feat(ch10): compose functions with a reducerRight

src/ch-10_abstraction-and-composition/10.1_abstraction-and-composition.test.js

@@ -1,5 +1,5 @@
 import {expect} from 'chai';
-import {add, addN, filterRed, map, mapRedu} from "./10.1_abstraction_and_composition";
+import {add, addN, compose, filterRed, map, mapRedu} from "./10.1_abstraction_and_composition";
 
 describe("Abstraction and Composition", () => {
     it("should fix a parameter in a function", () => {
@@ -47,4 +47,14 @@ describe("Abstraction and Composition", () => {
         expect(filterRed(gt5, terms)).to.deep.equal([6, 10]);
     });
 
+    it("should compose functions using a right to left reducer", () => {
+        const add1 = x => x + 1;
+        const add5 = x => x + 5;
+        const multiplyWith3 = x => x * 3;
+
+        const composedFn = compose(add1, add5, multiplyWith3);
+
+        expect(composedFn(5)).to.equal(21);
+    });
+
 });

src/ch-10_abstraction-and-composition/10.1_abstraction_and_composition.js

@@ -11,3 +11,5 @@ export const mapRedu = (fn, arr) => arr.reduce((acc, item) => {
 export const filterRed = (fn, arr) => arr.reduce((acc, item) => {
     return fn(item) ? acc.concat([item]) : acc;
 }, []);
+
+export const compose = (...fns) => x => fns.reduceRight((y, f) => f(y), x);