Solving TypeError: Cannot assign to read only property 'quantity' of object in Redux Toolkit

Learn how to handle the `TypeError` when modifying objects in Redux Toolkit by creating new instances instead of mutating existing ones.
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Understanding TypeError in Redux Toolkit

When you're working with Redux Toolkit in a React application, you may come across an error that states: TypeError: Cannot assign to read only property 'quantity' of object '-<Object>'. This error often arises when you attempt to directly modify an object that is being managed by your Redux state, which is immutable under the principles of Redux. But don't worry, in this guide, we’ll explain not only why this error occurs but also how to resolve it effectively.

The Problem: Why the Error Occurs

In Redux, particularly with libraries like Redux Toolkit, you are encouraged to treat your state's objects as immutable. This means rather than updating the properties of existing objects (which is what’s causing the TypeError), you should make a new copy of the object and update it accordingly. The error suggests that you are trying to mutate an object that is intended to remain unchanged, thus leading to potential bugs and unintended behavior in your application.

Scenario Explanation

In this case, the error was triggered while trying to update the quantity property of a meal object stored within an array called orderedList. Here’s a snippet from the problematic code:

[[See Video to Reveal this Text or Code Snippet]]

The Solution: Pushing New Objects to the Array

Here’s the Updated Code Approach

Replace the faulty code inside your condition (where mealIsInList is checked) with the following lines:

[[See Video to Reveal this Text or Code Snippet]]

Now, instead of trying to change an existing object, you are effectively adding a new instance to your orderedList where you have control over the properties.

Conclusion

The key takeaway here is that in Redux (and modern state management in general), you should avoid mutating existing state directly. Create new instances instead, and keep your state predictable and bug-free. This practice aligns with the principles of functional programming, enhancing the maintainability of your code.

By addressing the TypeError in this way, you'll ensure your application runs smoothly without unexpected errors arising from object mutations.

Thank you for joining us in this exploration of Redux Toolkit! Happy coding!