fix: skip sparse array holes in optimized filter

__tests__/array-methods-filter.js

@@ -0,0 +1,45 @@
+import {Immer, enableArrayMethods} from "../src/immer"
+import {clearPlugin, PluginArrayMethods} from "../src/internal"
+
+for (const useArrayMethods of [false, true]) {
+	for (const autoFreeze of [false, true]) {
+		describe(`array methods: plugin=${useArrayMethods}, autoFreeze=${autoFreeze}`, () => {
+			const {produce} = new Immer({autoFreeze})
+			beforeEach(() => {
+				if (useArrayMethods) enableArrayMethods()
+				else clearPlugin(PluginArrayMethods)
+			})
+			afterEach(() => clearPlugin(PluginArrayMethods))
+			test("skips holes but visits explicit undefined values", () => {
+				const items = new Array(5)
+				items[1] = {value: 10}
+				items[3] = undefined
+				const visited = []
+				const base = {items}
+				const result = produce(base, draft => {
+					const filtered = draft.items.filter((item, index) => {
+						visited.push(index)
+						return true
+					})
+					expect(visited).toEqual([1, 3])
+					expect(filtered).toHaveLength(2)
+					expect(filtered[1]).toBeUndefined()
+					filtered[0].value = 20
+				})
+				expect(result.items[1].value).toBe(20)
+				expect(base.items[1].value).toBe(10)
+				expect(Object.keys(result.items)).toEqual(["1", "3"])
+			})
+
+			test("does not call the filter predicate for an entirely sparse array", () => {
+				const base = new Array(3)
+				const predicate = vi.fn(() => true)
+				const result = produce(base, draft => {
+					expect(draft.filter(predicate)).toEqual([])
+				})
+				expect(predicate).not.toHaveBeenCalled()
+				expect(result).toBe(base)
+			})
+		})
+	}
+}

src/plugins/arrayMethods.ts

@@ -452,7 +452,7 @@ export function enableArrayMethods() {
 
 			// First pass: call predicate on base values to determine which items pass
 			for (let i = 0; i < source.length; i++) {
-				if (predicate.call(thisArg, source[i], i, source)) {
+				if (i in source && predicate.call(thisArg, source[i], i, source)) {
 					// Only create draft for items that passed the predicate
 					result.push(state.draft_[i])
 				}