fix: make deriveds on the server lazy again

.changeset/khaki-carrots-destroy.md

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

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

@@ -23,6 +23,7 @@ import { Identifier } from './visitors/Identifier.js';
 import { IfBlock } from './visitors/IfBlock.js';
 import { KeyBlock } from './visitors/KeyBlock.js';
 import { LabeledStatement } from './visitors/LabeledStatement.js';
+import { MemberExpression } from './visitors/MemberExpression.js';
 import { PropertyDefinition } from './visitors/PropertyDefinition.js';
 import { RegularElement } from './visitors/RegularElement.js';
 import { RenderTag } from './visitors/RenderTag.js';
@@ -48,6 +49,7 @@ const global_visitors = {
 	ExpressionStatement,
 	Identifier,
 	LabeledStatement,
+	MemberExpression,
 	PropertyDefinition,
 	UpdateExpression,
 	VariableDeclaration

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

@@ -44,6 +44,11 @@ function build_assignment(operator, left, right, context) {
 					/** @type {Expression} */ (context.visit(right))
 				);
 			}
+		} else if (field && (field.type === '$derived' || field.type === '$derived.by')) {
+			let value = /** @type {Expression} */ (
+				context.visit(build_assignment_value(operator, left, right))
+			);
+			return b.call(b.member(b.this, name), value);
 		}
 	}
 

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

@@ -31,7 +31,7 @@ export function CallExpression(node, context) {
 
 	if (rune === '$derived' || rune === '$derived.by') {
 		const fn = /** @type {Expression} */ (context.visit(node.arguments[0]));
-		return b.call('$.once', rune === '$derived' ? b.thunk(fn) : fn);
+		return b.call('$.derived', rune === '$derived' ? b.thunk(fn) : fn);
 	}
 
 	if (rune === '$state.snapshot') {

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

@@ -36,7 +36,8 @@ export function ClassBody(node, context) {
 
 			body.push(
 				b.prop_def(field.key, null),
-				b.method('get', b.key(name), [], [b.return(b.call(member))])
+				b.method('get', b.key(name), [], [b.return(b.call(member))]),
+				b.method('set', b.key(name), [b.id('$$value')], [b.return(b.call(member, b.id('$$value')))])
 			);
 		}
 	}
@@ -61,6 +62,7 @@ export function ClassBody(node, context) {
 		if (name[0] === '#' || field.type === '$state' || field.type === '$state.raw') {
 			body.push(/** @type {PropertyDefinition} */ (context.visit(definition, child_state)));
 		} else if (field.node === definition) {
+			// $derived / $derived.by
 			const member = b.member(b.this, field.key);
 
 			body.push(
@@ -69,7 +71,8 @@ export function ClassBody(node, context) {
 					/** @type {CallExpression} */ (context.visit(field.value, child_state))
 				),
 
-				b.method('get', definition.key, [], [b.return(b.call(member))])
+				b.method('get', definition.key, [], [b.return(b.call(member))]),
+				b.method('set', b.key(name), [b.id('$$value')], [b.return(b.call(member, b.id('$$value')))])
 			);
 		}
 	}

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

@@ -0,0 +1,23 @@
+/** @import { ClassBody, MemberExpression } from 'estree' */
+/** @import { Context } from '../types.js' */
+import * as b from '#compiler/builders';
+
+/**
+ * @param {MemberExpression} node
+ * @param {Context} context
+ */
+export function MemberExpression(node, context) {
+	if (
+		context.state.analysis.runes &&
+		node.object.type === 'ThisExpression' &&
+		node.property.type === 'PrivateIdentifier'
+	) {
+		const field = context.state.state_fields?.get(`#${node.property.name}`);
+
+		if (field?.type === '$derived' || field?.type === '$derived.by') {
+			return b.call(node);
+		}
+	}
+
+	context.next();
+}

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

@@ -11,7 +11,7 @@ export function PropertyDefinition(node, context) {
 	if (context.state.analysis.runes && node.value != null && node.value.type === 'CallExpression') {
 		const rune = get_rune(node.value, context.state.scope);
 
-		if (rune === '$state' || rune === '$state.raw' || rune === '$derived') {
+		if (rune === '$state' || rune === '$state.raw') {
 			return {
 				...node,
 				value:
@@ -21,13 +21,14 @@ export function PropertyDefinition(node, context) {
 			};
 		}
 
-		if (rune === '$derived.by') {
+		if (rune === '$derived.by' || rune === '$derived') {
+			const fn = /** @type {Expression} */ (context.visit(node.value.arguments[0]));
 			return {
 				...node,
 				value:
 					node.value.arguments.length === 0
 						? null
-						: b.call(/** @type {Expression} */ (context.visit(node.value.arguments[0])))
+						: b.call('$.derived', rune === '$derived' ? b.thunk(fn) : fn)
 			};
 		}
 	}

packages/svelte/src/internal/server/index.js

@@ -514,3 +514,24 @@ export {
 } from '../shared/validate.js';
 
 export { escape_html as escape };
+
+/**
+ * @template T
+ * @param {()=>T} fn
+ * @returns {(new_value?: T) => (T | void)}
+ */
+export function derived(fn) {
+	const get_value = once(fn);
+	/**
+	 * @type {T | undefined}
+	 */
+	let updated_value;
+
+	return function (new_value) {
+		if (arguments.length === 0) {
+			return updated_value ?? get_value();
+		}
+		updated_value = new_value;
+		return updated_value;
+	};
+}

packages/svelte/tests/runtime-runes/samples/class-state-derived-private/_config.js

@@ -5,6 +5,7 @@ export default test({
 	html: `
 		<button>0</button>
 		<p>doubled: 0</p>
+		<p>tripled: 0</p>
 	`,
 
 	test({ assert, target }) {
@@ -17,6 +18,7 @@ export default test({
 			`
 				<button>1</button>
 				<p>doubled: 2</p>
+				<p>tripled: 3</p>
 			`
 		);
 
@@ -27,6 +29,7 @@ export default test({
 			`
 				<button>2</button>
 				<p>doubled: 4</p>
+				<p>tripled: 6</p>
 			`
 		);
 	}

packages/svelte/tests/runtime-runes/samples/class-state-derived-private/main.svelte

@@ -2,14 +2,24 @@
 	class Counter {
 		count = $state(0);
 		#doubled = $derived(this.count * 2);
+		#tripled = $derived.by(() => this.count * this.by);
 
-		get embiggened() {
+		constructor(by) {
+			this.by = by;
+		}
+
+		get embiggened1() {
 			return this.#doubled;
 		}
+
+		get embiggened2() {
+			return this.#tripled;
+		}
 	}
 
-	const counter = new Counter();
+	const counter = new Counter(3);
 </script>
 
 <button on:click={() => counter.count++}>{counter.count}</button>
-<p>doubled: {counter.embiggened}</p>
+<p>doubled: {counter.embiggened1}</p>
+<p>tripled: {counter.embiggened2}</p>

packages/svelte/tests/runtime-runes/samples/writable-derived-3/_config.js

@@ -0,0 +1,5 @@
+import { test } from '../../test';
+
+export default test({
+	html: `3 3 3 3`
+});

packages/svelte/tests/runtime-runes/samples/writable-derived-3/main.svelte

@@ -0,0 +1,29 @@
+<script>
+	class X {
+    x = $state(1);
+    on_class = $derived(this.x * 2);
+    #on_class_private = $derived(this.x * 2);
+    #in_constructor_private
+
+    constructor() {
+      this.#in_constructor_private = $derived(this.x * 2);
+      this.in_constructor = $derived(this.x * 2);
+      this.#on_class_private = 3;
+      this.#in_constructor_private = 3;
+    }
+
+    get on_class_private() {
+      return this.#on_class_private;
+    }
+
+    get in_constructor_private() {
+      return this.#in_constructor_private;
+    }
+  }
+
+  const x = new X();
+  x.on_class = 3;
+  x.in_constructor = 3;
+</script>
+
+{x.on_class} {x.in_constructor} {x.on_class_private} {x.in_constructor_private}