feat: added reduce() operation

src/operations/index.js

@@ -14,6 +14,7 @@ const insert = require('./insert');
 const insertAll = require('./insertAll');
 const filter = require('./filter');
 const map = require('./map');
+const reduce = require('./reduce');
 
 module.exports = {
     update,
@@ -31,5 +32,6 @@ module.exports = {
     insert,
     insertAll,
     filter,
-    map
+    map,
+    reduce
 };

src/operations/reduce/index.js

@@ -0,0 +1,51 @@
+const { updateArray } = require('../common');
+
+module.exports = reduce;
+
+/**
+ * Reduces array-item selected by path using provided reducer
+ * @param {Object | any[]} object object to update
+ * @param {string | string[]} path path to be updated
+ * @param {function} reducer reducer function
+ * @returns {Object | any[]} updated object
+ * @description one of additional functions to work with array items
+ * @docs
+ * ```js
+ * import { reduce } from 'immutable-object-update';
+ *
+ * const state = {
+ *     a: {
+ *         b: [ 1, 2, 3, 4, 5 ]
+ *     }
+ * };
+ * const sum = (acc, item) => acc + item;
+ *
+ * const updated = reduce(state, [ 'a', 'b' ], sum);
+ *
+ * // or
+ *
+ * const updated = reduce(state, 'a.b', sum);
+ * ```
+ *
+ * As a result we will receive new object with structure below:
+ *
+ * ```js
+ * {
+ *     a: {
+ *         b: 15
+ *     }
+ * }
+ * ```
+ *
+ * > Please note: first element of array will be always used as initial value
+ */
+function reduce(object, path, reducer) {
+    return updateArray(
+        object,
+        path,
+        array => array.reduce(
+            reducer,
+            ...(array.length ? [] : [ undefined ])
+        )
+    );
+}