feat: $derived.by get current value as first argument

.changeset/gentle-mayflies-try.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+feat: `$derived.by` get current value as first argument

packages/svelte/src/ambient.d.ts

@@ -202,7 +202,7 @@ declare namespace $derived {
 	 *
 	 * https://svelte-5-preview.vercel.app/docs/runes#$derived-by
 	 */
-	export function by<T>(fn: () => T): T;
+	export function by<T>(fn: (current: T | null) => T): T;
 
 	// prevent intellisense from being unhelpful
 	/** @deprecated */

packages/svelte/src/compiler/utils/builders.js

@@ -419,16 +419,6 @@ export function template(elements, expressions) {
  * @returns {ESTree.Expression}
  */
 export function thunk(expression, async = false) {
-	if (
-		expression.type === 'CallExpression' &&
-		expression.callee.type !== 'Super' &&
-		expression.callee.type !== 'MemberExpression' &&
-		expression.callee.type !== 'CallExpression' &&
-		expression.arguments.length === 0
-	) {
-		return expression.callee;
-	}
-
 	const fn = arrow([], expression);
 	if (async) fn.async = true;
 	return fn;

packages/svelte/src/internal/client/reactivity/deriveds.js

@@ -17,7 +17,7 @@ import { inspect_effects, set_inspect_effects } from './sources.js';
 
 /**
  * @template V
- * @param {() => V} fn
+ * @param {(current: V | null ) => V} fn
  * @returns {Derived<V>}
  */
 /*#__NO_SIDE_EFFECTS__*/
@@ -47,7 +47,7 @@ export function derived(fn) {
 
 /**
  * @template V
- * @param {() => V} fn
+ * @param {(current: NoInfer<V> | null) => V} fn
  * @returns {Derived<V>}
  */
 /*#__NO_SIDE_EFFECTS__*/

packages/svelte/src/internal/client/reactivity/types.d.ts

@@ -25,7 +25,7 @@ export interface Reaction extends Signal {
 
 export interface Derived<V = unknown> extends Value<V>, Reaction {
 	/** The derived function */
-	fn: () => V;
+	fn: (current: V | null) => V;
 	/** Reactions created inside this signal */
 	children: null | Reaction[];
 }

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

@@ -309,7 +309,11 @@ export function update_reaction(reaction) {
 	derived_sources = null;
 
 	try {
-		var result = /** @type {Function} */ (0, reaction.fn)();
+		var args = [];
+		if (reaction.f & DERIVED) {
+			args.push(/**@type {Derived}*/ (reaction).v);
+		}
+		var result = /** @type {Function} */ (0, reaction.fn)(...args);
 		var deps = reaction.deps;
 
 		if (new_deps !== null) {
@@ -783,7 +787,6 @@ export function get(signal) {
 			update_derived(derived);
 		}
 	}
-
 	return signal.v;
 }
 

packages/svelte/tests/signals/test.ts

@@ -471,7 +471,7 @@ describe('signals', () => {
 	});
 
 	test('owned deriveds correctly cleanup when no longer connected to graph', () => {
-		let a: Derived<unknown>;
+		let a: Derived<void>;
 		let s = state(0);
 
 		const destroy = effect_root(() => {

packages/svelte/tests/snapshot/samples/svelte-element/_expected/client/index.svelte.js

@@ -6,6 +6,6 @@ export default function Svelte_element($$anchor, $$props) {
 	var fragment = $.comment();
 	var node = $.first_child(fragment);
 
-	$.element(node, tag, false);
+	$.element(node, () => tag(), false);
 	$.append($$anchor, fragment);
 }

packages/svelte/types/index.d.ts

@@ -2494,7 +2494,7 @@ declare namespace $derived {
 	 *
 	 * https://svelte-5-preview.vercel.app/docs/runes#$derived-by
 	 */
-	export function by<T>(fn: () => T): T;
+	export function by<T>(fn: (current: T | null) => T): T;
 
 	// prevent intellisense from being unhelpful
 	/** @deprecated */