feat(ch06): add functors example

src/ch-06_what-is-fp/6.1_what-is-fp.js

@@ -1,7 +1,9 @@
 const double = x => x * 2;
+const doublePoints = x => x.points * 2;
 
 export const incItem = x => ({...x, val: x.val + 1});
 
 export const doubleItem = x => ({...x, val: x.val * 2});
 
 export const doubleMap = numbers => numbers.map(double);
+export const doubleMapRecords = records => records.map(doublePoints);

src/ch-06_what-is-fp/6.1_what-is-fp.test.js

@@ -1,5 +1,5 @@
 import {assert, expect} from "chai";
-import {doubleItem, doubleMap, incItem} from "./6.1_what-is-fp";
+import {doubleItem, doubleMap, doubleMapRecords, incItem} from "./6.1_what-is-fp";
 
 describe("What is Functional Programming", () => {
     it("should avoid mutating shared state", () => {
@@ -34,4 +34,14 @@ describe("What is Functional Programming", () => {
     it("should encourage the usage of mappers", () => {
         expect(doubleMap([2, 3, 5])).to.deep.equal([4, 6, 10]);
     });
+
+    it("should encourage the usage of functors", () => {
+        const records = [
+            {name: 'foo', points: 2},
+            {name: 'bar', points: 3},
+            {name: 'baz', points: 5},
+        ];
+
+        expect(doubleMapRecords(records)).to.deep.equal([4, 6, 10]);
+    })
 });