sveltejs · Rich-Harris · Jul 18, 2025 · Jul 17, 2025 · Jul 17, 2025 · Jul 17, 2025
diff --git a/.changeset/old-dots-sin.md b/.changeset/old-dots-sin.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+chore: Switch `payload.out` to an array
diff --git a/packages/svelte/src/compiler/phases/3-transform/server/visitors/EachBlock.js b/packages/svelte/src/compiler/phases/3-transform/server/visitors/EachBlock.js
@@ -44,12 +44,12 @@ export function EachBlock(node, context) {
 	);
 
 	if (node.fallback) {
-		const open = b.stmt(b.assignment('+=', b.id('$$payload.out'), block_open));
+		const open = b.stmt(b.call(b.member(b.id('$$payload.out'), b.id('push')), block_open));
 
 		const fallback = /** @type {BlockStatement} */ (context.visit(node.fallback));
 
 		fallback.body.unshift(
-			b.stmt(b.assignment('+=', b.id('$$payload.out'), b.literal(BLOCK_OPEN_ELSE)))
+			b.stmt(b.call(b.member(b.id('$$payload.out'), b.id('push')), b.literal(BLOCK_OPEN_ELSE)))
 		);
 
 		state.template.push(

diff --git a/packages/svelte/src/compiler/phases/3-transform/server/visitors/IfBlock.js b/packages/svelte/src/compiler/phases/3-transform/server/visitors/IfBlock.js
@@ -17,10 +17,12 @@ export function IfBlock(node, context) {
 		? /** @type {BlockStatement} */ (context.visit(node.alternate))
 		: b.block([]);
 
-	consequent.body.unshift(b.stmt(b.assignment('+=', b.id('$$payload.out'), block_open)));
+	consequent.body.unshift(
+		b.stmt(b.call(b.member(b.id('$$payload.out'), b.id('push')), block_open))
+	);
 
 	alternate.body.unshift(
-		b.stmt(b.assignment('+=', b.id('$$payload.out'), b.literal(BLOCK_OPEN_ELSE)))
+		b.stmt(b.call(b.member(b.id('$$payload.out'), b.id('push')), b.literal(BLOCK_OPEN_ELSE)))
 	);
 
 	context.state.template.push(b.if(test, consequent, alternate), block_close);

diff --git a/packages/svelte/src/compiler/phases/3-transform/server/visitors/shared/utils.js b/packages/svelte/src/compiler/phases/3-transform/server/visitors/shared/utils.js
@@ -96,10 +96,10 @@ function is_statement(node) {
 /**
  * @param {Array<Statement | Expression>} template
  * @param {Identifier} out
- * @param {AssignmentOperator} operator
+ * @param {AssignmentOperator | 'push'} operator
  * @returns {Statement[]}
  */
-export function build_template(template, out = b.id('$$payload.out'), operator = '+=') {
+export function build_template(template, out = b.id('$$payload.out'), operator = 'push') {
 	/** @type {string[]} */
 	let strings = [];
 
@@ -110,18 +110,32 @@ export function build_template(template, out = b.id('$$payload.out'), operator =
 	const statements = [];
 
 	const flush = () => {
-		statements.push(
-			b.stmt(
-				b.assignment(
-					operator,
-					out,
-					b.template(
-						strings.map((cooked, i) => b.quasi(cooked, i === strings.length - 1)),
-						expressions
+		if (operator === 'push') {
+			statements.push(
+				b.stmt(
+					b.call(
+						b.member(out, b.id('push')),
+						b.template(
+							strings.map((cooked, i) => b.quasi(cooked, i === strings.length - 1)),
+							expressions
+						)
 					)
 				)
-			)
-		);
+			);
+		} else {
+			statements.push(
+				b.stmt(
+					b.assignment(
+						operator,
+						out,
+						b.template(
+							strings.map((cooked, i) => b.quasi(cooked, i === strings.length - 1)),
+							expressions
+						)
+					)
+				)
+			);
+		}
 		strings = [];
 		expressions = [];
 	};

diff --git a/packages/svelte/src/internal/server/blocks/snippet.js b/packages/svelte/src/internal/server/blocks/snippet.js
@@ -15,8 +15,10 @@ export function createRawSnippet(fn) {
 	// @ts-expect-error the types are a lie
 	return (/** @type {Payload} */ payload, /** @type {Params} */ ...args) => {
 		var getters = /** @type {Getters<Params>} */ (args.map((value) => () => value));
-		payload.out += fn(...getters)
-			.render()
-			.trim();
+		payload.out.push(
+			fn(...getters)
+				.render()
+				.trim()
+		);
 	};
 }
diff --git a/packages/svelte/src/internal/server/dev.js b/packages/svelte/src/internal/server/dev.js
@@ -40,7 +40,7 @@ function print_error(payload, message) {
 
 	// eslint-disable-next-line no-console
 	console.error(message);
-	payload.head.out += `<script>console.error(${JSON.stringify(message)})</script>`;
+	payload.head.out.push(`<script>console.error(${JSON.stringify(message)})</script>`);
 }
 
 export function reset_elements() {

diff --git a/packages/svelte/src/internal/server/index.js b/packages/svelte/src/internal/server/index.js
@@ -33,23 +33,23 @@ const INVALID_ATTR_NAME_CHAR_REGEX =
  * @returns {void}
  */
 export function element(payload, tag, attributes_fn = noop, children_fn = noop) {
-	payload.out += '<!---->';
+	payload.out.push('<!---->');
 
 	if (tag) {
-		payload.out += `<${tag}`;
+		payload.out.push(`<${tag}`);
 		attributes_fn();
-		payload.out += `>`;
+		payload.out.push(`>`);
 
 		if (!is_void(tag)) {
 			children_fn();
 			if (!is_raw_text_element(tag)) {
-				payload.out += EMPTY_COMMENT;
+				payload.out.push(EMPTY_COMMENT);
 			}
-			payload.out += `</${tag}>`;
+			payload.out.push(`</${tag}>`);
 		}
 	}
 
-	payload.out += '<!---->';
+	payload.out.push('<!---->');
 }
 
 /**
@@ -72,7 +72,7 @@ export function render(component, options = {}) {
 
 		const prev_on_destroy = on_destroy;
 		on_destroy = [];
-		payload.out += BLOCK_OPEN;
+		payload.out.push(BLOCK_OPEN);
 
 		let reset_reset_element;
 
@@ -97,20 +97,22 @@ export function render(component, options = {}) {
 			reset_reset_element();
 		}
 
-		payload.out += BLOCK_CLOSE;
+		payload.out.push(BLOCK_CLOSE);
 		for (const cleanup of on_destroy) cleanup();
 		on_destroy = prev_on_destroy;
 
-		let head = payload.head.out + payload.head.title;
+		let head = payload.head.out.join('') + payload.head.title;
 
 		for (const { hash, code } of payload.css) {
 			head += `<style id="${hash}">${code}</style>`;
 		}
 
+		const body = payload.out.join('');
+
 		return {
 			head,
-			html: payload.out,
-			body: payload.out
+			html: body,
+			body: body
 		};
 	} finally {
 		abort();
@@ -124,9 +126,9 @@ export function render(component, options = {}) {
  */
 export function head(payload, fn) {
 	const head_payload = payload.head;
-	head_payload.out += BLOCK_OPEN;
+	head_payload.out.push(BLOCK_OPEN);
 	fn(head_payload);
-	head_payload.out += BLOCK_CLOSE;
+	head_payload.out.push(BLOCK_CLOSE);
 }
 
 /**
@@ -141,21 +143,21 @@ export function css_props(payload, is_html, props, component, dynamic = false) {
 	const styles = style_object_to_string(props);
 
 	if (is_html) {
-		payload.out += `<svelte-css-wrapper style="display: contents; ${styles}">`;
+		payload.out.push(`<svelte-css-wrapper style="display: contents; ${styles}">`);
 	} else {
-		payload.out += `<g style="${styles}">`;
+		payload.out.push(`<g style="${styles}">`);
 	}
 
 	if (dynamic) {
-		payload.out += '<!---->';
+		payload.out.push('<!---->');
 	}
 
 	component();
 
 	if (is_html) {
-		payload.out += `<!----></svelte-css-wrapper>`;
+		payload.out.push(`<!----></svelte-css-wrapper>`);
 	} else {
-		payload.out += `<!----></g>`;
+		payload.out.push(`<!----></g>`);
 	}
 }
 
@@ -440,13 +442,13 @@ export function bind_props(props_parent, props_now) {
  */
 function await_block(payload, promise, pending_fn, then_fn) {
 	if (is_promise(promise)) {
-		payload.out += BLOCK_OPEN;
+		payload.out.push(BLOCK_OPEN);
 		promise.then(null, noop);
 		if (pending_fn !== null) {
 			pending_fn();
 		}
 	} else if (then_fn !== null) {
-		payload.out += BLOCK_OPEN_ELSE;
+		payload.out.push(BLOCK_OPEN_ELSE);
 		then_fn(promise);
 	}
 }
@@ -493,7 +495,7 @@ export function once(get_value) {
  */
 export function props_id(payload) {
 	const uid = payload.uid();
-	payload.out += '<!--#' + uid + '-->';
+	payload.out.push('<!--#' + uid + '-->');
 	return uid;
 }
 
@@ -562,10 +564,13 @@ export function valueless_option(payload, children) {
 
 	children();
 
-	var body = payload.out.slice(i);
+	var body = payload.out.slice(i).join('');
 
 	if (body.replace(/<!---->/g, '') === payload.select_value) {
 		// replace '>' with ' selected>' (closing tag will be added later)
-		payload.out = payload.out.slice(0, i - 1) + ' selected>' + body;
+		var last_item = payload.out[i - 1];
+		payload.out[i - 1] = last_item.slice(0, -1) + ' selected>';
+		// Remove the old items after position i and add the body as a single item
+		payload.out.splice(i, payload.out.length - i, body);
 	}
 }
diff --git a/packages/svelte/src/internal/server/payload.js b/packages/svelte/src/internal/server/payload.js
@@ -1,11 +1,12 @@
 export class HeadPayload {
 	/** @type {Set<{ hash: string; code: string }>} */
 	css = new Set();
-	out = '';
+	/** @type {string[]} */
+	out = [];
 	uid = () => '';
 	title = '';
 
-	constructor(css = new Set(), out = '', title = '', uid = () => '') {
+	constructor(css = new Set(), /** @type {string[]} */ out = [], title = '', uid = () => '') {
 		this.css = css;
 		this.out = out;
 		this.title = title;
@@ -16,7 +17,8 @@ export class HeadPayload {
 export class Payload {
 	/** @type {Set<{ hash: string; code: string }>} */
 	css = new Set();
-	out = '';
+	/** @type {string[]} */
+	out = [];
 	uid = () => '';
 	select_value = undefined;
 
@@ -36,12 +38,12 @@ export class Payload {
 export function copy_payload({ out, css, head, uid }) {
 	const payload = new Payload();
 
-	payload.out = out;
+	payload.out = [...out];
 	payload.css = new Set(css);
 	payload.uid = uid;
 
 	payload.head = new HeadPayload();
-	payload.head.out = head.out;
+	payload.head.out = [...head.out];
 	payload.head.css = new Set(head.css);
 	payload.head.title = head.title;
 	payload.head.uid = head.uid;
@@ -56,7 +58,7 @@ export function copy_payload({ out, css, head, uid }) {
  * @returns {void}
  */
 export function assign_payload(p1, p2) {
-	p1.out = p2.out;
+	p1.out = [...p2.out];
 	p1.css = p2.css;
 	p1.head = p2.head;
 	p1.uid = p2.uid;

diff --git a/packages/svelte/tests/snapshot/samples/await-block-scope/_expected/server/index.svelte.js b/packages/svelte/tests/snapshot/samples/await-block-scope/_expected/server/index.svelte.js
@@ -8,7 +8,7 @@ export default function Await_block_scope($$payload) {
 		counter.count += 1;
 	}
 
-	$$payload.out += `<button>clicks: ${$.escape(counter.count)}</button> `;
+	$$payload.out.push(`<button>clicks: ${$.escape(counter.count)}</button> `);
 	$.await($$payload, promise, () => {}, (counter) => {});
-	$$payload.out += `<!--]--> ${$.escape(counter.count)}`;
+	$$payload.out.push(`<!--]--> ${$.escape(counter.count)}`);
 }
diff --git a/...ges/svelte/tests/snapshot/samples/bind-component-snippet/_expected/server/index.svelte.js b/...ges/svelte/tests/snapshot/samples/bind-component-snippet/_expected/server/index.svelte.js
@@ -2,7 +2,7 @@ import * as $ from 'svelte/internal/server';
 import TextInput from './Child.svelte';
 
 function snippet($$payload) {
-	$$payload.out += `<!---->Something`;
+	$$payload.out.push(`<!---->Something`);
 }
 
 export default function Bind_component_snippet($$payload) {
@@ -23,7 +23,7 @@ export default function Bind_component_snippet($$payload) {
 			}
 		});
 
-		$$payload.out += `<!----> value: ${$.escape(value)}`;
+		$$payload.out.push(`<!----> value: ${$.escape(value)}`);
 	}
 
 	do {

diff --git a/...sts/snapshot/samples/delegated-locally-declared-shadowed/_expected/server/index.svelte.js b/...sts/snapshot/samples/delegated-locally-declared-shadowed/_expected/server/index.svelte.js
@@ -3,11 +3,11 @@ import * as $ from 'svelte/internal/server';
 export default function Delegated_locally_declared_shadowed($$payload) {
 	const each_array = $.ensure_array_like({ length: 1 });
 
-	$$payload.out += `<!--[-->`;
+	$$payload.out.push(`<!--[-->`);
 
 	for (let index = 0, $$length = each_array.length; index < $$length; index++) {
-		$$payload.out += `<button type="button"${$.attr('data-index', index)}>B</button>`;
+		$$payload.out.push(`<button type="button"${$.attr('data-index', index)}>B</button>`);
 	}
 
-	$$payload.out += `<!--]-->`;
+	$$payload.out.push(`<!--]-->`);
 }
diff --git a/...s/svelte/tests/snapshot/samples/dynamic-attributes-casing/_expected/server/main.svelte.js b/...s/svelte/tests/snapshot/samples/dynamic-attributes-casing/_expected/server/main.svelte.js
@@ -6,5 +6,5 @@ export default function Main($$payload) {
 
 	let y = () => 'test';
 
-	$$payload.out += `<div${$.attr('foobar', x)}></div> <svg${$.attr('viewBox', x)}></svg> <custom-element${$.attr('foobar', x)}></custom-element> <div${$.attr('foobar', y())}></div> <svg${$.attr('viewBox', y())}></svg> <custom-element${$.attr('foobar', y())}></custom-element>`;
+	$$payload.out.push(`<div${$.attr('foobar', x)}></div> <svg${$.attr('viewBox', x)}></svg> <custom-element${$.attr('foobar', x)}></custom-element> <div${$.attr('foobar', y())}></div> <svg${$.attr('viewBox', y())}></svg> <custom-element${$.attr('foobar', y())}></custom-element>`);
 }
diff --git a/packages/svelte/tests/snapshot/samples/each-index-non-null/_expected/server/index.svelte.js b/packages/svelte/tests/snapshot/samples/each-index-non-null/_expected/server/index.svelte.js
@@ -3,11 +3,11 @@ import * as $ from 'svelte/internal/server';
 export default function Each_index_non_null($$payload) {
 	const each_array = $.ensure_array_like(Array(10));
 
-	$$payload.out += `<!--[-->`;
+	$$payload.out.push(`<!--[-->`);
 
 	for (let i = 0, $$length = each_array.length; i < $$length; i++) {
-		$$payload.out += `<p>index: ${$.escape(i)}</p>`;
+		$$payload.out.push(`<p>index: ${$.escape(i)}</p>`);
 	}
 
-	$$payload.out += `<!--]-->`;
+	$$payload.out.push(`<!--]-->`);
 }
diff --git a/packages/svelte/tests/snapshot/samples/each-string-template/_expected/server/index.svelte.js b/packages/svelte/tests/snapshot/samples/each-string-template/_expected/server/index.svelte.js
@@ -3,13 +3,13 @@ import * as $ from 'svelte/internal/server';
 export default function Each_string_template($$payload) {
 	const each_array = $.ensure_array_like(['foo', 'bar', 'baz']);
 
-	$$payload.out += `<!--[-->`;
+	$$payload.out.push(`<!--[-->`);
 
 	for (let $$index = 0, $$length = each_array.length; $$index < $$length; $$index++) {
 		let thing = each_array[$$index];
 
-		$$payload.out += `<!---->${$.escape(thing)}, `;
+		$$payload.out.push(`<!---->${$.escape(thing)}, `);
 	}
 
-	$$payload.out += `<!--]-->`;
+	$$payload.out.push(`<!--]-->`);
 }