feat(ch05): add example for no side effects

src/ch-05_pure-functions/5.1_pure-functions.js

@@ -1 +1,8 @@
 export const double = x => x * 2;
+
+export const addToCart = (cart, product, quantity) => {
+    return {
+        ...cart,
+        products: cart.products.concat([{product, quantity}])
+    }
+};

src/ch-05_pure-functions/5.1_pure-functions.test.js

@@ -1,5 +1,5 @@
 import {expect} from "chai";
-import {double} from "./5.1_pure-functions";
+import {addToCart, double} from "./5.1_pure-functions";
 
 describe("Pure Functions", () => {
     it("should always return the same output for the same input", () => {
@@ -8,4 +8,13 @@ describe("Pure Functions", () => {
         expect(double(3)).to.equal(6);
         expect(double(3)).to.equal(6);
     });
+
+    it("should have no side effects", () => {
+        const originalCart = {
+            products: []
+        };
+        const newCart = addToCart(originalCart, {name: "product A"}, 3);
+
+        expect(originalCart).to.be.not.deep.equal(newCart);
+    })
 });