In a Flux architecture, you have a store used to hold application state. You know that when state changes, it happens synchronously and unidirectionally, making the system as a whole more predictable and easy to reason about. However, there's still one more thing you can do to ensure that side-effects aren't introduced.
You're keeping all your application state in a store, which is great, but you can still break things by mutating data in other places. These mutations might seem innocent at first glance, but they're toxic to your architecture. For example, the callback function that handles a fetch() call might manipulate the data before passing it to the store. An event handler might generate some structure and pass it to the store. There are limitless possibilities.
The problem with performing these state transformations outside the store is that you don't necessarily know that they're happening. Think of mutating...
