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Render children passed to "backwards" SuspenseList in reverse mount order (#35021)
Stacked on #35018. This mounts the children of SuspenseList backwards. Meaning the first child is mounted last in the DOM (and effect list). It's like calling reverse() on the children. This is meant to set us up for allowing AsyncIterable children where the unknown number of children streams in at the end (which is the beginning in a backwards SuspenseList). For consistency we do that with other children too. `unstable_legacy-backwards` still exists for the old mode but is meant to be deprecated. <img width="100" alt="image" src="https://github.com/user-attachments/assets/5c2a95d7-34c4-4a4e-b602-3646a834d779" />
Sebastian Markbåge committed
Oct 31, 2025 at 13:33 UTC
488d88b018ee8fd1fac56cab22dfa8796ebce30b
5 files changed
+140
-33
packages/react-dom/src/__tests__/ReactDOMFizzSuspenseList-test.js
+75
-6
@@ -656,6 +656,77 @@ describe('ReactDOMFizzSuspenseList', () => {
656
);
657
});
658
659
+ // @gate enableSuspenseList
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+ it('displays each items in "backwards" mount order', async () => {
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+ const A = createAsyncText('A');
662
+ const B = createAsyncText('B');
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+ const C = createAsyncText('C');
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+
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+ function Foo() {
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+ return (
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+ <div>
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+ <SuspenseList revealOrder="backwards" tail="visible">
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+ <Suspense fallback={<Text text="Loading C" />}>
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+ <C />
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+ </Suspense>
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+ <Suspense fallback={<Text text="Loading B" />}>
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+ <B />
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+ </Suspense>
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+ <Suspense fallback={<Text text="Loading A" />}>
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+ <A />
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+ </Suspense>
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+ </SuspenseList>
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+ </div>
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+ );
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+ }
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+
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+ await A.resolve();
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+
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+ await serverAct(async () => {
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+ const {pipe} = ReactDOMFizzServer.renderToPipeableStream(<Foo />);
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+ pipe(writable);
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+ });
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+
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+ assertLog([
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+ 'Suspend! [C]',
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+ 'Suspend! [B]', // TODO: Defer rendering the content after fallback if previous suspended,
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+ 'A',
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+ 'Loading C',
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+ 'Loading B',
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+ 'Loading A',
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+ ]);
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+
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+ expect(getVisibleChildren(container)).toEqual(
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+ <div>
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+ <span>Loading A</span>
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+ <span>Loading B</span>
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+ <span>Loading C</span>
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+ </div>,
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+ );
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+
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+ await serverAct(() => C.resolve());
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+ assertLog(['C']);
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+
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+ expect(getVisibleChildren(container)).toEqual(
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+ <div>
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+ <span>Loading A</span>
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+ <span>Loading B</span>
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+ <span>C</span>
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+ </div>,
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+ );
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+
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+ await serverAct(() => B.resolve());
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+ assertLog(['B']);
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+
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+ expect(getVisibleChildren(container)).toEqual(
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+ <div>
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+ <span>A</span>
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+ <span>B</span>
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+ <span>C</span>
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+ </div>,
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+ );
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+ });
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+
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// @gate enableSuspenseList
731
it('displays each items in "backwards" order in legacy mode', async () => {
732
const A = createAsyncText('A');
@@ -737,15 +808,13 @@ describe('ReactDOMFizzSuspenseList', () => {
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return (
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<div>
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<SuspenseList revealOrder="forwards" tail="visible">
740
- <SuspenseList
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- revealOrder="unstable_legacy-backwards"
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- tail="visible">
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- <Suspense fallback={<Text text="Loading A" />}>
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- <A />
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- </Suspense>
811
+ <SuspenseList revealOrder="backwards" tail="visible">
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<Suspense fallback={<Text text="Loading B" />}>
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<B />
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</Suspense>
815
+ <Suspense fallback={<Text text="Loading A" />}>
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+ <A />
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+ </Suspense>
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</SuspenseList>
819
<Suspense fallback={<Text text="Loading C" />}>
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<C />
packages/react-reconciler/src/ReactFiberBeginWork.js
+54
-8
@@ -3247,18 +3247,14 @@ function validateRevealOrder(revealOrder: SuspenseListRevealOrder) {
3247
if (
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revealOrder != null &&
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revealOrder !== 'forwards' &&
3250
+ revealOrder !== 'backwards' &&
3251
revealOrder !== 'unstable_legacy-backwards' &&
3252
revealOrder !== 'together' &&
3253
revealOrder !== 'independent' &&
3254
!didWarnAboutRevealOrder[cacheKey]
3255
) {
3256
didWarnAboutRevealOrder[cacheKey] = true;
3256
- if (revealOrder === 'backwards') {
3257
- console.error(
3258
- 'The rendering order of <SuspenseList revealOrder="backwards"> is changing. ' +
3259
- 'To be future compatible you must specify revealOrder="legacy_unstable-backwards" instead.',
3260
- );
3261
- } else if (typeof revealOrder === 'string') {
3257
+ if (typeof revealOrder === 'string') {
3258
switch (revealOrder.toLowerCase()) {
3259
case 'together':
3260
case 'forwards':
@@ -3371,6 +3367,17 @@ function initSuspenseListRenderState(
3367
}
3368
}
3369
3370
+function reverseChildren(fiber: Fiber): void {
3371
+ let row = fiber.child;
3372
+ fiber.child = null;
3373
+ while (row !== null) {
3374
+ const nextRow = row.sibling;
3375
+ row.sibling = fiber.child;
3376
+ fiber.child = row;
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+ row = nextRow;
3378
+ }
3379
+}
3380
+
3381
// This can end up rendering this component multiple passes.
3382
// The first pass splits the children fibers into two sets. A head and tail.
3383
// We first render the head. If anything is in fallback state, we do another
@@ -3409,7 +3416,16 @@ function updateSuspenseListComponent(
3416
validateTailOptions(tailMode, revealOrder);
3417
validateSuspenseListChildren(newChildren, revealOrder);
3418
3412
- reconcileChildren(current, workInProgress, newChildren, renderLanes);
3419
+ if (revealOrder === 'backwards' && current !== null) {
3420
+ // For backwards the current mounted set will be backwards. Reconciling against it
3421
+ // will lead to mismatches and reorders. We need to swap the original set first
3422
+ // and then restore it afterwards.
3423
+ reverseChildren(current);
3424
+ reconcileChildren(current, workInProgress, newChildren, renderLanes);
3425
+ reverseChildren(current);
3426
+ } else {
3427
+ reconcileChildren(current, workInProgress, newChildren, renderLanes);
3428
+ }
3429
// Read how many children forks this set pushed so we can push it every time we retry.
3430
const treeForkCount = getIsHydrating() ? getForksAtLevel(workInProgress) : 0;
3431
@@ -3434,7 +3450,37 @@ function updateSuspenseListComponent(
3450
workInProgress.memoizedState = null;
3451
} else {
3452
switch (revealOrder) {
3437
- case 'backwards':
3453
+ case 'backwards': {
3454
+ // We're going to find the first row that has existing content.
3455
+ // We are also going to reverse the order of anything in the existing content
3456
+ // since we want to actually render them backwards from the reconciled set.
3457
+ // The tail is left in order, because it'll be added to the front as we
3458
+ // complete each item.
3459
+ const lastContentRow = findLastContentRow(workInProgress.child);
3460
+ let tail;
3461
+ if (lastContentRow === null) {
3462
+ // The whole list is part of the tail.
3463
+ tail = workInProgress.child;
3464
+ workInProgress.child = null;
3465
+ } else {
3466
+ // Disconnect the tail rows after the content row.
3467
+ // We're going to render them separately later in reverse order.
3468
+ tail = lastContentRow.sibling;
3469
+ lastContentRow.sibling = null;
3470
+ // We have to now reverse the main content so it renders backwards too.
3471
+ reverseChildren(workInProgress);
3472
+ }
3473
+ // TODO: If workInProgress.child is null, we can continue on the tail immediately.
3474
+ initSuspenseListRenderState(
3475
+ workInProgress,
3476
+ true, // isBackwards
3477
+ tail,
3478
+ null, // last
3479
+ tailMode,
3480
+ treeForkCount,
3481
+ );
3482
+ break;
3483
+ }
3484
case 'unstable_legacy-backwards': {
3485
// We're going to find the first row that has existing content.
3486
// At the same time we're going to reverse the list of everything
packages/react-reconciler/src/ReactFiberCompleteWork.js
-4
@@ -1838,10 +1838,6 @@ function completeWork(
1838
}
1839
}
1840
if (renderState.isBackwards) {
1841
- // The effect list of the backwards tail will have been added
1842
- // to the end. This breaks the guarantee that life-cycles fire in
1843
- // sibling order but that isn't a strong guarantee promised by React.
1844
- // Especially since these might also just pop in during future commits.
1841
// Append to the beginning of the list.
1842
renderedTail.sibling = workInProgress.child;
1843
workInProgress.child = renderedTail;
packages/react-reconciler/src/__tests__/ReactSuspenseList-test.js
+6
-14
@@ -1022,7 +1022,7 @@ describe('ReactSuspenseList', () => {
1022
});
1023
1024
// @gate enableSuspenseList
1025
- it('warns if revealOrder="backwards" is specified', async () => {
1025
+ it('displays each items in "backwards" order', async () => {
1026
const A = createAsyncText('A');
1027
const B = createAsyncText('B');
1028
const C = createAsyncText('C');
@@ -1030,14 +1030,14 @@ describe('ReactSuspenseList', () => {
1030
function Foo() {
1031
return (
1032
<SuspenseList revealOrder="backwards" tail="visible">
1033
- <Suspense fallback={<Text text="Loading A" />}>
1034
- <A />
1033
+ <Suspense fallback={<Text text="Loading C" />}>
1034
+ <C />
1035
</Suspense>
1036
<Suspense fallback={<Text text="Loading B" />}>
1037
<B />
1038
</Suspense>
1039
- <Suspense fallback={<Text text="Loading C" />}>
1040
- <C />
1039
+ <Suspense fallback={<Text text="Loading A" />}>
1040
+ <A />
1041
</Suspense>
1042
</SuspenseList>
1043
);
@@ -1056,14 +1056,6 @@ describe('ReactSuspenseList', () => {
1056
'Suspend! [C]',
1057
]);
1058
1059
- assertConsoleErrorDev([
1060
- 'The rendering order of <SuspenseList revealOrder="backwards"> is changing. ' +
1061
- 'To be future compatible you must specify ' +
1062
- 'revealOrder="legacy_unstable-backwards" instead.' +
1063
- '\n in SuspenseList (at **)' +
1064
- '\n in Foo (at **)',
1065
- ]);
1066
-
1059
expect(ReactNoop).toMatchRenderedOutput(
1060
<>
1061
<span>Loading A</span>
@@ -1101,7 +1093,7 @@ describe('ReactSuspenseList', () => {
1093
});
1094
1095
// @gate enableSuspenseList
1104
- it('displays each items in "backwards" order', async () => {
1096
+ it('displays each items in "backwards" order (legacy)', async () => {
1097
const A = createAsyncText('A');
1098
const B = createAsyncText('B');
1099
const C = createAsyncText('C');
packages/react-server/src/ReactFizzServer.js
+5
-1
@@ -2017,7 +2017,11 @@ function renderSuspenseListRows(
2017
const parentSegment = task.blockedSegment;
2018
const childIndex = parentSegment.children.length;
2019
const insertionIndex = parentSegment.chunks.length;
2020
- for (let i = totalChildren - 1; i >= 0; i--) {
2020
+ for (let n = 0; n < totalChildren; n++) {
2021
+ const i =
2022
+ revealOrder === 'unstable_legacy-backwards'
2023
+ ? totalChildren - 1 - n
2024
+ : n;
2025
const node = rows[i];
2026
task.row = previousSuspenseListRow = createSuspenseListRow(
2027
previousSuspenseListRow,