fix: handle basic assignment of deriveds on the server

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

@@ -44,6 +44,8 @@ function build_assignment(operator, left, right, context) {
 					/** @type {Expression} */ (context.visit(right))
 				);
 			}
+		} else if (field && (field.type === '$derived' || field.type === '$derived.by')) {
+			return b.call(b.member(b.this, name), right);
 		}
 	}
 

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

@@ -22,6 +22,10 @@ export function ClassBody(node, context) {
 	const child_state = { ...context.state, state_fields };
 
 	for (const [name, field] of state_fields) {
+		if (name[0] === '#') {
+			continue;
+		}
+
 		// insert backing fields for stuff declared in the constructor
 		if (
 			field &&

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

@@ -28,7 +28,7 @@ export function PropertyDefinition(node, context) {
 				value:
 					node.value.arguments.length === 0
 						? null
-						: b.call('$.once', rune === '$derived' ? b.thunk(fn) : fn)
+						: b.call('$.derived', rune === '$derived' ? b.thunk(fn) : fn)
 			};
 		}
 	}

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

@@ -514,3 +514,23 @@ 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) {
+	/**
+	 * @type {T | undefined}
+	 */
+	let updated_value;
+
+	return function (new_value) {
+		if (arguments.length === 0) {
+			return updated_value ?? fn();
+		}
+		updated_value = new_value;
+		return updated_value;
+	};
+}

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

@@ -5,7 +5,7 @@ export default test({
 	html: `
 		<button>0</button>
 		<p>doubled: 0</p>
-		<p>trippled: 0</p>
+		<p>tripled: 0</p>
 	`,
 
 	test({ assert, target }) {
@@ -18,7 +18,7 @@ export default test({
 			`
 				<button>1</button>
 				<p>doubled: 2</p>
-				<p>trippled: 3</p>
+				<p>tripled: 3</p>
 			`
 		);
 
@@ -29,7 +29,7 @@ export default test({
 			`
 				<button>2</button>
 				<p>doubled: 4</p>
-				<p>trippled: 9</p>
+				<p>tripled: 6</p>
 			`
 		);
 	}

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

@@ -2,7 +2,7 @@
 	class Counter {
 		count = $state(0);
 		#doubled = $derived(this.count * 2);
-		#trippled = $derived.by(() => this.count * this.by);
+		#tripled = $derived.by(() => this.count * this.by);
 
 		constructor(by) {
 			this.by = by;
@@ -13,7 +13,7 @@
 		}
 
 		get embiggened2() {
-			return this.#trippled;
+			return this.#tripled;
 		}
 	}
 
@@ -22,4 +22,4 @@
 
 <button on:click={() => counter.count++}>{counter.count}</button>
 <p>doubled: {counter.embiggened1}</p>
-<p>trippled: {counter.embiggened2}</p>
+<p>tripled: {counter.embiggened2}</p>