks due the extra abstractions we use
to check the various properties. This uses a flatter and more inline
model which makes it easier to see what each property does. The tradeoff
is that a change might need changes in more places.
The general structure is that there's a switch for tag first, then a
switch for each attribute special case, then a switch for the value. So
it's easy to follow where each scenario will end up and there shouldn't
be any unnecessary code executed along the way.
My goal is to eventually get rid of the meta-programming in DOMProperty
and CSSProperty but I'm leaving that in for now - in line with Fizz.
My next step is moving around things a bit in the diff/commit phases.
This is the first step to more refactors for perf and size, but also
because I'm adding more special cases so I need to have a flatter
structure that I can reason about for those special cases.
Rewrite ArrowFunctionExpression to FunctionDeclaration and compile it. This lets
us reuse all the export gating logic, rather than writing separate, specific
logic for ArrowFunctionExpression.
Cleans up duplicated code for processing call/constructor arguments. As a side
benefit, we now support spread elements for constructor arguments (and if we
want to change how we represent that, we can do it in one place).
When rendering a suspensey resource that we haven't seen before, it may
have loaded in the background while we were rendering. We should yield
to the main thread to see if the load event fires in an immediate task.
For example, if the resource for a link element has already loaded, its
load event will fire in a task right after React yields to the main
thread. Because the continuation task is not scheduled until right
before React yields, the load event will ping React before it resumes.
If this happens, we can resume rendering without showing a fallback.
I don't think this matters much for images, because the `completed`
property tells us whether the image has loaded, and during a non-urgent
render, we never block the main thread for more than 5ms at a time (for
now — we might increase this in the future). It matters more for
stylesheets because the only way to check if it has loaded is by
listening for the load event.
This is essentially the same trick that `use` does for userspace
promises, but a bit simpler because we don't need to replay the host
component's begin phase; the work-in-progress fiber already completed,
so we can just continue onto the next sibling without any additional
work.
As part of this change, I split the `shouldSuspendCommit` host config
method into separate `maySuspendCommit` and `preloadInstance` methods.
Previously `shouldSuspendCommit` was used for both.
This raised a question of whether we should preload resources during a
synchronous render. My initial instinct was that we shouldn't, because
we're going to synchronously block the main thread until the resource is
inserted into the DOM, anyway. But I wonder if the browser is able to
initiate the preload even while the main thread is blocked. It's
probably a micro-optimization either way because most resources will be
loaded during transitions, not urgent renders.
## Summary
We are going to move the definition of public instances from React to
React Native to have them together with the native methods in Fabric
that they invoke. This will allow us to have a better type safety
between them and iterate faster on the implementation of this proposal:
https://github.com/react-native-community/discussions-and-proposals/pull/607
The interface between React and React Native would look like this after
this change and a following PR (#26418):
React → React Native:
```javascript
ReactNativePrivateInterface.createPublicInstance // to provide via refs
ReactNativePrivateInterface.getNodeFromPublicInstance // for DevTools, commands, etc.
ReactNativePrivateInterface.getNativeTagFromPublicInstance // to implement `findNodeHandle`
```
React Native → React (ReactFabric):
```javascript
ReactFabric.getNodeFromInternalInstanceHandle // to get most recent node to call into native
ReactFabric.getPublicInstanceFromInternalInstanceHandle // to get public instances from results from native
```
## How did you test this change?
Flow
Existing unit tests
Sizebot works by fetching the base artifacts from CI. CircleCI recently
updated this endpoint to require an auth token. This is a problem for PR
branches, where sizebot runs, because we don't want to leak the token to
arbitrary code written by an outside contributor.
This only affects PR branches. CI workflows that run on the main branch
are allowed to access environment variables, because only those with
push access can land code in main.
As a temporary workaround, we'll fetch the assets from a mirror,
react-builds.vercel.app. This is the same app that hosts the sizebot
diff previews.
Need to figure out a longer term solution. Perhaps by converting sizebot
into a proper GitHub app.
This adds a new capability for renderers (React DOM, React Native):
prevent a tree from being displayed until it is ready, showing a
fallback if necessary, but without blocking the React components from
being evaluated in the meantime.
A concrete example is CSS loading: React DOM can block a commit from
being applied until the stylesheet has loaded. This allows us to load
the CSS asynchronously, while also preventing a flash of unstyled
content. Images and fonts are some of the other use cases.
You can think of this as "Suspense for the commit phase". Traditional
Suspense, i.e. with `use`, blocking during the render phase: React
cannot proceed with rendering until the data is available. But in the
case of things like stylesheets, you don't need the CSS in order to
evaluate the component. It just needs to be loaded before the tree is
committed. Because React buffers its side effects and mutations, it can
do work in parallel while the stylesheets load in the background.
Like regular Suspense, a "suspensey" stylesheet or image will trigger
the nearest Suspense fallback if it hasn't loaded yet. For now, though,
we only do this for non-urgent updates, like with startTransition. If
you render a suspensey resource during an urgent update, it will revert
to today's behavior. (We may or may not add a way to suspend the commit
during an urgent update in the future.)
In this PR, I have implemented this capability in the reconciler via new
methods added to the host config. I've used our internal React "no-op"
renderer to write tests that demonstrate the feature. I have not yet
implemented Suspensey CSS, images, etc in React DOM. @gnoff and I will
work on that in subsequent PRs.
CircleCI now enforces passing a token when fetching artifacts. I'm also
deleting the old request-promise-json dependency because AFAIK we were
only using it to fetch json from circleci about the list of available
artifacts – which we can just do using node-fetch. Plus, the underlying
request package it uses has been deprecated since 2019.
## Summary
We had to revert the last React sync to React Native because we saw
issues with Responder events using stale event handlers instead of
recent versions.
I reviewed the merged PRs and realized the problem was in the refactor I
did in #26321. In that PR, we moved `currentProps` from `canonical`,
which is a singleton referenced by all versions of the same fiber, to
the fiber itself. This is causing the staleness we observed in events.
This PR does a partial revert of the refactor in #26321, bringing back
the `canonical` object but moving `publicInstance` to one of its fields,
instead of being the `canonical` object itself.
## How did you test this change?
Existing unit tests continue working (I didn't manage to get a repro
using the test renderer).
I manually tested this change in Meta infra and saw the problem was
fixed.
In concurrent mode we error if child nodes mismatches which triggers a
recreation of the whole hydration boundary. This ensures that we don't
replay the wrong thing, transform state or other security issues.
For text content, we respect `suppressedHydrationWarning` to allow for
things like `<div suppressedHydrationWarning>{timestamp}</div>` to
ignore the timestamp. This mode actually still patches up the text
content to be the client rendered content.
In principle we shouldn't have to do that because either value should be
ok, and arguably it's better not to trigger layout thrash after the
fact.
We do have a lot of code still to deal with patching up the tree because
that's what legacy mode does which is still in the code base. When we
delete legacy mode we would still be stuck with a lot of it just to deal
with this case.
Therefore I propose that we change the semantics to not patch up
hydration errors for text nodes. We already don't for attributes.
We currently are not lowering property calls in evaluation order.
I wonder if the following lowering for a PropertyCall (with static or computed
property) is semantically equivalent to source:
1. eval + resolve receiver (store in t0)
2. eval computed property (if present)
3. resolve t0.property (binding the call to receiver and storing in t1)
4. eval args
5. eval t1(args)
Although codegen might then generate something like this (if args are named
temporaries)
```js
const tmp = receiver.property.bind(receiver);
// lower args to temporaries
tmp(arg1, arg2);
```
Just noticed the test isn't testing what it is meant to test properly.
The error `Warning: ReactDOM.render is no longer supported in React 18.
Use createRoot instead. Until you switch to the new API, your app will
behave as if it's running React 17. Learn more:
https://reactjs.org/link/switch-to-createroot` is thrown, the inner
`expect(error).toContain('Warning: Maximum update depth exceeded.');`
failed and threw jest error, and the outer `.toThrow('Maximum update
depth exceeded.')` happens to catch it and makes the test pass.
As I understand it this isn't used at Meta and it would let us get rid
of at least legacy mode hydration code when we remove legacy mode from
OSS builds.
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## Summary
<!--
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does the pull request solve?
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`react-dom/server` in Bun (correctly) chooses `react-dom/server.bun`,
but `react-dom/server.bun` currently can't be imported because it is not
included in package.json `"exports"` (`react-dom/server` works,
`react-dom/server.bun` doesn't). Previously, I didn't think it was
necessary to do that, but it is too easy to accidentally run the browser
build in unit tests when importing `react-dom/server`
This also aligns behavior of package.json `"exports"` of
`react-dom/server.bun` with `react-dom/server.browser`,
`react-dom/server.node`, and the rest.
## How did you test this change?
Manually edited package.json in node_modules in a separate folder and
ran tests in Bun with `react-dom/server.bun` as the import specifier
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Adds a `DeclareLocal` instruction which represents declaring a named variable
without initializing it. Currently declarations without an initializer (`let x`)
are transformed into a declaration to undefined (`let x = undefined`) which
changes the semantics due to hoisting and TDZ (temporary dead zone). The correct
thing is to represent declaration without initialization.
These examples previously errored all the way in codegen, when we detected that
a value block (eg a `while` test expression) was declaring a new variable. We
now detect this in LeaveSSA and error. The actual fix is a bit tricky, we'd need
to add a new declaration in the nearest block scope (or selectively not DCE the
declaration if its reassigned in just this way).
If a logical or conditional expression is unused, then a phi node isn't created
for the identifier it assigns to. Then when we leave SSA form the two branches
will assign to separate values, and we aren't sure which identifier to use as
the lvalue of the resulting ReactiveInstruction (remember that
logicals/conditionals decompose into control flow in HIR, but are a single
compound instruction in ReactiveFunction). If the two sides don't assign to the
same location, it could be because of a bug in the compiler or because the value
wasn't used. Ideally we'd represent this explicitly, but for now i'm just making
this a TODO since most logicals/conditionals should have their value used.
## Summary
I refactored this code in #26290 but forgot to add guards when the fiber
or the state node where null, and this is typed as `any` so Flow didn't
catch it.
This restores the same logic to guard against null.
## How did you test this change?
Existing tests.
Enables support for assignment expressions in value blocks (which includes in
loop init/test/update blocks). This was pretty straightforward, the main changes
are:
* During PropagateScopeDependencies, we currently record scope reassignments
based on `Identifier` object identity. In the case where a variable is
reassigned in multiple control-flow paths of a value block, however, there can
be multiple object identities. So we now de-dupe reassignments based on
identifier id.
* MergeOverlappingScopes now treats value blocks as regular blocks, allowing it
to correctly merge scopes from the value with other scopes from the outer block.
Otherwise this is mostly just lots of tests. Note that there is an outstanding
todo, which is that we currently error for ternaries and logicals whose value is
unused (eg `cond ? (x = 1) : null`). I'll address that in a follow-up.
There's currently a giant cycle between the event system, through
react-dom-bindings, reconciler and then react-dom. We resolve this cycle
using dependency injection. However, this all ends up in the same
bundle. It can be reordered to resolve the cycles. If we avoid
side-effects and avoid reading from module exports during
initialization, this should be resolvable in a more optimal way by the
compiler.
I'm trying to get rid of all meta programming in the module scope so
that closure can do a better job figuring out cyclic dependencies and
ability to reorder.
This is converting a lot of the patterns that assign functions
conditionally to using function declarations instead.
```
let fn;
if (__DEV__) {
fn = function() {
...
};
}
```
->
```
function fn() {
if (__DEV__) {
...
}
}
```
These used to be used by partial render.
ReactDOMDispatcher ended up not being used in this way.
Move shared DOM files to client. These are only used by client
abstractions now. They're inlined in the Fizz code so they're no longer
shared.
Joe Savona
Samuel Susla
Andrew Clark
Sathya Gunasekaran
Sebastian Markbåge
Rubén Norte
mofeiZ
lauren
Jan Kassens
Ibrahim Amin
Tianyu Yao
Ricky
Jarred Sumner
Sophie Alpert