feat: add `getAbortSignal()` (#16266)

* WIP getAbortSignal

* add test

* regenerate

* add error code

* changeset

* regenerate

* try this

* { stale: true }

* fix test

* lint

* abort synchronously in SSR

* make STALE_REACTION a `StaleReactionError extends Error`

* make non-optional

* Update packages/svelte/src/internal/server/abort-signal.js

Co-authored-by: Elliott Johnson <sejohnson@torchcloudconsulting.com>

---------

Co-authored-by: Elliott Johnson <sejohnson@torchcloudconsulting.com>

.changeset/short-fireants-flow.md

@@ -0,0 +1,5 @@
+---
+'svelte': minor
+---
+
+feat: add `getAbortSignal()`

documentation/docs/98-reference/.generated/client-errors.md

@@ -74,6 +74,12 @@ Effect cannot be created inside a `$derived` value that was not itself created i
 Maximum update depth exceeded. This can happen when a reactive block or effect repeatedly sets a new value. Svelte limits the number of nested updates to prevent infinite loops
 ```
 
+### get_abort_signal_outside_reaction
+
+```
+`getAbortSignal()` can only be called inside an effect or derived
+```
+
 ### hydration_failed
 
 ```

packages/svelte/messages/client-errors/errors.md

@@ -48,6 +48,10 @@ See the [migration guide](/docs/svelte/v5-migration-guide#Components-are-no-long
 
 > Maximum update depth exceeded. This can happen when a reactive block or effect repeatedly sets a new value. Svelte limits the number of nested updates to prevent infinite loops
 
+## get_abort_signal_outside_reaction
+
+> `getAbortSignal()` can only be called inside an effect or derived
+
 ## hydration_failed
 
 > Failed to hydrate the application

packages/svelte/src/index-client.js

@@ -1,7 +1,7 @@
 /** @import { ComponentContext, ComponentContextLegacy } from '#client' */
 /** @import { EventDispatcher } from './index.js' */
 /** @import { NotFunction } from './internal/types.js' */
-import { untrack } from './internal/client/runtime.js';
+import { active_reaction, untrack } from './internal/client/runtime.js';
 import { is_array } from './internal/shared/utils.js';
 import { user_effect } from './internal/client/index.js';
 import * as e from './internal/client/errors.js';
@@ -44,6 +44,37 @@ if (DEV) {
 	throw_rune_error('$bindable');
 }
 
+/**
+ * Returns an [`AbortSignal`](https://developer.mozilla.org/en-US/docs/Web/API/AbortSignal) that aborts when the current [derived](https://svelte.dev/docs/svelte/$derived) or [effect](https://svelte.dev/docs/svelte/$effect) re-runs or is destroyed.
+ *
+ * Must be called while a derived or effect is running.
+ *
+ * ```svelte
+ * <script>
+ * 	import { getAbortSignal } from 'svelte';
+ *
+ * 	let { id } = $props();
+ *
+ * 	async function getData(id) {
+ * 		const response = await fetch(`/items/${id}`, {
+ * 			signal: getAbortSignal()
+ * 		});
+ *
+ * 		return await response.json();
+ * 	}
+ *
+ * 	const data = $derived(await getData(id));
+ * </script>
+ * ```
+ */
+export function getAbortSignal() {
+	if (active_reaction === null) {
+		e.get_abort_signal_outside_reaction();
+	}
+
+	return (active_reaction.ac ??= new AbortController()).signal;
+}
+
 /**
  * `onMount`, like [`$effect`](https://svelte.dev/docs/svelte/$effect), schedules a function to run as soon as the component has been mounted to the DOM.
  * Unlike `$effect`, the provided function only runs once.

packages/svelte/src/index-server.js

@@ -35,6 +35,8 @@ export function unmount() {
 
 export async function tick() {}
 
+export { getAbortSignal } from './internal/server/abort-signal.js';
+
 export { getAllContexts, getContext, hasContext, setContext } from './internal/server/context.js';
 
 export { createRawSnippet } from './internal/server/blocks/snippet.js';

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

@@ -27,6 +27,12 @@ export const LEGACY_PROPS = Symbol('legacy props');
 export const LOADING_ATTR_SYMBOL = Symbol('');
 export const PROXY_PATH_SYMBOL = Symbol('proxy path');
 
+// allow users to ignore aborted signal errors if `reason.stale`
+export const STALE_REACTION = new (class StaleReactionError extends Error {
+	name = 'StaleReactionError';
+	message = 'The reaction that called `getAbortSignal()` was re-run or destroyed';
+})();
+
 export const ELEMENT_NODE = 1;
 export const TEXT_NODE = 3;
 export const COMMENT_NODE = 8;

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

@@ -195,6 +195,22 @@ export function effect_update_depth_exceeded() {
 	}
 }
 
+/**
+ * `getAbortSignal()` can only be called inside an effect or derived
+ * @returns {never}
+ */
+export function get_abort_signal_outside_reaction() {
+	if (DEV) {
+		const error = new Error(`get_abort_signal_outside_reaction\n\`getAbortSignal()\` can only be called inside an effect or derived\nhttps://svelte.dev/e/get_abort_signal_outside_reaction`);
+
+		error.name = 'Svelte error';
+
+		throw error;
+	} else {
+		throw new Error(`https://svelte.dev/e/get_abort_signal_outside_reaction`);
+	}
+}
+
 /**
  * Failed to hydrate the application
  * @returns {never}

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

@@ -53,7 +53,8 @@ export function derived(fn) {
 		rv: 0,
 		v: /** @type {V} */ (null),
 		wv: 0,
-		parent: parent_derived ?? active_effect
+		parent: parent_derived ?? active_effect,
+		ac: null
 	};
 
 	if (DEV && tracing_mode_flag) {

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

@@ -32,7 +32,8 @@ import {
 	HEAD_EFFECT,
 	MAYBE_DIRTY,
 	EFFECT_HAS_DERIVED,
-	BOUNDARY_EFFECT
+	BOUNDARY_EFFECT,
+	STALE_REACTION
 } from '#client/constants';
 import { set } from './sources.js';
 import * as e from '../errors.js';
@@ -106,7 +107,8 @@ function create_effect(type, fn, sync, push = true) {
 		prev: null,
 		teardown: null,
 		transitions: null,
-		wv: 0
+		wv: 0,
+		ac: null
 	};
 
 	if (DEV) {
@@ -397,6 +399,8 @@ export function destroy_effect_children(signal, remove_dom = false) {
 	signal.first = signal.last = null;
 
 	while (effect !== null) {
+		effect.ac?.abort(STALE_REACTION);
+
 		var next = effect.next;
 
 		if ((effect.f & ROOT_EFFECT) !== 0) {
@@ -478,6 +482,7 @@ export function destroy_effect(effect, remove_dom = true) {
 		effect.fn =
 		effect.nodes_start =
 		effect.nodes_end =
+		effect.ac =
 			null;
 }
 

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

@@ -40,6 +40,8 @@ export interface Reaction extends Signal {
 	fn: null | Function;
 	/** Signals that this signal reads from */
 	deps: null | Value[];
+	/** An AbortController that aborts when the signal is destroyed */
+	ac: null | AbortController;
 }
 
 export interface Derived<V = unknown> extends Value<V>, Reaction {

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

@@ -22,7 +22,8 @@ import {
 	ROOT_EFFECT,
 	LEGACY_DERIVED_PROP,
 	DISCONNECTED,
-	EFFECT_IS_UPDATING
+	EFFECT_IS_UPDATING,
+	STALE_REACTION
 } from './constants.js';
 import { flush_tasks } from './dom/task.js';
 import { internal_set, old_values } from './reactivity/sources.js';
@@ -276,6 +277,11 @@ export function update_reaction(reaction) {
 
 	reaction.f |= EFFECT_IS_UPDATING;
 
+	if (reaction.ac !== null) {
+		reaction.ac.abort(STALE_REACTION);
+		reaction.ac = null;
+	}
+
 	try {
 		var result = /** @type {Function} */ (0, reaction.fn)();
 		var deps = reaction.deps;

packages/svelte/src/internal/server/abort-signal.js

@@ -0,0 +1,13 @@
+import { STALE_REACTION } from '#client/constants';
+
+/** @type {AbortController | null} */
+export let controller = null;
+
+export function abort() {
+	controller?.abort(STALE_REACTION);
+	controller = null;
+}
+
+export function getAbortSignal() {
+	return (controller ??= new AbortController()).signal;
+}

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

@@ -18,6 +18,7 @@ import { validate_store } from '../shared/validate.js';
 import { is_boolean_attribute, is_raw_text_element, is_void } from '../../utils.js';
 import { reset_elements } from './dev.js';
 import { Payload } from './payload.js';
+import { abort } from './abort-signal.js';
 
 // https://html.spec.whatwg.org/multipage/syntax.html#attributes-2
 // https://infra.spec.whatwg.org/#noncharacter
@@ -66,50 +67,54 @@ export let on_destroy = [];
  * @returns {RenderOutput}
  */
 export function render(component, options = {}) {
-	const payload = new Payload(options.idPrefix ? options.idPrefix + '-' : '');
+	try {
+		const payload = new Payload(options.idPrefix ? options.idPrefix + '-' : '');
 
-	const prev_on_destroy = on_destroy;
-	on_destroy = [];
-	payload.out += BLOCK_OPEN;
+		const prev_on_destroy = on_destroy;
+		on_destroy = [];
+		payload.out += BLOCK_OPEN;
 
-	let reset_reset_element;
+		let reset_reset_element;
 
-	if (DEV) {
-		// prevent parent/child element state being corrupted by a bad render
-		reset_reset_element = reset_elements();
-	}
+		if (DEV) {
+			// prevent parent/child element state being corrupted by a bad render
+			reset_reset_element = reset_elements();
+		}
 
-	if (options.context) {
-		push();
-		/** @type {Component} */ (current_component).c = options.context;
-	}
+		if (options.context) {
+			push();
+			/** @type {Component} */ (current_component).c = options.context;
+		}
 
-	// @ts-expect-error
-	component(payload, options.props ?? {}, {}, {});
+		// @ts-expect-error
+		component(payload, options.props ?? {}, {}, {});
 
-	if (options.context) {
-		pop();
-	}
+		if (options.context) {
+			pop();
+		}
 
-	if (reset_reset_element) {
-		reset_reset_element();
-	}
+		if (reset_reset_element) {
+			reset_reset_element();
+		}
 
-	payload.out += BLOCK_CLOSE;
-	for (const cleanup of on_destroy) cleanup();
-	on_destroy = prev_on_destroy;
+		payload.out += BLOCK_CLOSE;
+		for (const cleanup of on_destroy) cleanup();
+		on_destroy = prev_on_destroy;
 
-	let head = payload.head.out + payload.head.title;
+		let head = payload.head.out + payload.head.title;
 
-	for (const { hash, code } of payload.css) {
-		head += `<style id="${hash}">${code}</style>`;
-	}
+		for (const { hash, code } of payload.css) {
+			head += `<style id="${hash}">${code}</style>`;
+		}
 
-	return {
-		head,
-		html: payload.out,
-		body: payload.out
-	};
+		return {
+			head,
+			html: payload.out,
+			body: payload.out
+		};
+	} finally {
+		abort();
+	}
 }
 
 /**

packages/svelte/tests/runtime-runes/samples/get-abort-signal/_config.js

@@ -0,0 +1,34 @@
+import { test } from '../../test';
+
+export default test({
+	html: `<button>increment</button><p>loading...</p>`,
+
+	async test({ assert, target, variant, logs }) {
+		await new Promise((f) => setTimeout(f, 50));
+
+		if (variant === 'hydrate') {
+			assert.deepEqual(logs, [
+				'aborted',
+				'StaleReactionError',
+				'The reaction that called `getAbortSignal()` was re-run or destroyed'
+			]);
+		}
+
+		logs.length = 0;
+
+		const [button] = target.querySelectorAll('button');
+
+		await new Promise((f) => setTimeout(f, 50));
+		assert.htmlEqual(target.innerHTML, '<button>increment</button><p>0</p>');
+
+		button.click();
+		await new Promise((f) => setTimeout(f, 50));
+		assert.htmlEqual(target.innerHTML, '<button>increment</button><p>2</p>');
+
+		assert.deepEqual(logs, [
+			'aborted',
+			'StaleReactionError',
+			'The reaction that called `getAbortSignal()` was re-run or destroyed'
+		]);
+	}
+});

packages/svelte/tests/runtime-runes/samples/get-abort-signal/main.svelte

@@ -0,0 +1,33 @@
+<script>
+	import { getAbortSignal } from 'svelte';
+
+	let count = $state(0);
+
+	let delayed_count = $derived.by(async () => {
+		let c = count;
+
+		const signal = getAbortSignal();
+
+		await new Promise((f) => setTimeout(f));
+
+		if (signal.aborted) {
+			console.log('aborted', signal.reason.name, signal.reason.message);
+		}
+
+		return c;
+	});
+</script>
+
+<button onclick={async () => {
+	count += 1;
+	await Promise.resolve();
+	count += 1;
+}}>increment</button>
+
+{#await delayed_count}
+	<p>loading...</p>
+{:then count}
+	<p>{count}</p>
+{:catch error}
+	{console.log('this should never be rendered')}
+{/await}

packages/svelte/types/index.d.ts

@@ -348,6 +348,30 @@ declare module 'svelte' {
 				 */
 				props: Props;
 			});
+	/**
+	 * Returns an [`AbortSignal`](https://developer.mozilla.org/en-US/docs/Web/API/AbortSignal) that aborts when the current [derived](https://svelte.dev/docs/svelte/$derived) or [effect](https://svelte.dev/docs/svelte/$effect) re-runs or is destroyed.
+	 *
+	 * Must be called while a derived or effect is running.
+	 *
+	 * ```svelte
+	 * <script>
+	 * 	import { getAbortSignal } from 'svelte';
+	 *
+	 * 	let { id } = $props();
+	 *
+	 * 	async function getData(id) {
+	 * 		const response = await fetch(`/items/${id}`, {
+	 * 			signal: getAbortSignal()
+	 * 		});
+	 *
+	 * 		return await response.json();
+	 * 	}
+	 *
+	 * 	const data = $derived(await getData(id));
+	 * </script>
+	 * ```
+	 */
+	export function getAbortSignal(): AbortSignal;
 	/**
 	 * `onMount`, like [`$effect`](https://svelte.dev/docs/svelte/$effect), schedules a function to run as soon as the component has been mounted to the DOM.
 	 * Unlike `$effect`, the provided function only runs once.