Why does a Redux reducer have to be side-effect free?

Question

I have been using Redux in my React app and something has been bothering me. The documentation for Redux makes it very clear that the reducer should be state free. You often see examples like this:

function reducer(state = { exampleState: true }, action) {
  switch(action.type) {
  case "ACTION_EXAMPLE":
    return Object.assign({}, state, { exampleState: false });
  default:
    return state;
  }
}

My question is why is this required at all? JavaScript is single threaded. There is no chance of a race condition inside the reducer. As far as I can tell, a Redux store is only capable of returning the current state of the store, so it seems weird that there is so much focus on pure functions.

Answer 1

The case for pure functions is made in the author's documentation. Sure, you can write reducers with impure functions, but:

Development features like time travel, record/replay, or hot reloading will break.

If none of those features provide a benefit or are of interest you, then, by all means, write impure functions. However, the question then would become, why use Redux?