correctly inspect derived values (#9731)

Co-authored-by: Rich Harris <rich.harris@vercel.com>

.changeset/lemon-geese-drum.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: improve `$inspect` type definition

.changeset/two-dragons-yell.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: correctly inspect derived values

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

@@ -191,9 +191,36 @@ function create_source_signal(flags, value) {
  * @param {import('./types.js').SignalFlags} flags
  * @param {V} value
  * @param {import('./types.js').Block | null} block
- * @returns {import('./types.js').ComputationSignal<V>}
+ * @returns {import('./types.js').ComputationSignal<V> | import('./types.js').ComputationSignal<V> & import('./types.js').SourceSignalDebug}
  */
 function create_computation_signal(flags, value, block) {
+	if (DEV) {
+		return {
+			// block
+			b: block,
+			// consumers
+			c: null,
+			// destroy
+			d: null,
+			// equals
+			e: null,
+			// flags
+			f: flags,
+			// init
+			i: null,
+			// references
+			r: null,
+			// value
+			v: value,
+			// context: We can remove this if we get rid of beforeUpdate/afterUpdate
+			x: null,
+			// destroy
+			y: null,
+			// this is for DEV only
+			inspect: new Set()
+		};
+	}
+
 	return {
 		// block
 		b: block,
@@ -671,6 +698,12 @@ function update_derived(signal, force_schedule) {
 	if (!equals(value, signal.v)) {
 		signal.v = value;
 		mark_signal_consumers(signal, DIRTY, force_schedule);
+
+		// @ts-expect-error
+		if (DEV && signal.inspect && force_schedule) {
+			// @ts-expect-error
+			for (const fn of signal.inspect) fn();
+		}
 	}
 }
 
@@ -836,7 +869,15 @@ export function get(signal) {
 	}
 
 	if ((flags & DERIVED) !== 0 && is_signal_dirty(signal)) {
-		update_derived(/** @type {import('./types.js').ComputationSignal<V>} **/ (signal), false);
+		if (DEV) {
+			// we want to avoid tracking indirect dependencies
+			const previous_inspect_fn = inspect_fn;
+			inspect_fn = null;
+			update_derived(/** @type {import('./types.js').ComputationSignal<V>} **/ (signal), false);
+			inspect_fn = previous_inspect_fn;
+		} else {
+			update_derived(/** @type {import('./types.js').ComputationSignal<V>} **/ (signal), false);
+		}
 	}
 	return signal.v;
 }

packages/svelte/src/main/ambient.d.ts

@@ -132,12 +132,23 @@ declare namespace $effect {
 declare function $props<T>(): T;
 
 /**
- * Logs the arguments whenever they, or the properties they contain, change. Example:
+ * Inspects a value whenever it, or the properties it contains, change. Example:
  *
  * ```ts
- * $inspect(someValue, someOtherValue)
+ * $inspect({ someValue, someOtherValue })
+ * ```
+ *
+ * If a second argument is provided, it will be called with the value and the event type
+ * (`'init'` or `'update'`), otherwise the value will be logged to the console.
+ *
+ * ```ts
+ * $inspect(x, console.trace);
+ * $inspect(y, (y) => { debugger; });
  * ```
  *
  * https://svelte-5-preview.vercel.app/docs/runes#$inspect
  */
-declare function $inspect(): void;
+declare function $inspect<T>(
+	value: T,
+	callback?: (value: T, type: 'init' | 'update') => void
+): void;

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

@@ -0,0 +1,31 @@
+import { test } from '../../test';
+
+/**
+ * @type {any[]}
+ */
+let log;
+
+export default test({
+	compileOptions: {
+		dev: true
+	},
+
+	get props() {
+		log = [];
+		return {
+			push: (/** @type {any} */ ...v) => log.push(...v)
+		};
+	},
+
+	async test({ assert, target }) {
+		const button = target.querySelector('button');
+
+		button?.click();
+		await Promise.resolve();
+
+		button?.click();
+		await Promise.resolve();
+
+		assert.deepEqual(log, ['X', 'init', 'XX', 'update', 'XXX', 'update']);
+	}
+});

packages/svelte/tests/runtime-runes/samples/inspect-derived/main.svelte

@@ -0,0 +1,11 @@
+<script>
+	/** @type {{ push: (v: any) => void }} */
+	let { push } = $props();
+
+	let x = $state('x');
+	let y = $derived(x.toUpperCase());
+
+	$inspect(y, push);
+</script>
+
+<button on:click={() => x += 'x'}>{x}</button>