@@ -514,7 +514,7 @@ function createTempFile(name) {
 ```
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-### Avoid Side Effects
+### Avoid Side Effects (part 1)
 A function produces a side effect if it does anything other than take a value in
 and return another value or values. A side effect could be writing to a file,
 modifying some global variable, or accidentally wiring all your money to a
@@ -559,6 +559,38 @@ console.log(newName); // ['Ryan', 'McDermott'];
 ```
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+### Avoid Side Effects (part 2)
+Side effects could also occur from inside a function. In JavaScript, primitives are
+passed by value and objects are passed by reference. In the later case, we should be
+careful not to change any of these argument's properties.
+
+A possible solution would be to always clone the variable, edit it and return the
+clone. There would be cases where you actually want to modify the input object
+and this should not be taken as a silver bullet. Furthermore, cloning big objects can
+be very expensive in terms of performance.
+
+**Bad:**
+```javascript
+const addItemToCart = (cart, item) => {
+cart.push({ item, date: Date.now() });
+
+return cart;
+};
+```
+
+**Good:**
+```javascript
+const addItemToCart = (cart, item) => {
+const c = Object.assign({}, cart);
+
+c.push({ item, date: Date.now() });
+
+return c;
+};
+```
+
+**[⬆ back to top](#table-of-contents)**
+
 ### Don't write to global functions
 Polluting globals is a bad practice in JavaScript because you could clash with another
 library and the user of your API would be none-the-wiser until they get an