fix: polyfill the builtins missing from Lynx's JS engine

.changeset/lynx-builtin-polyfills.md

@@ -0,0 +1,5 @@
+---
+'@react-navigation/lynx': patch
+---
+
+Polyfill the builtins Lynx's JS engine doesn't implement. `@react-navigation/core`, `@react-navigation/routers` and `query-string` call `Array.prototype.findLast`, `findLastIndex` and `at`, `Object.hasOwn`, `String.prototype.replaceAll` and `queueMicrotask`, so navigating a stack crashed with `findLast is not a function`.

packages/lynx/README.md

@@ -23,6 +23,11 @@ host app as well.
 `@lynx-js/react` has to be 0.126 or later, where `ref` reaches components as a
 prop the way it does in React 19; `NavigationContainer` depends on that.
 
+Lynx's JS engine is missing builtins that `@react-navigation/core`,
+`@react-navigation/routers` and `query-string` call, so importing this package
+installs conditional polyfills for `Array.prototype.findLast`, `findLastIndex`
+and `at`, `Object.hasOwn`, `String.prototype.replaceAll` and `queueMicrotask`.
+
 `@react-navigation/core` is written against React, so `react` has to resolve to
 ReactLynx's compat entry, which adds `use`, `useInsertionEffect` and
 `startTransition`:

packages/lynx/src/index.tsx

@@ -5,6 +5,8 @@
 // The platform layer, mirroring what `@react-navigation/native` does for
 // React Native: it owns everything Lynx-specific that every navigator needs,
 // and re-exports core so apps have a single import surface.
+import './polyfills';
+
 export { createStaticNavigation } from './createStaticNavigation';
 export { LinkingContext } from './LinkingContext';
 export { LocaleDirContext } from './LocaleDirContext';

packages/lynx/src/polyfills.ts

@@ -0,0 +1,91 @@
+// Copyright 2026 The Lynx Authors. All rights reserved.
+// Licensed under the Apache License Version 2.0 that can be found in the
+// LICENSE file in the root directory of this source tree.
+
+type Predicate<T> = (value: T, index: number, array: T[]) => boolean;
+
+function define(target: object, name: string, value: unknown) {
+  Object.defineProperty(target, name, {
+    value,
+    configurable: true,
+    writable: true,
+  });
+}
+
+if (typeof Array.prototype.findLast !== 'function') {
+  define(Array.prototype, 'findLast', function<T>(
+    this: T[],
+    predicate: Predicate<T>,
+    thisArg?: unknown
+  ) {
+    for (let i = this.length - 1; i >= 0; i--) {
+      if (predicate.call(thisArg, this[i] as T, i, this)) {
+        return this[i];
+      }
+    }
+
+    return undefined;
+  });
+}
+
+if (typeof Array.prototype.findLastIndex !== 'function') {
+  define(Array.prototype, 'findLastIndex', function<T>(
+    this: T[],
+    predicate: Predicate<T>,
+    thisArg?: unknown
+  ) {
+    for (let i = this.length - 1; i >= 0; i--) {
+      if (predicate.call(thisArg, this[i] as T, i, this)) {
+        return i;
+      }
+    }
+
+    return -1;
+  });
+}
+
+if (typeof Array.prototype.at !== 'function') {
+  define(Array.prototype, 'at', function<T>(this: T[], index: number) {
+    const i = Math.trunc(index) || 0;
+
+    return this[i < 0 ? this.length + i : i];
+  });
+}
+
+if (typeof Object.hasOwn !== 'function') {
+  define(Object, 'hasOwn', function(target: object, key: PropertyKey) {
+    return Object.prototype.hasOwnProperty.call(target, key);
+  });
+}
+
+if (typeof String.prototype.replaceAll !== 'function') {
+  define(String.prototype, 'replaceAll', function(
+    this: string,
+    pattern: string | RegExp,
+    replacement: string | ((...args: string[]) => string)
+  ) {
+    if (pattern instanceof RegExp) {
+      if (!pattern.global) {
+        throw new TypeError(
+          'replaceAll must be called with a global RegExp'
+        );
+      }
+
+      return this.replace(pattern, replacement as string);
+    }
+
+    const escaped = String(pattern).replace(/[.*+?^${}()|[\]\\]/g, '\\$&');
+
+    return this.replace(new RegExp(escaped, 'g'), replacement as string);
+  });
+}
+
+const globals = globalThis as {
+  queueMicrotask?: (callback: () => void) => void;
+};
+
+if (typeof globals.queueMicrotask !== 'function') {
+  globals.queueMicrotask = (callback: () => void) => {
+    Promise.resolve().then(callback);
+  };
+}

packages/lynx/src/stack/index.tsx

@@ -2,6 +2,8 @@
 // Licensed under the Apache License Version 2.0 that can be found in the
 // LICENSE file in the root directory of this source tree.
 
+import '../polyfills';
+
 /**
  * Navigators
  */