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fix: reuse DOM nodes inside <template> during hydration #4803

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fix: reuse DOM nodes inside <template> during hydration #4803

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1dae489
fix: reuse DOM nodes inside <template> during hydration
frontman-git Jun 18, 2025
e17ec02
Merge branch 'main' into fix/template-hydration-reuse-dom
frontman-git Jun 18, 2025
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10 changes: 9 additions & 1 deletion src/diff/children.js
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Original file line number Diff line number Diff line change
Expand Up @@ -8,9 +8,10 @@ import {
UNDEFINED,
NULL
} from '../constants';
import { isArray } from '../util';
import { isArray, slice } from '../util';
import { getDomSibling } from '../component';


/**
* @typedef {import('../internal').ComponentChildren} ComponentChildren
* @typedef {import('../internal').Component} Component
Expand Down Expand Up @@ -53,6 +54,13 @@ export function diffChildren(
isHydrating,
refQueue
) {
if (
newParentVNode.type === 'template' &&
excessDomChildren?.length === 0 &&
parentDom instanceof DocumentFragment
) {
Comment on lines +57 to +61
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@JoviDeCroock JoviDeCroock Jun 18, 2025 •
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Let's make this fast by adding the isHydrating check, we don't need to check excessDomChildren length as we expect this to be 0 as per your assumptions in this PR. I would assume that we are in a DocumentFragment when we are dealing with a template.

Suggested change
if (
newParentVNode.type === 'template' &&
excessDomChildren?.length === 0 &&
parentDom instanceof DocumentFragment
) {
if (isHydrating && newParentVNode.type === 'template') {

I assume you could save even more bytes by moving this to

preact/src/diff/index.js

Lines 596 to 611 in 1dae489

diffChildren(
// @ts-expect-error
newVNode.type == 'template' ? dom.content : dom,
isArray(newChildren) ? newChildren : [newChildren],
newVNode,
oldVNode,
globalContext,
nodeType == 'foreignObject' ? XHTML_NAMESPACE : namespace,
excessDomChildren,
commitQueue,
excessDomChildren
? excessDomChildren[0]
: oldVNode._children && getDomSibling(oldVNode, 0),
isHydrating,
refQueue
);
and changing this condition to use newVNode.type === 'template' as well as hoisting the duplicated check

excessDomChildren = slice.call(parentDom.childNodes);
}
let i,
/** @type {VNode} */
oldVNode,
Expand Down
7 changes: 7 additions & 0 deletions test/browser/render.test.js
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Original file line number Diff line number Diff line change
Expand Up @@ -2013,4 +2013,11 @@ describe('render()', () => {
render(<App />, scratch);
expect(scratch.innerHTML).to.equal('hello world');
});
it('should hydrate <template> tags ', () => {
const App = () => <template><h1>it works</h1></template>;
scratch.innerHTML = `<template><h1>it works</h1></template>`;

render(<App />, scratch);
expect(scratch.innerHTML).to.equal(`<template><h1>it works</h1></template>`);
});
});
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