fix: Throw if key is empty in set(...) (#919)

* fix: Throw if key is empty in `set(...)`

* docs: Add docs for empty key

* Add harness test for empty key

* Also throw in mocks and web

Author: mrousavy

example/__tests__/MMKV.harness.ts

@@ -825,9 +825,10 @@ describe('MMKV Error Handling & Edge Cases', () => {
 
   describe('Error Scenarios', () => {
     it('should handle invalid key operations gracefully', () => {
-      // Empty string key should work
-      storage.set('', 'empty-key-value');
-      expect(storage.getString('')).toStrictEqual('empty-key-value');
+      // Empty string key should throw
+      expect(() => {
+        storage.set('', 'empty-key-value');
+      }).toThrow()
     });
 
     it('should handle concurrent operations', () => {

packages/react-native-mmkv/cpp/HybridMMKV.cpp

@@ -71,6 +71,10 @@ template <class... Ts>
 overloaded(Ts...) -> overloaded<Ts...>;
 
 void HybridMMKV::set(const std::string& key, const std::variant<std::string, double, bool, std::shared_ptr<ArrayBuffer>>& value) {
+  if (key.empty()) [[unlikely]] {
+    throw std::runtime_error("Cannot set a value for an empty key!");
+  }
+
   // Pattern-match each potential value in std::variant
   std::visit(overloaded{[&](const std::string& string) {
                           // string

packages/react-native-mmkv/src/createMMKV/createMMKV.web.ts

@@ -91,6 +91,7 @@ export function createMMKV(
     },
     remove: (key) => storage().removeItem(prefixedKey(key)) ?? false,
     set: (key, value) => {
+      if (key === '') throw new Error('Cannot set a value for an empty key!')
       storage().setItem(prefixedKey(key), value.toString())
     },
     getString: (key) => storage().getItem(prefixedKey(key)) ?? undefined,

packages/react-native-mmkv/src/createMMKV/createMockMMKV.ts

@@ -30,6 +30,7 @@ export function createMockMMKV(): MMKV {
       return deleted
     },
     set: (key, value) => {
+      if (key === '') throw new Error('Cannot set a value for an empty key!')
       storage.set(key, value)
       notifyListeners(key)
     },

packages/react-native-mmkv/src/specs/MMKV.nitro.ts

@@ -6,9 +6,10 @@ export interface Listener {
 
 export interface MMKV extends HybridObject<{ ios: 'c++'; android: 'c++' }> {
   /**
-   * Set a value for the given `key`.
+   * Set a {@linkcode value} for the given {@linkcode key}.
    *
-   * @throws an Error if the value cannot be set.
+   * @throws an Error if the {@linkcode key} is empty.
+   * @throws an Error if the {@linkcode value} cannot be set.
    */
   set(key: string, value: boolean | string | number | ArrayBuffer): void
   /**