add types, DRY out some logic

packages/svelte/src/ambient.d.ts

@@ -237,6 +237,34 @@ declare namespace $derived {
 declare function $effect(fn: () => void | (() => void)): void;
 
 declare namespace $effect {
+	/**
+	 * The `$effect.active` rune is an advanced feature that indicates whether an effect or async `$derived` can be created in the current context.
+	 * Effects and async deriveds can only be created in root effects, which are created during component setup, or can be programmatically created via `$effect.root`.
+	 * 
+	 * Example:
+	 * ```svelte
+	 * <script>
+	 *   console.log('in component setup', $effect.active()); // true
+	 * 
+	 *   function onclick() {
+	 *     console.log('after component setup', $effect.active()); // false
+	 *   }
+	 *   function ondblclick() {
+	 *     $effect.root(() => {
+	 *       console.log('in root effect', $effect.active()); // true
+	 *       return () => {
+	 *         console.log('in effect teardown', $effect.active()); // false
+	 *       }
+	 *     })();
+	 *   }
+	 * </script>
+	 * <button {onclick}>Click me!</button>
+	 * <button {ondblclick}>Click me twice!</button>
+	 * ```
+	 * 
+	 * https://svelte.dev/docs/svelte/$effect#$effect.active
+	 */
+	export function active(): boolean;
 	/**
 	 * Runs code right before a component is mounted to the DOM, and then whenever its dependencies change, i.e. `$state` or `$derived` values.
 	 * The timing of the execution is right before the DOM is updated.

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

@@ -41,3 +41,8 @@ export const ELEMENT_NODE = 1;
 export const TEXT_NODE = 3;
 export const COMMENT_NODE = 8;
 export const DOCUMENT_FRAGMENT_NODE = 11;
+
+export const VALID_EFFECT_PARENT = 0;
+export const EFFECT_ORPHAN = 1;
+export const UNOWNED_DERIVED_PARENT = 2;
+export const EFFECT_TEARDOWN = 3;

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

@@ -33,7 +33,11 @@ import {
 	EFFECT_PRESERVED,
 	STALE_REACTION,
 	USER_EFFECT,
-	ASYNC
+	ASYNC,
+	EFFECT_ORPHAN,
+	EFFECT_TEARDOWN,
+	UNOWNED_DERIVED_PARENT,
+	VALID_EFFECT_PARENT
 } from '#client/constants';
 import * as e from '../errors.js';
 import { DEV } from 'esm-env';
@@ -44,19 +48,38 @@ import { Batch, schedule_effect } from './batch.js';
 import { flatten } from './async.js';
 
 /**
- * @param {'$effect' | '$effect.pre' | '$inspect'} rune
+ * @returns {number}
  */
-export function validate_effect(rune) {
+function active_root_effect() {
 	if (active_effect === null && active_reaction === null) {
-		e.effect_orphan(rune);
+		return EFFECT_ORPHAN;
 	}
 
 	if (active_reaction !== null && (active_reaction.f & UNOWNED) !== 0 && active_effect === null) {
-		e.effect_in_unowned_derived();
+		return UNOWNED_DERIVED_PARENT;
 	}
 
 	if (is_destroying_effect) {
-		e.effect_in_teardown(rune);
+		return EFFECT_TEARDOWN;
+	}
+
+	return VALID_EFFECT_PARENT;
+}
+
+/**
+ * @param {'$effect' | '$effect.pre' | '$inspect'} rune
+ */
+export function validate_effect(rune) {
+	const valid_effect_parent = active_root_effect();
+	switch(valid_effect_parent) {
+		case VALID_EFFECT_PARENT:
+			return;
+		case EFFECT_ORPHAN:
+			e.effect_orphan(rune);
+		case UNOWNED_DERIVED_PARENT:
+			e.effect_in_unowned_derived();
+		case EFFECT_TEARDOWN:
+			e.effect_in_teardown(rune);
 	}
 }
 
@@ -170,13 +193,7 @@ export function effect_tracking() {
  * @returns {boolean}
  */
 export function effect_active() {
-	if (active_reaction === null && active_effect === null) {
-		return false;
-	}
-	if (is_destroying_effect) {
-		return false;
-	}
-	return true;
+	return active_root_effect() === VALID_EFFECT_PARENT;
 }
 
 /**

packages/svelte/types/index.d.ts

@@ -3311,6 +3311,34 @@ declare namespace $derived {
 declare function $effect(fn: () => void | (() => void)): void;
 
 declare namespace $effect {
+	/**
+	 * The `$effect.active` rune is an advanced feature that indicates whether an effect or async `$derived` can be created in the current context.
+	 * Effects and async deriveds can only be created in root effects, which are created during component setup, or can be programmatically created via `$effect.root`.
+	 * 
+	 * Example:
+	 * ```svelte
+	 * <script>
+	 *   console.log('in component setup', $effect.active()); // true
+	 * 
+	 *   function onclick() {
+	 *     console.log('after component setup', $effect.active()); // false
+	 *   }
+	 *   function ondblclick() {
+	 *     $effect.root(() => {
+	 *       console.log('in root effect', $effect.active()); // true
+	 *       return () => {
+	 *         console.log('in effect teardown', $effect.active()); // false
+	 *       }
+	 *     })();
+	 *   }
+	 * </script>
+	 * <button {onclick}>Click me!</button>
+	 * <button {ondblclick}>Click me twice!</button>
+	 * ```
+	 * 
+	 * https://svelte.dev/docs/svelte/$effect#$effect.active
+	 */
+	export function active(): boolean;
 	/**
 	 * Runs code right before a component is mounted to the DOM, and then whenever its dependencies change, i.e. `$state` or `$derived` values.
 	 * The timing of the execution is right before the DOM is updated.