Avoid Side Effects (part 2)


const addItemToCart = (cart, item) => {
  cart.push({ item, date: Date.now() });


const addItemToCart = (cart, item) => {
  return [...cart, { item, date: Date.now() }];

In JavaScript, primitives are passed by value and objects/arrays are passed by reference. In the case of objects and arrays, if your function makes a change in a shopping cart array, for example, by adding an item to purchase, then any other function that uses that cart array will be affected by this addition. That may be great, however it can be bad too. Let’s imagine a bad situation:

The user clicks the “Purchase”, button which calls a purchase function that spawns a network request and sends the cart array to the server. Because of a bad network connection, the purchase function has to keep retrying the request. Now, what if in the meantime the user accidentally clicks “Add to Cart” button on an item they don’t actually want before the network request begins? If that happens and the network request begins, then that purchase function will send the accidentally added item because it has a reference to a shopping cart array that the addItemToCart function modified by adding an unwanted item.

A great solution would be for the addItemToCart to always clone the cart, edit it, and return the clone. This ensures that no other functions that are holding onto a reference of the shopping cart will be affected by any changes.

Two caveats to mention to this approach:

  1. There might be cases where you actually want to modify the input object, but when you adopt this programming practice you will find that those cases are pretty rare. Most things can be refactored to have no side effects!

  2. Cloning big objects can be very expensive in terms of performance. Luckily, this isn’t a big issue in practice because there are great libraries that allow this kind of programming approach to be fast and not as memory intensive as it would be for you to manually clone objects and arrays.