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Warn for Child Iterator of all types but allow Generator Components #28853

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Apr 21, 2024
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Warn for Child Iterator of all types but allow Generator Components #28853

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81 changes: 81 additions & 0 deletions packages/react-dom/src/__tests__/ReactDOMFizzServer-test.js
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Original file line number Diff line number Diff line change
Expand Up @@ -7913,4 +7913,85 @@ describe('ReactDOMFizzServer', () => {
]);
expect(postpones).toEqual([]);
});

it('should NOT warn for using generator functions as components', async () => {
function* Foo() {
yield <h1 key="1">Hello</h1>;
yield <h1 key="2">World</h1>;
}

await act(() => {
const {pipe} = renderToPipeableStream(<Foo />);
pipe(writable);
});

expect(document.body.textContent).toBe('HelloWorld');
});

it('should warn for using generators as children props', async () => {
function* getChildren() {
yield <h1 key="1">Hello</h1>;
yield <h1 key="2">World</h1>;
}

function Foo() {
const children = getChildren();
return <div>{children}</div>;
}

await expect(async () => {
await act(() => {
const {pipe} = renderToPipeableStream(<Foo />);
pipe(writable);
});
}).toErrorDev(
'Using Iterators as children is unsupported and will likely yield ' +
'unexpected results because enumerating a generator mutates it. ' +
'You may convert it to an array with `Array.from()` or the ' +
'`[...spread]` operator before rendering. You can also use an ' +
'Iterable that can iterate multiple times over the same items.\n' +
' in div (at **)\n' +
' in Foo (at **)',
);

expect(document.body.textContent).toBe('HelloWorld');
});

it('should warn for using other types of iterators as children', async () => {
function Foo() {
let i = 0;
const iterator = {
[Symbol.iterator]() {
return iterator;
},
next() {
switch (i++) {
case 0:
return {done: false, value: <h1 key="1">Hello</h1>};
case 1:
return {done: false, value: <h1 key="2">World</h1>};
default:
return {done: true, value: undefined};
}
},
};
return iterator;
}

await expect(async () => {
await act(() => {
const {pipe} = renderToPipeableStream(<Foo />);
pipe(writable);
});
}).toErrorDev(
'Using Iterators as children is unsupported and will likely yield ' +
'unexpected results because enumerating a generator mutates it. ' +
'You may convert it to an array with `Array.from()` or the ' +
'`[...spread]` operator before rendering. You can also use an ' +
'Iterable that can iterate multiple times over the same items.\n' +
' in Foo (at **)',
);

expect(document.body.textContent).toBe('HelloWorld');
});
});
78 changes: 73 additions & 5 deletions packages/react-dom/src/__tests__/ReactMultiChild-test.js
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Original file line number Diff line number Diff line change
Expand Up @@ -328,26 +328,94 @@ describe('ReactMultiChild', () => {
);
});

it('should warn for using generators as children', async () => {
it('should NOT warn for using generator functions as components', async () => {
function* Foo() {
yield <h1 key="1">Hello</h1>;
yield <h1 key="2">World</h1>;
}

const container = document.createElement('div');
const root = ReactDOMClient.createRoot(container);
await act(async () => {
root.render(<Foo />);
});

expect(container.textContent).toBe('HelloWorld');
});

it('should warn for using generators as children props', async () => {
function* getChildren() {
yield <h1 key="1">Hello</h1>;
yield <h1 key="2">World</h1>;
}

function Foo() {
const children = getChildren();
return <div>{children}</div>;
}

const container = document.createElement('div');
const root = ReactDOMClient.createRoot(container);
await expect(async () => {
await act(async () => {
root.render(<Foo />);
});
}).toErrorDev(
'Using Iterators as children is unsupported and will likely yield ' +
'unexpected results because enumerating a generator mutates it. ' +
'You may convert it to an array with `Array.from()` or the ' +
'`[...spread]` operator before rendering. You can also use an ' +
'Iterable that can iterate multiple times over the same items.\n' +
' in div (at **)\n' +
' in Foo (at **)',
);

expect(container.textContent).toBe('HelloWorld');

// Test de-duplication
await act(async () => {
root.render(<Foo />);
});
});

it('should warn for using other types of iterators as children', async () => {
function Foo() {
let i = 0;
const iterator = {
[Symbol.iterator]() {
return iterator;
},
next() {
switch (i++) {
case 0:
return {done: false, value: <h1 key="1">Hello</h1>};
case 1:
return {done: false, value: <h1 key="2">World</h1>};
default:
return {done: true, value: undefined};
}
},
};
return iterator;
}

const container = document.createElement('div');
const root = ReactDOMClient.createRoot(container);
await expect(async () => {
await act(async () => {
root.render(<Foo />);
});
}).toErrorDev(
'Using Generators as children is unsupported and will likely yield ' +
'unexpected results because enumerating a generator mutates it. You may ' +
'convert it to an array with `Array.from()` or the `[...spread]` operator ' +
'before rendering. Keep in mind you might need to polyfill these features for older browsers.\n' +
'Using Iterators as children is unsupported and will likely yield ' +
'unexpected results because enumerating a generator mutates it. ' +
'You may convert it to an array with `Array.from()` or the ' +
'`[...spread]` operator before rendering. You can also use an ' +
'Iterable that can iterate multiple times over the same items.\n' +
' in Foo (at **)',
);

expect(container.textContent).toBe('HelloWorld');

// Test de-duplication
await act(async () => {
root.render(<Foo />);
Expand Down
103 changes: 64 additions & 39 deletions packages/react-reconciler/src/ReactChildFiber.js
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Original file line number Diff line number Diff line change
Expand Up @@ -33,7 +33,13 @@ import {
REACT_LAZY_TYPE,
REACT_CONTEXT_TYPE,
} from 'shared/ReactSymbols';
import {HostRoot, HostText, HostPortal, Fragment} from './ReactWorkTags';
import {
HostRoot,
HostText,
HostPortal,
Fragment,
FunctionComponent,
} from './ReactWorkTags';
import isArray from 'shared/isArray';
import {enableRefAsProp} from 'shared/ReactFeatureFlags';

Expand Down Expand Up @@ -1114,52 +1120,46 @@ function createChildReconciler(
);
}

const newChildren = iteratorFn.call(newChildrenIterable);

if (__DEV__) {
// We don't support rendering Generators because it's a mutation.
// See https://github.com/facebook/react/issues/12995
if (
typeof Symbol === 'function' &&
// $FlowFixMe[prop-missing] Flow doesn't know about toStringTag
newChildrenIterable[Symbol.toStringTag] === 'Generator'
) {
if (!didWarnAboutGenerators) {
console.error(
'Using Generators as children is unsupported and will likely yield ' +
'unexpected results because enumerating a generator mutates it. ' +
'You may convert it to an array with `Array.from()` or the ' +
'`[...spread]` operator before rendering. Keep in mind ' +
'you might need to polyfill these features for older browsers.',
);
if (newChildren === newChildrenIterable) {
// We don't support rendering Generators as props because it's a mutation.
// See https://github.com/facebook/react/issues/12995
// We do support generators if they were created by a GeneratorFunction component
// as its direct child since we can recreate those by rerendering the component
// as needed.
const isGeneratorComponent =
returnFiber.tag === FunctionComponent &&
// $FlowFixMe[method-unbinding]
Object.prototype.toString.call(returnFiber.type) ===
'[object GeneratorFunction]' &&
// $FlowFixMe[method-unbinding]
Object.prototype.toString.call(newChildren) === '[object Generator]';
if (!isGeneratorComponent) {
if (!didWarnAboutGenerators) {
console.error(
'Using Iterators as children is unsupported and will likely yield ' +
'unexpected results because enumerating a generator mutates it. ' +
'You may convert it to an array with `Array.from()` or the ' +
'`[...spread]` operator before rendering. You can also use an ' +
'Iterable that can iterate multiple times over the same items.',
);
}
didWarnAboutGenerators = true;
}
didWarnAboutGenerators = true;
}

// Warn about using Maps as children
if ((newChildrenIterable: any).entries === iteratorFn) {
} else if ((newChildrenIterable: any).entries === iteratorFn) {
// Warn about using Maps as children
if (!didWarnAboutMaps) {
console.error(
'Using Maps as children is not supported. ' +
'Use an array of keyed ReactElements instead.',
);
}
didWarnAboutMaps = true;
}

// First, validate keys.
// We'll get a different iterator later for the main pass.
const newChildren = iteratorFn.call(newChildrenIterable);
if (newChildren) {
let knownKeys: Set<string> | null = null;
let step = newChildren.next();
for (; !step.done; step = newChildren.next()) {
const child = step.value;
knownKeys = warnOnInvalidKey(child, knownKeys, returnFiber);
didWarnAboutMaps = true;
}
}
}

const newChildren = iteratorFn.call(newChildrenIterable);

if (newChildren == null) {
throw new Error('An iterable object provided no iterator.');
}
Expand All @@ -1172,11 +1172,20 @@ function createChildReconciler(
let newIdx = 0;
let nextOldFiber = null;

let knownKeys: Set<string> | null = null;

let step = newChildren.next();
if (__DEV__) {
knownKeys = warnOnInvalidKey(step.value, knownKeys, returnFiber);
}
for (
;
oldFiber !== null && !step.done;
newIdx++, step = newChildren.next()
newIdx++,
step = newChildren.next(),
knownKeys = __DEV__
? warnOnInvalidKey(step.value, knownKeys, returnFiber)
: null
) {
if (oldFiber.index > newIdx) {
nextOldFiber = oldFiber;
Expand Down Expand Up @@ -1236,7 +1245,15 @@ function createChildReconciler(
if (oldFiber === null) {
// If we don't have any more existing children we can choose a fast path
// since the rest will all be insertions.
for (; !step.done; newIdx++, step = newChildren.next()) {
for (
;
!step.done;
newIdx++,
step = newChildren.next(),
knownKeys = __DEV__
? warnOnInvalidKey(step.value, knownKeys, returnFiber)
: null
Comment on lines +1253 to +1255
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Might DCE better if you move this to the first line of the for block. It's not big but it leaves behind a null in the prod builds for each instance

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The way this is structured, that doesn't mean the same thing because this should only execute at the beginning of the next iteration loop - not the first one. A proper structure would need to change the way the continue calls are placed.

) {
const newFiber = createChild(returnFiber, step.value, lanes, debugInfo);
if (newFiber === null) {
continue;
Expand All @@ -1261,7 +1278,15 @@ function createChildReconciler(
const existingChildren = mapRemainingChildren(oldFiber);

// Keep scanning and use the map to restore deleted items as moves.
for (; !step.done; newIdx++, step = newChildren.next()) {
for (
;
!step.done;
newIdx++,
step = newChildren.next(),
knownKeys = __DEV__
? warnOnInvalidKey(step.value, knownKeys, returnFiber)
: null
) {
const newFiber = updateFromMap(
existingChildren,
returnFiber,
Expand Down
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