There are certain rules while using Redux Reducers which we need to follow to avoid certain bugs or surprises.

- Reducers should use only the `state` and `action` arguments to calculate the new state. They shouldn't access any external values;
- Reducers must not perform asynchronous logic or side effects such as making API calls or running timers etc.;
- The state should not be directly modified inside the reducer functions, instead, the reducer function should return a duplicate state object with updated values. This rule only applies when creating reducers manually. If we use `createSlice` to create a slice, we can modify the `state` directly.

It is important to note that in the case of `createSlice` reducers, we directly modify the state object, however, in the backend, Redux creates a new duplicate object and assigns it the new changes, so the rule is being followed, but in the backend.


In which case can we directly modify the `state`?

In this section we will learn what Redux is, why it is useful, and most importantly when it is useful. We will also learn how to set up a basic react-redux application.

In this section we will go through the fundamental concepts of Redux that are necessary to understand the workings of a Redux application. We will use code examples to understand the concepts and in the following section we will use them practically.

In the previous section we looked at the basic concepts. In this section we use those basic concepts to understand some more complicated concepts and the structure of a Redux application. In the end we will also look at the Counter application to put together all the pieces of our knowledge together.

This chapter is focused on practice hence we will utilize the knowledge we acquired in previous sections to create a practical application using Redux along with React.