Allow MERGE operations into empty array values

Author: chuckdries

lib/OnyxUtils.ts

@@ -1060,7 +1060,7 @@ function mergeInternal<TValue extends OnyxInput<OnyxKey> | undefined, TChange ex
                 return modifiedData;
             },
             {
-                result: (existingValue ?? {}) as TChange,
+                result: ((Array.isArray(existingValue) && existingValue.length === 0 ? {} : existingValue) ?? {}) as TChange,
                 replaceNullPatches: [],
             },
         );

lib/utils.ts

@@ -213,6 +213,12 @@ function checkCompatibilityWithExistingValue(value: unknown, existingValue: unkn
             isCompatible: true,
         };
     }
+    // Empty arrays are compatible with objects — PHP's json_encode produces []
+    // for empty associative arrays that should be {}.
+    if (Array.isArray(existingValue) && existingValue.length === 0 && !Array.isArray(value)) {
+        return {isCompatible: true};
+    }
+
     const existingValueType = Array.isArray(existingValue) ? 'array' : 'non-array';
     const newValueType = Array.isArray(value) ? 'array' : 'non-array';
     if (existingValueType !== newValueType) {

tests/unit/onyxTest.ts

@@ -182,6 +182,48 @@ describe('Onyx', () => {
             });
     });
 
+    it('should merge an object into an empty array (treating [] as {})', () => {
+        let testKeyValue: unknown;
+
+        connection = Onyx.connect({
+            key: ONYX_KEYS.TEST_KEY,
+            initWithStoredValues: false,
+            callback: (value) => {
+                testKeyValue = value;
+            },
+        });
+
+        return Onyx.set(ONYX_KEYS.TEST_KEY, [])
+            .then(() => {
+                expect(testKeyValue).toStrictEqual([]);
+                return Onyx.merge(ONYX_KEYS.TEST_KEY, {test: 'value'});
+            })
+            .then(() => {
+                expect(testKeyValue).toStrictEqual({test: 'value'});
+            });
+    });
+
+    it('should still reject merging an object into a non-empty array', () => {
+        let testKeyValue: unknown;
+
+        connection = Onyx.connect({
+            key: ONYX_KEYS.TEST_KEY,
+            initWithStoredValues: false,
+            callback: (value) => {
+                testKeyValue = value;
+            },
+        });
+
+        return Onyx.merge(ONYX_KEYS.TEST_KEY, ['existing'])
+            .then(() => {
+                expect(testKeyValue).toStrictEqual(['existing']);
+                return Onyx.merge(ONYX_KEYS.TEST_KEY, {test: 'value'});
+            })
+            .then(() => {
+                expect(testKeyValue).toStrictEqual(['existing']);
+            });
+    });
+
     it('should notify subscribers when data has been cleared', () => {
         let testKeyValue: unknown;
         connection = Onyx.connect({