read old values in teardown

Author: Rich-Harris

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

@@ -159,6 +159,8 @@ export function set(source, value) {
 	return internal_set(source, value);
 }
 
+export const old_values = new Map();
+
 /**
  * @template V
  * @param {Source<V>} source
@@ -167,6 +169,8 @@ export function set(source, value) {
  */
 export function internal_set(source, value) {
 	if (!source.equals(value)) {
+		old_values.set(source, source.v);
+
 		var old_value = source.v;
 		source.v = value;
 		source.wv = increment_write_version();

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

@@ -25,7 +25,7 @@ import {
 	BOUNDARY_EFFECT
 } from './constants.js';
 import { flush_tasks } from './dom/task.js';
-import { internal_set } from './reactivity/sources.js';
+import { internal_set, old_values } from './reactivity/sources.js';
 import { destroy_derived_effects, update_derived } from './reactivity/deriveds.js';
 import * as e from './errors.js';
 import { FILENAME } from '../../constants.js';
@@ -673,6 +673,8 @@ function flush_queued_root_effects() {
 		if (DEV) {
 			dev_effect_stack = [];
 		}
+
+		old_values.clear();
 	}
 }
 
@@ -923,6 +925,10 @@ export function get(signal) {
 		}
 	}
 
+	if (is_destroying_effect && old_values.has(signal)) {
+		return old_values.get(signal);
+	}
+
 	return signal.v;
 }