feat: more efficient text-only fragments (#12864)

* feat: more efficient text-only fragments

* set_text always receives a string now

* another optimisation

* revert sandbox change

* fix test

Author: Rich-Harris

.changeset/hot-tips-appear.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+feat: more efficient text-only fragments

packages/svelte/src/compiler/phases/3-transform/client/visitors/Fragment.js

@@ -128,6 +128,14 @@ export function Fragment(node, context) {
 	} else if (is_single_child_not_needing_template) {
 		context.visit(trimmed[0], state);
 		body.push(...state.before_init, ...state.init);
+	} else if (trimmed.length === 1 && trimmed[0].type === 'Text') {
+		const id = b.id(context.state.scope.generate('text'));
+		body.push(
+			b.var(id, b.call('$.text', b.literal(trimmed[0].data))),
+			...state.before_init,
+			...state.init
+		);
+		close = b.stmt(b.call('$.append', b.id('$$anchor'), id));
 	} else if (trimmed.length > 0) {
 		const id = b.id(context.state.scope.generate('fragment'));
 

packages/svelte/src/compiler/phases/3-transform/client/visitors/RegularElement.js

@@ -27,7 +27,12 @@ import {
 	build_style_directives
 } from './shared/element.js';
 import { process_children } from './shared/fragment.js';
-import { build_render_statement, build_update, build_update_assignment } from './shared/utils.js';
+import {
+	build_render_statement,
+	build_template_literal,
+	build_update,
+	build_update_assignment
+} from './shared/utils.js';
 import { visit_event_attribute } from './shared/events.js';
 
 /**
@@ -320,28 +325,43 @@ export function RegularElement(node, context) {
 		context.visit(node, child_state);
 	}
 
-	/** @type {Expression} */
-	let arg = context.state.node;
-
-	// If `hydrate_node` is set inside the element, we need to reset it
-	// after the element has been hydrated
-	let needs_reset = trimmed.some((node) => node.type !== 'Text');
+	// special case — if an element that only contains text, we don't need
+	// to descend into it if the text is non-reactive
+	const text_content =
+		trimmed.every((node) => node.type === 'Text' || node.type === 'ExpressionTag') &&
+		trimmed.some((node) => node.type === 'ExpressionTag') &&
+		build_template_literal(trimmed, context.visit, child_state);
 
-	// The same applies if it's a `<template>` element, since we need to
-	// set the value of `hydrate_node` to `node.content`
-	if (node.name === 'template') {
-		needs_reset = true;
-		child_state.init.push(b.stmt(b.call('$.hydrate_template', arg)));
-		arg = b.member(arg, b.id('content'));
-	}
+	if (text_content && !text_content.has_state) {
+		child_state.init.push(
+			b.stmt(
+				b.assignment('=', b.member(context.state.node, b.id('textContent')), text_content.value)
+			)
+		);
+	} else {
+		/** @type {Expression} */
+		let arg = context.state.node;
+
+		// If `hydrate_node` is set inside the element, we need to reset it
+		// after the element has been hydrated
+		let needs_reset = trimmed.some((node) => node.type !== 'Text');
+
+		// The same applies if it's a `<template>` element, since we need to
+		// set the value of `hydrate_node` to `node.content`
+		if (node.name === 'template') {
+			needs_reset = true;
+			child_state.init.push(b.stmt(b.call('$.hydrate_template', arg)));
+			arg = b.member(arg, b.id('content'));
+		}
 
-	process_children(trimmed, () => b.call('$.child', arg), true, {
-		...context,
-		state: child_state
-	});
+		process_children(trimmed, () => b.call('$.child', arg), true, {
+			...context,
+			state: child_state
+		});
 
-	if (needs_reset) {
-		child_state.init.push(b.stmt(b.call('$.reset', context.state.node)));
+		if (needs_reset) {
+			child_state.init.push(b.stmt(b.call('$.reset', context.state.node)));
+		}
 	}
 
 	if (has_declaration) {

packages/svelte/src/compiler/phases/3-transform/client/visitors/shared/fragment.js

@@ -34,53 +34,26 @@ export function process_children(nodes, expression, is_element, { visit, state }
 				state.template.push(node.raw);
 				return;
 			}
+		}
 
-			state.template.push(' ');
+		const text_id = get_node_id(expression(true), state, 'text');
 
-			const text_id = get_node_id(expression(true), state, 'text');
+		state.template.push(' ');
 
-			const update = b.stmt(
-				b.call('$.set_text', text_id, /** @type {Expression} */ (visit(node.expression, state)))
-			);
+		const { has_state, has_call, value } = build_template_literal(sequence, visit, state);
 
-			if (node.metadata.expression.has_call && !within_bound_contenteditable) {
-				state.init.push(build_update(update));
-			} else if (node.metadata.expression.has_state && !within_bound_contenteditable) {
-				state.update.push(update);
-			} else {
-				state.init.push(
-					b.stmt(
-						b.assignment(
-							'=',
-							b.member(text_id, b.id('nodeValue')),
-							/** @type {Expression} */ (visit(node.expression))
-						)
-					)
-				);
-			}
+		const update = b.stmt(b.call('$.set_text', text_id, value));
 
-			expression = (is_text) =>
-				is_text ? b.call('$.sibling', text_id, b.true) : b.call('$.sibling', text_id);
+		if (has_call && !within_bound_contenteditable) {
+			state.init.push(build_update(update));
+		} else if (has_state && !within_bound_contenteditable) {
+			state.update.push(update);
 		} else {
-			const text_id = get_node_id(expression(true), state, 'text');
-
-			state.template.push(' ');
-
-			const { has_state, has_call, value } = build_template_literal(sequence, visit, state);
-
-			const update = b.stmt(b.call('$.set_text', text_id, value));
-
-			if (has_call && !within_bound_contenteditable) {
-				state.init.push(build_update(update));
-			} else if (has_state && !within_bound_contenteditable) {
-				state.update.push(update);
-			} else {
-				state.init.push(b.stmt(b.assignment('=', b.member(text_id, b.id('nodeValue')), value)));
-			}
-
-			expression = (is_text) =>
-				is_text ? b.call('$.sibling', text_id, b.true) : b.call('$.sibling', text_id);
+			state.init.push(b.stmt(b.assignment('=', b.member(text_id, b.id('nodeValue')), value)));
 		}
+
+		expression = (is_text) =>
+			is_text ? b.call('$.sibling', text_id, b.true) : b.call('$.sibling', text_id);
 	}
 
 	for (let i = 0; i < nodes.length; i += 1) {

packages/svelte/src/internal/client/dom/blocks/each.js

@@ -16,7 +16,7 @@ import {
 	set_hydrate_node,
 	set_hydrating
 } from '../hydration.js';
-import { clear_text_content, empty } from '../operations.js';
+import { clear_text_content, create_text } from '../operations.js';
 import {
 	block,
 	branch,
@@ -117,7 +117,7 @@ export function each(node, flags, get_collection, get_key, render_fn, fallback_f
 
 		anchor = hydrating
 			? set_hydrate_node(/** @type {Comment | Text} */ (parent_node.firstChild))
-			: parent_node.appendChild(empty());
+			: parent_node.appendChild(create_text());
 	}
 
 	if (hydrating) {

packages/svelte/src/internal/client/dom/blocks/svelte-element.js

@@ -7,7 +7,7 @@ import {
 	set_hydrate_node,
 	set_hydrating
 } from '../hydration.js';
-import { empty } from '../operations.js';
+import { create_text } from '../operations.js';
 import {
 	block,
 	branch,
@@ -119,7 +119,7 @@ export function element(node, get_tag, is_svg, render_fn, get_namespace, locatio
 					// If hydrating, use the existing ssr comment as the anchor so that the
 					// inner open and close methods can pick up the existing nodes correctly
 					var child_anchor = /** @type {TemplateNode} */ (
-						hydrating ? element.firstChild : element.appendChild(empty())
+						hydrating ? element.firstChild : element.appendChild(create_text())
 					);
 
 					if (hydrating) {

packages/svelte/src/internal/client/dom/blocks/svelte-head.js

@@ -1,6 +1,6 @@
 /** @import { TemplateNode } from '#client' */
 import { hydrate_node, hydrating, set_hydrate_node, set_hydrating } from '../hydration.js';
-import { empty } from '../operations.js';
+import { create_text } from '../operations.js';
 import { block } from '../../reactivity/effects.js';
 import { HEAD_EFFECT } from '../../constants.js';
 import { HYDRATION_START } from '../../../../constants.js';
@@ -52,7 +52,7 @@ export function head(render_fn) {
 	}
 
 	if (!hydrating) {
-		anchor = document.head.appendChild(empty());
+		anchor = document.head.appendChild(create_text());
 	}
 
 	try {

packages/svelte/src/internal/client/dom/operations.js

@@ -45,9 +45,12 @@ export function init_operations() {
 	}
 }
 
-/** @returns {Text} */
-export function empty() {
-	return document.createTextNode('');
+/**
+ * @param {string} value
+ * @returns {Text}
+ */
+export function create_text(value = '') {
+	return document.createTextNode(value);
 }
 
 /**
@@ -65,7 +68,7 @@ export function child(node) {
 
 	// Child can be null if we have an element with a single child, like `<p>{text}</p>`, where `text` is empty
 	if (child === null) {
-		child = hydrate_node.appendChild(empty());
+		child = hydrate_node.appendChild(create_text());
 	}
 
 	set_hydrate_node(child);
@@ -92,7 +95,7 @@ export function first_child(fragment, is_text) {
 	// if an {expression} is empty during SSR, there might be no
 	// text node to hydrate — we must therefore create one
 	if (is_text && hydrate_node?.nodeType !== 3) {
-		var text = empty();
+		var text = create_text();
 
 		hydrate_node?.before(text);
 		set_hydrate_node(text);
@@ -121,7 +124,7 @@ export function sibling(node, is_text = false) {
 	// if a sibling {expression} is empty during SSR, there might be no
 	// text node to hydrate — we must therefore create one
 	if (is_text && type !== 3) {
-		var text = empty();
+		var text = create_text();
 		next_sibling?.before(text);
 		set_hydrate_node(text);
 		return text;

packages/svelte/src/internal/client/dom/template.js

@@ -1,6 +1,6 @@
 /** @import { Effect, EffectNodes, TemplateNode } from '#client' */
 import { hydrate_next, hydrate_node, hydrating, set_hydrate_node } from './hydration.js';
-import { empty } from './operations.js';
+import { create_text } from './operations.js';
 import { create_fragment_from_html } from './reconciler.js';
 import { current_effect } from '../runtime.js';
 import { TEMPLATE_FRAGMENT, TEMPLATE_USE_IMPORT_NODE } from '../../../constants.js';
@@ -209,10 +209,11 @@ function run_scripts(node) {
 
 /**
  * Don't mark this as side-effect-free, hydration needs to walk all nodes
+ * @param {any} value
  */
-export function text() {
+export function text(value = '') {
 	if (!hydrating) {
-		var t = empty();
+		var t = create_text(value + '');
 		assign_nodes(t, t);
 		return t;
 	}
@@ -221,7 +222,7 @@ export function text() {
 
 	if (node.nodeType !== 3) {
 		// if an {expression} is empty during SSR, we need to insert an empty text node
-		node.before((node = empty()));
+		node.before((node = create_text()));
 		set_hydrate_node(node);
 	}
 
@@ -238,7 +239,7 @@ export function comment() {
 
 	var frag = document.createDocumentFragment();
 	var start = document.createComment('');
-	var anchor = empty();
+	var anchor = create_text();
 	frag.append(start, anchor);
 
 	assign_nodes(start, anchor);

packages/svelte/src/internal/client/render.js

@@ -1,7 +1,7 @@
 /** @import { ComponentContext, Effect, EffectNodes, TemplateNode } from '#client' */
 /** @import { Component, ComponentType, SvelteComponent } from '../../index.js' */
 import { DEV } from 'esm-env';
-import { clear_text_content, empty, init_operations } from './dom/operations.js';
+import { clear_text_content, create_text, init_operations } from './dom/operations.js';
 import { HYDRATION_END, HYDRATION_ERROR, HYDRATION_START } from '../../constants.js';
 import { push, pop, current_component_context, current_effect } from './runtime.js';
 import { effect_root, branch } from './reactivity/effects.js';
@@ -43,13 +43,9 @@ export function set_should_intro(value) {
  */
 export function set_text(text, value) {
 	// @ts-expect-error
-	const prev = (text.__t ??= text.nodeValue);
-
-	if (prev !== value) {
+	if (value !== (text.__t ??= text.nodeValue)) {
 		// @ts-expect-error
-		text.__t = value;
-		// It's faster to make the value a string rather than passing a non-string to nodeValue
-		text.nodeValue = value == null ? '' : value + '';
+		text.nodeValue = text.__t = value;
 	}
 }
 
@@ -78,7 +74,7 @@ export function set_text(text, value) {
  * @returns {Exports}
  */
 export function mount(component, options) {
-	const anchor = options.anchor ?? options.target.appendChild(empty());
+	const anchor = options.anchor ?? options.target.appendChild(create_text());
 	return _mount(component, { ...options, anchor });
 }