Add merge reducer option

README.md

@@ -78,6 +78,7 @@ An interface defining the configuration attributes to bootstrap `localStorageSyn
 * `restoreDates` \(*boolean? = true*): Restore serialized date objects. If you work directly with ISO date strings, set this option to `false`.
 * `syncCondition` (optional) `(state) => boolean`: When set, sync to storage medium will only occur when this function returns a true boolean. Example: `(state) => state.config.syncToStorage` will check the state tree under config.syncToStorage and if true, it will sync to the storage. If undefined or false it will not sync to storage. Often useful for "remember me" options in login.
 * `checkStorageAvailability` \(*boolean? = false*): Specify if the storage availability checking is expected, i.e. for server side rendering / Universal.
+* `mergeReducer` (optional) `(state: any, rehydratedState: any, action: any) => any`: Defines the reducer to use to merge the rehydrated state from storage with the state from the ngrx store. If unspecified, defaults to performing a full deepmerge on an `INIT_ACTION` or an `UPDATE_ACTION`.
 
 Usage: `localStorageSync({keys: ['todos', 'visibilityFilter'], storageKeySerializer: (key) => 'cool_' + key, ... })`. In this example `Storage` will use keys `cool_todos` and `cool_visibilityFilter` keys to store `todos` and `visibilityFilter` slices of state). The key itself is used by default - `(key) => key`.
 

spec/index_spec.ts

@@ -4,6 +4,7 @@ import { syncStateUpdate, rehydrateApplicationState, dateReviver, localStorageSy
 import *  as CryptoJS from 'crypto-js';
 import 'localstorage-polyfill';
 const INIT_ACTION = '@ngrx/store/init';
+import * as deepmerge from 'deepmerge';
 
 // Very simple classes to test serialization options.  They cover string, number, date, and nested classes
 // The top level class has static functions to help test reviver, replacer, serialize and deserialize
@@ -486,4 +487,45 @@ describe('ngrxLocalStorage', () => {
           feature2: { slice21: true, slice22: [1, 2], slice23: {} },
         });
     });
+
+    it('should enable a complex merge of rehydrated storage and state', () => {
+        const initialState = {
+            app: { app1: false, app2: [], app3: {} },
+            feature1: { slice11: false, slice12: [], slice13: {} },
+            feature2: { slice21: false, slice22: [], slice23: {} },
+          };
+
+          // A legit case where state is saved in chunks rather than as a single object
+          localStorage.setItem('feature1', JSON.stringify({ slice11: true, slice12: [1, 2] }));
+          localStorage.setItem('feature2', JSON.stringify({ slice21: true, slice22: [1, 2] }));
+
+          // Set up reducers
+          const reducer = (state = initialState, action) => state;
+          const mergeReducer = (state, rehydratedState, action) => {
+            // Perform a merge where we only want a single property from feature1
+            // but a deepmerge with feature2
+
+            return {                
+                ...state,
+                feature1: {
+                    slice11: rehydratedState.feature1.slice11
+                },
+                feature2: deepmerge(state.feature2, rehydratedState.feature2)
+            }
+          }
+          const metaReducer = localStorageSync({keys: [
+            {'feature1': ['slice11', 'slice12']},
+            {'feature2': ['slice21', 'slice22']},
+          ], rehydrate: true, mergeReducer});
+
+          const action = {type: INIT_ACTION};
+
+          // Resultant state should merge the rehydrated partial state and our initial state
+          const finalState = metaReducer(reducer)(initialState, action);
+          expect(finalState).toEqual({
+            app: { app1: false, app2: [], app3: {} },
+            feature1: { slice11: true },
+            feature2: { slice21: true, slice22: [1, 2], slice23: {} },
+          });
+    });    
 });

src/index.ts

@@ -219,6 +219,21 @@ export const syncStateUpdate = (
   });
 };
 
+// Default merge strategy is a full deep merge.
+export const defaultMergeReducer = (state: any, rehydratedState: any, action: any) => { 
+
+  if ((action.type === INIT_ACTION || action.type === UPDATE_ACTION) && rehydratedState) {
+    const overwriteMerge = (destinationArray, sourceArray, options) => sourceArray;
+    const options: deepmerge.Options = {
+      arrayMerge: overwriteMerge
+    };
+
+    state = deepmerge(state, rehydratedState, options);
+  }
+
+  return state;
+};
+
 export const localStorageSync = (config: LocalStorageConfig) => (
   reducer: any
 ) => {
@@ -236,6 +251,13 @@ export const localStorageSync = (config: LocalStorageConfig) => (
     config.restoreDates = true;
   }
 
+  // Use default merge reducer.
+  let mergeReducer = config.mergeReducer;
+
+  if (mergeReducer === undefined || typeof(mergeReducer) !== 'function') {
+    mergeReducer = defaultMergeReducer;
+  }
+
   const stateKeys = validateStateKeys(config.keys);
   const rehydratedState = config.rehydrate
     ? rehydrateApplicationState(
@@ -257,14 +279,10 @@ export const localStorageSync = (config: LocalStorageConfig) => (
       nextState = { ...state };
     }
 
-    if ((action.type === INIT_ACTION || action.type === UPDATE_ACTION) && rehydratedState) {
-      const overwriteMerge = (destinationArray, sourceArray, options) => sourceArray;
-      const options: deepmerge.Options = {
-        arrayMerge: overwriteMerge
-      };
-      nextState = deepmerge(nextState, rehydratedState, options);
-    }
-
+    // Merge the store state with the rehydrated state using
+    // either a user-defined reducer or the default.
+    nextState = mergeReducer(nextState, rehydratedState, action);
+
     nextState = reducer(nextState, action);
 
     if (action.type !== INIT_ACTION) {
@@ -313,4 +331,5 @@ export interface LocalStorageConfig {
   storageKeySerializer?: (key: string) => string;
   syncCondition?: (state: any) => any;
   checkStorageAvailability?: boolean;
+  mergeReducer?: (state: any, rehydratedState: any, action: any) => any;
 }