Consistently assign defaults when prop parsing fails

ReactCommon/react/debug/react_native_assert.h

@@ -12,6 +12,12 @@
 // test before moving on. When all issues have been found, maybe we can use
 // `UNDEBUG` flag to disable NDEBUG in debug builds on Android.
 
+// Asserting is appropriate for conditions that:
+//   1. May or may not be recoverable, and
+//   2. imply there is a bug in React Native when violated.
+// For recoverable conditions that can be violated by user mistake (e.g. JS
+// code passes an unexpected prop value), consider react_native_expect instead.
+
 #include "flags.h"
 
 #undef react_native_assert

ReactCommon/react/debug/react_native_expect.h

@@ -0,0 +1,41 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+// No header guards since it is legitimately possible to include this file more
+// than once with and without REACT_NATIVE_DEBUG.
+
+// react_native_expect is a non-fatal counterpart of react_native_assert.
+// In debug builds, when an expectation fails, we log and move on.
+// In release builds, react_native_expect is a noop.
+
+// react_native_expect is appropriate for recoverable conditions that can be
+// violated by user mistake (e.g. JS code passes an unexpected prop value).
+// To enforce invariants that are internal to React Native, consider
+// react_native_assert (or a stronger mechanism).
+// Calling react_native_expect does NOT, by itself, guarantee that the user
+// will see a helpful diagnostic (beyond a low level log). That concern is the
+// caller's responsibility.
+
+#include "flags.h"
+
+#undef react_native_expect
+
+#ifndef REACT_NATIVE_DEBUG
+
+#define react_native_expect(e) ((void)0)
+
+#else // REACT_NATIVE_DEBUG
+
+#include <glog/logging.h>
+#include <cassert>
+
+#define react_native_expect(cond)                           \
+  if (!(cond)) {                                            \
+    LOG(ERROR) << "react_native_expect failure: " << #cond; \
+  }
+
+#endif // REACT_NATIVE_DEBUG