fix: snapshot array length before optimized filter callbacks

__tests__/array-methods-filter-length.js

@@ -0,0 +1,32 @@
+import {Immer, enableArrayMethods} from "../src/immer"
+import {clearPlugin, PluginArrayMethods} from "../src/internal"
+
+for (const enabled of [false, true]) {
+	for (const autoFreeze of [false, true]) {
+		describe(`filter length: plugin=${enabled}, autoFreeze=${autoFreeze}`, () => {
+			beforeEach(() => {
+				if (enabled) enableArrayMethods()
+				else clearPlugin(PluginArrayMethods)
+			})
+			afterEach(() => clearPlugin(PluginArrayMethods))
+
+			test("does not visit items appended by the predicate", () => {
+				const {produce} = new Immer({autoFreeze})
+				const base = [1, 2]
+				const visited = []
+				const result = produce(base, draft => {
+					draft.push(3)
+					const filtered = draft.filter((value, index) => {
+						visited.push(index)
+						if (index === 0) draft.push(4)
+						return true
+					})
+					expect(filtered).toEqual([1, 2, 3])
+				})
+				expect(visited).toEqual([0, 1, 2])
+				expect(result).toEqual([1, 2, 3, 4])
+				expect(base).toEqual([1, 2])
+			})
+		})
+	}
+}

src/plugins/arrayMethods.ts

@@ -451,7 +451,8 @@ export function enableArrayMethods() {
 			const result: any[] = []
 
 			// First pass: call predicate on base values to determine which items pass
-			for (let i = 0; i < source.length; i++) {
+			const length = source.length
+			for (let i = 0; i < length; i++) {
 				if (predicate.call(thisArg, source[i], i, source)) {
 					// Only create draft for items that passed the predicate
 					result.push(state.draft_[i])