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Normalize className as token sets for hydration comparison #34940

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21 changes: 21 additions & 0 deletions packages/react-dom-bindings/src/client/ReactDOMComponent.js
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Original file line number Diff line number Diff line change
Expand Up @@ -230,6 +230,13 @@ function warnForPropDifference(
if (normalizedServerValue === normalizedClientValue) {
return;
}
if (propName === 'class' || propName === 'className') {
const normalizedClientClass = normalizeClassForHydration(clientValue);
const normalizedServerClass = normalizeClassForHydration(serverValue);
if (normalizedServerClass === normalizedClientClass) {
return;
}
}

serverDifferences[propName] = serverValue;
}
Expand Down Expand Up @@ -300,6 +307,12 @@ function normalizeHTML(parent: Element, html: string) {
const NORMALIZE_NEWLINES_REGEX = /\r\n?/g;
const NORMALIZE_NULL_AND_REPLACEMENT_REGEX = /\u0000|\uFFFD/g;

// HTML: `class` is a space-separated token list split on ASCII whitespace
// (TAB U+0009, LF U+000A, FF U+000C, CR U+000D, SPACE U+0020).
// Specs: https://infra.spec.whatwg.org/#ascii-whitespace
// https://html.spec.whatwg.org/multipage/dom.html#global-attributes
const HTML_SPACE_CLASS_SEPARATOR = /[ \t\n\f\r]+/;

function normalizeMarkupForTextOrAttribute(markup: mixed): string {
if (__DEV__) {
checkHtmlStringCoercion(markup);
Expand All @@ -310,6 +323,14 @@ function normalizeMarkupForTextOrAttribute(markup: mixed): string {
.replace(NORMALIZE_NULL_AND_REPLACEMENT_REGEX, '');
}

function normalizeClassForHydration(markup: mixed): string {
const s = normalizeMarkupForTextOrAttribute(markup);
const tokens = s.trim().split(HTML_SPACE_CLASS_SEPARATOR).filter(Boolean);
const unique = Array.from(new Set(tokens));
unique.sort();
return unique.join(' ');
}

function checkForUnmatchedText(
serverText: string,
clientText: string | number | bigint,
Expand Down
29 changes: 29 additions & 0 deletions packages/react-dom/src/__tests__/ReactServerRenderingHydration-test.js
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Original file line number Diff line number Diff line change
Expand Up @@ -333,6 +333,35 @@ describe('ReactDOMServerHydration', () => {
]);
});

it('should not warn when class differs only by whitespace/order/duplicates', async () => {
const element = document.createElement('div');
element.innerHTML = ReactDOMServer.renderToString(
<div className="foo bar bar" />,
);

await act(() => {
ReactDOMClient.hydrateRoot(element, <div className="bar foo" />);
});

assertConsoleErrorDev([]);
});

it('should not warn for class with CRLF (Windows line endings)', async () => {
const element = document.createElement('div');
element.innerHTML = ReactDOMServer.renderToString(
<div className={'flex\r\nitems-center'} />,
);

await act(() => {
ReactDOMClient.hydrateRoot(
element,
<div className="flex items-center" />,
);
});

assertConsoleErrorDev([]);
});

it('should throw rendering portals on the server', () => {
const div = document.createElement('div');
expect(() => {
Expand Down