fix: more informative error when effects run in an infinite loop

.changeset/slimy-doors-fetch.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: more informative error when effects run in an infinite loop

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

@@ -89,9 +89,47 @@ Effect cannot be created inside a `$derived` value that was not itself created i
 ### effect_update_depth_exceeded
 
 ```
-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
+Maximum update depth exceeded. This typically indicates that an effect reads and writes the same piece of state
 ```
 
+If an effect updates some state that it also depends on, it will re-run, potentially in a loop:
+
+```js
+let count = $state(0);
+
+$effect(() => {
+	// this both reads and writes `count`,
+	// so will run in an infinite loop
+	count += 1;
+});
+```
+
+(Svelte intervenes before this can crash your browser tab.)
+
+The same applies to array mutations, since these both read and write to the array:
+
+```js
+let array = $state([]);
+
+$effect(() => {
+	array.push('hello');
+});
+```
+
+Note that it's fine for an effect to re-run itself as long as it 'settles':
+
+```js
+let array = ['a', 'b', 'c'];
+// ---cut---
+$effect(() => {
+	// this is okay, because sorting an already-sorted array
+	// won't result in a mutation
+	array.sort();
+});
+```
+
+Often when encountering this issue, the value in question shouldn't be state (for example, if you are pushing to a `logs` array in an effect, make `logs` a normal array rather than `$state([])`). In the rare cases where you really _do_ need to write to state in an effect — [which you should avoid]($effect#When-not-to-use-$effect) — you can read the state with [untrack](svelte#untrack) to avoid adding it as a dependency.
+
 ### flush_sync_in_effect
 
 ```

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

@@ -60,7 +60,45 @@ See the [migration guide](/docs/svelte/v5-migration-guide#Components-are-no-long
 
 ## effect_update_depth_exceeded
 
-> 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
+> Maximum update depth exceeded. This typically indicates that an effect reads and writes the same piece of state
+
+If an effect updates some state that it also depends on, it will re-run, potentially in a loop:
+
+```js
+let count = $state(0);
+
+$effect(() => {
+	// this both reads and writes `count`,
+	// so will run in an infinite loop
+	count += 1;
+});
+```
+
+(Svelte intervenes before this can crash your browser tab.)
+
+The same applies to array mutations, since these both read and write to the array:
+
+```js
+let array = $state([]);
+
+$effect(() => {
+	array.push('hello');
+});
+```
+
+Note that it's fine for an effect to re-run itself as long as it 'settles':
+
+```js
+let array = ['a', 'b', 'c'];
+// ---cut---
+$effect(() => {
+	// this is okay, because sorting an already-sorted array
+	// won't result in a mutation
+	array.sort();
+});
+```
+
+Often when encountering this issue, the value in question shouldn't be state (for example, if you are pushing to a `logs` array in an effect, make `logs` a normal array rather than `$state([])`). In the rare cases where you really _do_ need to write to state in an effect — [which you should avoid]($effect#When-not-to-use-$effect) — you can read the state with [untrack](svelte#untrack) to avoid adding it as a dependency.
 
 ## flush_sync_in_effect
 

packages/svelte/src/internal/client/dev/tracing.js

@@ -56,8 +56,10 @@ function log_entry(signal, entry) {
 	}
 
 	if (dirty && signal.updated) {
-		// eslint-disable-next-line no-console
-		console.log(signal.updated);
+		for (const updated of signal.updated.values()) {
+			// eslint-disable-next-line no-console
+			console.log(updated.error);
+		}
 	}
 
 	if (entry) {
@@ -120,44 +122,46 @@ export function trace(label, fn) {
 
 /**
  * @param {string} label
+ * @returns {Error & { stack: string } | null}
  */
 export function get_stack(label) {
 	let error = Error();
 	const stack = error.stack;
 
-	if (stack) {
-		const lines = stack.split('\n');
-		const new_lines = ['\n'];
-
-		for (let i = 0; i < lines.length; i++) {
-			const line = lines[i];
-
-			if (line === 'Error') {
-				continue;
-			}
-			if (line.includes('validate_each_keys')) {
-				return null;
-			}
-			if (line.includes('svelte/src/internal')) {
-				continue;
-			}
-			new_lines.push(line);
-		}
+	if (!stack) return null;
 
-		if (new_lines.length === 1) {
+	const lines = stack.split('\n');
+	const new_lines = ['\n'];
+
+	for (let i = 0; i < lines.length; i++) {
+		const line = lines[i];
+
+		if (line === 'Error') {
+			continue;
+		}
+		if (line.includes('validate_each_keys')) {
 			return null;
 		}
+		if (line.includes('svelte/src/internal')) {
+			continue;
+		}
+		new_lines.push(line);
+	}
 
-		define_property(error, 'stack', {
-			value: new_lines.join('\n')
-		});
-
-		define_property(error, 'name', {
-			// 'Error' suffix is required for stack traces to be rendered properly
-			value: `${label}Error`
-		});
+	if (new_lines.length === 1) {
+		return null;
 	}
-	return error;
+
+	define_property(error, 'stack', {
+		value: new_lines.join('\n')
+	});
+
+	define_property(error, 'name', {
+		// 'Error' suffix is required for stack traces to be rendered properly
+		value: `${label}Error`
+	});
+
+	return /** @type {Error & { stack: string }} */ (error);
 }
 
 /**

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

@@ -214,12 +214,12 @@ export function effect_pending_outside_reaction() {
 }
 
 /**
- * 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
+ * Maximum update depth exceeded. This typically indicates that an effect reads and writes the same piece of state
  * @returns {never}
  */
 export function effect_update_depth_exceeded() {
 	if (DEV) {
-		const error = new Error(`effect_update_depth_exceeded\nMaximum 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\nhttps://svelte.dev/e/effect_update_depth_exceeded`);
+		const error = new Error(`effect_update_depth_exceeded\nMaximum update depth exceeded. This typically indicates that an effect reads and writes the same piece of state\nhttps://svelte.dev/e/effect_update_depth_exceeded`);
 
 		error.name = 'Svelte error';
 

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

@@ -46,9 +46,6 @@ export let current_batch = null;
  */
 export let batch_deriveds = null;
 
-/** @type {Effect[]} Stack of effects, dev only */
-export let dev_effect_stack = [];
-
 /** @type {Set<() => void>} */
 export let effect_pending_updates = new Set();
 
@@ -345,6 +342,28 @@ export class Batch {
 
 			while (queued_root_effects.length > 0) {
 				if (flush_count++ > 1000) {
+					if (DEV) {
+						var updates = new Map();
+
+						for (const source of this.#current.keys()) {
+							for (const [stack, update] of source.updated ?? []) {
+								var entry = updates.get(stack);
+
+								if (!entry) {
+									entry = { error: update.error, count: 0 };
+									updates.set(stack, entry);
+								}
+
+								entry.count += update.count;
+							}
+						}
+
+						for (const update of updates.values()) {
+							// eslint-disable-next-line no-console
+							console.error(update.error);
+						}
+					}
+
 					infinite_loop_guard();
 				}
 
@@ -356,9 +375,6 @@ export class Batch {
 			set_is_updating_effect(was_updating_effect);
 
 			last_scheduled_effect = null;
-			if (DEV) {
-				dev_effect_stack = [];
-			}
 		}
 	}
 
@@ -471,56 +487,25 @@ export function flushSync(fn) {
 			// we need to reset it here as well in case the first time there's 0 queued root effects
 			last_scheduled_effect = null;
 
-			if (DEV) {
-				dev_effect_stack = [];
-			}
-
 			return /** @type {T} */ (result);
 		}
 
 		batch.flush_effects();
 	}
 }
 
-function log_effect_stack() {
-	// eslint-disable-next-line no-console
-	console.error(
-		'Last ten effects were: ',
-		dev_effect_stack.slice(-10).map((d) => d.fn)
-	);
-	dev_effect_stack = [];
-}
-
 function infinite_loop_guard() {
 	try {
 		e.effect_update_depth_exceeded();
 	} catch (error) {
 		if (DEV) {
-			// stack is garbage, ignore. Instead add a console.error message.
-			define_property(error, 'stack', {
-				value: ''
-			});
-		}
-		// Try and handle the error so it can be caught at a boundary, that's
-		// if there's an effect available from when it was last scheduled
-		if (last_scheduled_effect !== null) {
-			if (DEV) {
-				try {
-					invoke_error_boundary(error, last_scheduled_effect);
-				} catch (e) {
-					// Only log the effect stack if the error is re-thrown
-					log_effect_stack();
-					throw e;
-				}
-			} else {
-				invoke_error_boundary(error, last_scheduled_effect);
-			}
-		} else {
-			if (DEV) {
-				log_effect_stack();
-			}
-			throw error;
+			// stack contains no useful information, replace it
+			define_property(error, 'stack', { value: '' });
 		}
+
+		// Best effort: invoke the boundary nearest the most recent
+		// effect and hope that it's relevant to the infinite loop
+		invoke_error_boundary(error, last_scheduled_effect);
 	}
 }
 

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

@@ -182,8 +182,22 @@ export function internal_set(source, value) {
 		const batch = Batch.ensure();
 		batch.capture(source, old_value);
 
-		if (DEV && tracing_mode_flag) {
-			source.updated = get_stack('UpdatedAt');
+		if (DEV) {
+			if (tracing_mode_flag || active_effect !== null) {
+				const error = get_stack('UpdatedAt');
+
+				if (error !== null) {
+					source.updated ??= new Map();
+					let entry = source.updated.get(error.stack);
+
+					if (!entry) {
+						entry = { error, count: 0 };
+						source.updated.set(error.stack, entry);
+					}
+
+					entry.count++;
+				}
+			}
 
 			if (active_effect !== null) {
 				source.set_during_effect = true;

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

@@ -29,8 +29,8 @@ export interface Value<V = unknown> extends Signal {
 	label?: string;
 	/** An error with a stack trace showing when the source was created */
 	created?: Error | null;
-	/** An error with a stack trace showing when the source was last updated */
-	updated?: Error | null;
+	/** An map of errors with stack traces showing when the source was updated, keyed by the stack trace */
+	updated?: Map<string, { error: Error; count: number }> | null;
 	/**
 	 * Whether or not the source was set while running an effect — if so, we need to
 	 * increment the write version so that it shows up as dirty when the effect re-runs

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

@@ -42,13 +42,7 @@ import {
 	set_dev_stack
 } from './context.js';
 import * as w from './warnings.js';
-import {
-	Batch,
-	batch_deriveds,
-	dev_effect_stack,
-	flushSync,
-	schedule_effect
-} from './reactivity/batch.js';
+import { Batch, batch_deriveds, flushSync, schedule_effect } from './reactivity/batch.js';
 import { handle_error } from './error-handling.js';
 import { UNINITIALIZED } from '../../constants.js';
 
@@ -491,10 +485,6 @@ export function update_effect(effect) {
 				}
 			}
 		}
-
-		if (DEV) {
-			dev_effect_stack.push(effect);
-		}
 	} finally {
 		is_updating_effect = was_updating_effect;
 		active_effect = previous_effect;