fix: thunkify deriveds on the server

.changeset/curvy-toes-warn.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: thunkify deriveds on the server

packages/svelte/src/compiler/phases/3-transform/server/visitors/ClassBody.js

@@ -85,8 +85,7 @@ export function ClassBody(node, context) {
 				const init = /** @type {Expression} **/ (
 					context.visit(definition.value.arguments[0], child_state)
 				);
-				const value =
+				const value = field.kind === 'derived_by' ? init : b.thunk(init);
-					field.kind === 'derived_by' ? b.call('$.once', init) : b.call('$.once', b.thunk(init));
 
 				if (is_private) {
 					body.push(b.prop_def(field.id, value));

packages/svelte/src/compiler/phases/3-transform/server/visitors/VariableDeclaration.js

@@ -1,11 +1,11 @@
-/** @import { VariableDeclaration, VariableDeclarator, Expression, CallExpression, Pattern, Identifier } from 'estree' */
+/** @import { VariableDeclaration, VariableDeclarator, Expression, CallExpression, Pattern, Identifier, ObjectPattern, ArrayPattern, Property } from 'estree' */
 /** @import { Binding } from '#compiler' */
 /** @import { Context } from '../types.js' */
 /** @import { Scope } from '../../../scope.js' */
-import { build_fallback, extract_paths } from '../../../../utils/ast.js';
+import { walk } from 'zimmerframe';
+import { build_fallback, extract_identifiers, extract_paths } from '../../../../utils/ast.js';
 import * as b from '../../../../utils/builders.js';
 import { get_rune } from '../../../scope.js';
-import { walk } from 'zimmerframe';
 
 /**
  * @param {VariableDeclaration} node
@@ -16,6 +16,9 @@ export function VariableDeclaration(node, context) {
 	const declarations = [];
 
 	if (context.state.analysis.runes) {
+		/** @type {VariableDeclarator[]} */
+		const destructured_reassigns = [];
+
 		for (const declarator of node.declarations) {
 			const init = declarator.init;
 			const rune = get_rune(init, context.state.scope);
@@ -73,27 +76,72 @@ export function VariableDeclaration(node, context) {
 			const value =
 				args.length === 0 ? b.id('undefined') : /** @type {Expression} */ (context.visit(args[0]));
 
+			const is_destructuring =
+				declarator.id.type === 'ObjectPattern' || declarator.id.type === 'ArrayPattern';
+
+			/**
+			 *
+			 * @param {()=>Expression} get_generated_init
+			 */
+			function add_destructured_reassign(get_generated_init) {
+				// to keep everything that the user destructure as a function we need to change the original
+				// assignment to a generated value and then reassign a variable with the original name
+				if (declarator.id.type === 'ObjectPattern' || declarator.id.type === 'ArrayPattern') {
+					const id = /** @type {ObjectPattern | ArrayPattern} */ (context.visit(declarator.id));
+					const modified = walk(
+						/**@type {Identifier|Property}*/ (/**@type {unknown}*/ (id)),
+						{},
+						{
+							Identifier(id, { path }) {
+								const parent = path.at(-1);
+								// we only want the identifiers for the value
+								if (parent?.type === 'Property' && parent.value !== id) return;
+								const generated = context.state.scope.generate(id.name);
+								destructured_reassigns.push(b.declarator(b.id(id.name), b.thunk(b.id(generated))));
+								return b.id(generated);
+							}
+						}
+					);
+					declarations.push(b.declarator(/**@type {Pattern}*/ (modified), get_generated_init()));
+				}
+			}
+
 			if (rune === '$derived.by') {
+				if (is_destructuring) {
+					add_destructured_reassign(() => b.call(value));
+					continue;
+				}
 				declarations.push(
-					b.declarator(/** @type {Pattern} */ (context.visit(declarator.id)), b.call(value))
+					b.declarator(/** @type {Pattern} */ (context.visit(declarator.id)), value)
 				);
 				continue;
 			}
 
 			if (declarator.id.type === 'Identifier') {
-				declarations.push(b.declarator(declarator.id, value));
+				if (is_destructuring && rune === '$derived') {
+					add_destructured_reassign(() => value);
+					continue;
+				}
+				declarations.push(
+					b.declarator(declarator.id, rune === '$derived' ? b.thunk(value) : value)
+				);
 				continue;
 			}
 
 			if (rune === '$derived') {
+				if (is_destructuring) {
+					add_destructured_reassign(() => value);
+					continue;
+				}
 				declarations.push(
-					b.declarator(/** @type {Pattern} */ (context.visit(declarator.id)), value)
+					b.declarator(/** @type {Pattern} */ (context.visit(declarator.id)), b.thunk(value))
 				);
 				continue;
 			}
 
 			declarations.push(...create_state_declarators(declarator, context.state.scope, value));
 		}
+		declarations.push(...destructured_reassigns);
 	} else {
 		for (const declarator of node.declarations) {
 			const bindings = /** @type {Binding[]} */ (context.state.scope.get_bindings(declarator));

packages/svelte/src/compiler/phases/3-transform/server/visitors/shared/utils.js

@@ -237,6 +237,10 @@ export function build_getter(node, state) {
 			b.literal(node.name),
 			build_getter(store_id, state)
 		);
+	} else if (binding.kind === 'derived') {
+		// we need a maybe_call because in case of `var`
+		// the user might use the variable before the initialization
+		return b.maybe_call(node.name);
 	}
 
 	return node;

packages/svelte/tests/snapshot/samples/await-block-scope/_expected/server/index.svelte.js

@@ -2,13 +2,13 @@ import * as $ from 'svelte/internal/server';
 
 export default function Await_block_scope($$payload) {
 	let counter = { count: 0 };
-	const promise = Promise.resolve(counter);
+	const promise = () => Promise.resolve(counter);
 
 	function increment() {
 		counter.count += 1;
 	}
 
 	$$payload.out += `<button>clicks: ${$.escape(counter.count)}</button> <!---->`;
-	$.await(promise, () => {}, (counter) => {}, () => {});
+	$.await(promise?.(), () => {}, (counter) => {}, () => {});
 	$$payload.out += `<!----> ${$.escape(counter.count)}`;
 }

packages/svelte/tests/snapshot/samples/server-deriveds/_config.js

@@ -0,0 +1,5 @@
+import { test } from '../../test';
+
+export default test({
+	mode: ['server']
+});

packages/svelte/tests/snapshot/samples/server-deriveds/_expected/server/index.svelte.js

@@ -0,0 +1,59 @@
+import * as $ from "svelte/internal/server";
+
+export default function Server_deriveds($$payload, $$props) {
+	$.push();
+
+	// destructuring stuff on the server needs a bit more code
+	// so that every identifier is a function
+	let stuff = {
+		foo: true,
+		bar: [1, 2, { baz: 'baz' }]
+	};
+
+	let {
+			foo: foo_1,
+			bar: [a_1, b_1, { baz: baz_1 }]
+		} = stuff,
+		foo = () => foo_1,
+		a = () => a_1,
+		b = () => b_1,
+		baz = () => baz_1;
+
+	let stuff2 = [1, 2, 3];
+
+	let [d_1, e_1, f_1] = stuff2,
+		d = () => d_1,
+		e = () => e_1,
+		f = () => f_1;
+
+	let count = 0;
+	let double = () => count * 2;
+	let identifier = () => count;
+	let dot_by = () => () => count;
+
+	class Test {
+		state = 0;
+		#der = () => this.state * 2;
+
+		get der() {
+			return this.#der();
+		}
+
+		#der_by = () => this.state;
+
+		get der_by() {
+			return this.#der_by();
+		}
+
+		#identifier = () => this.state;
+
+		get identifier() {
+			return this.#identifier();
+		}
+	}
+
+	const test = new Test();
+
+	$$payload.out += `<!---->${$.escape(foo?.())} ${$.escape(a?.())} ${$.escape(b?.())} ${$.escape(baz?.())} ${$.escape(d?.())} ${$.escape(e?.())} ${$.escape(f?.())} ${$.escape(double?.())} ${$.escape(identifier?.())} ${$.escape(dot_by?.())} ${$.escape(test.der)} ${$.escape(test.der_by)} ${$.escape(test.identifier)}`;
+	$.pop();
+}

packages/svelte/tests/snapshot/samples/server-deriveds/index.svelte

@@ -0,0 +1,25 @@
+<script>
+	// destructuring stuff on the server needs a bit more code
+	// so that every identifier is a function
+	let stuff = $state({ foo: true, bar: [1, 2, {baz: 'baz'}] });
+	let { foo, bar: [a, b, { baz }]} = $derived(stuff);
+
+	let stuff2 = $state([1, 2, 3]);
+	let [d, e, f] = $derived(stuff2);
+
+	let count = $state(0);
+	let double = $derived(count * 2);
+	let identifier = $derived(count);
+	let dot_by = $derived(()=>count);
+
+	class Test{
+		state = $state(0);
+		der = $derived(this.state * 2);
+		der_by = $derived.by(()=>this.state);
+		identifier = $derived(this.state);
+	}
+
+	const test = new Test();
+</script>
+
+{foo} {a} {b} {baz} {d} {e} {f} {double} {identifier} {dot_by} {test.der} {test.der_by} {test.identifier}

packages/svelte/tests/snapshot/test.ts

@@ -7,11 +7,13 @@ import { VERSION } from 'svelte/compiler';
 
 interface SnapshotTest extends BaseTest {
 	compileOptions?: Partial<import('#compiler').CompileOptions>;
+	mode?: ('client' | 'server')[];
 }
 
 const { test, run } = suite<SnapshotTest>(async (config, cwd) => {
-	await compile_directory(cwd, 'client', config.compileOptions);
+	for (const mode of config?.mode ?? ['server', 'client']) {
-	await compile_directory(cwd, 'server', config.compileOptions);
+		await compile_directory(cwd, mode, config.compileOptions);
+	}
 
 	// run `UPDATE_SNAPSHOTS=true pnpm test snapshot` to update snapshot tests
 	if (process.env.UPDATE_SNAPSHOTS) {