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.