Immutable error

`cart` is an Array. the ES6 spread operator can be used : https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Spread_operator

Author: Gregoirevda

README.md

@@ -602,11 +602,7 @@ const addItemToCart = (cart, item) => {
 **Good:**
 ```javascript
 const addItemToCart = (cart, item) => {
-  const c = Object.assign({}, cart);
-
-  c.push({ item, date: Date.now() });
-
-  return c;
+  return [...cart, { item, date : Date.now() }];
 };
 ```