feat(ch14): add object Aggregation

src/ch-14_object_composition/14.1_object-composition.test.js

@@ -1,6 +1,11 @@
-import {assert, expect} from 'chai';
+import {expect} from 'chai';
 
 describe("Object Composition", () => {
+    const objs = [
+        {a: "a", b: "ab"},
+        {b: "b"},
+        {c: "c", b: "cb"},
+    ];
 
     it("should implement basic object composition using a reducer", () => {
         const assing = (acum, current) => ({...acum, ...current});
@@ -11,4 +16,28 @@ describe("Object Composition", () => {
 
         expect(orig[0]).to.deep.equal(composed);
     });
+
+    it("should describe Aggregation using a reducer", () => {
+        const collection = (array, e) => array.concat([e]);
+
+        const a = objs.reduce(collection, []);
+
+        expect(objs).to.deep.equal(a);
+    });
+
+    it("should describe Aggregation using a reducer, linked list example", () => {
+        const tuple = (a, b) => [b, a];
+
+        const linkedList = objs.reduceRight(tuple, []);
+
+        expect(linkedList)
+            .to.deep.equal([
+                {a: "a", b: "ab"}, [
+                    {b: "b"}, [
+                        {c: "c", b: "cb"}, []
+                    ]
+                ]
+            ]
+        );
+    })
 });

src/ch-14_object_composition/README.md

@@ -10,7 +10,7 @@ Class inheritance is the strongest kind of coupling in OO Design.
 - Duplication by need
 
 ## Main forms of Object Composition
-- **Aggregation**. 
+- **Aggregation**. An Object that contains other objects. Each subobject can be destructured from the parent. 
 - **Concatenation**.
 - **Delegation**.