feat(ch09): add function composition example

to compose two functions only

src/ch-09_curry-and-fn-composition/9.1_curry-and-fn-composition.test.js

@@ -2,13 +2,24 @@ import {expect} from 'chai';
 import {sum} from "./9.1_curry-and-fn-composition";
 
 describe("Curry and Function Composition", () => {
-    it("should apply two functions in a curried form", ()=> {
+    it("should apply two functions in a curried form", () => {
         expect(sum(2)(4)).to.equal(6);
     });
 
     it("should allow to use point-free style", () => {
         // inc10 is a specialized version of sum with param 'a' fixed to 1
         const inc10 = sum(10);
         expect(inc10(5)).to.equal(15);
-    })
+    });
+
+    it("should allow function composition of two functions", () => {
+        const g = a => a + 1;
+        const f = b => b * 2;
+        const h = x => f(g(x));
+
+        const composeFn = (f, g) => x => f(g(x));
+
+        expect(h(30)).to.equal(62);
+        expect(composeFn(f,g)(30)).to.equal(62);
+    });
 });

src/ch-09_curry-and-fn-composition/README.md

@@ -29,3 +29,6 @@ All curried fns return partial apps. However, not all partial apps result from c
 - Define functions without declaring formal parameters
     - function and arrow functions (=>) cannot be used
     - instead call a function that returns a function with its own closure and fixed variable
+
+## Why to curry
+- For function composition