isDispatching really needed?

[anler]

Hi! Just a comment/question about something I've seen in the dispatch's code in createStore.js (see the code below). I was wondering why is variable isDispatching needed, if there are two concurrent calls to dispatch the isDispatching check is pointless because is not inside a mutex:

But as far as I know, JS' runtime can only execute one thing/function at a time so two concurrent calls to dispatch will never happen and if that's the case I don't see the need for isDispatching right?

Can you tell me if I'm missing something? Thanks!

https://github.com/reactjs/redux/blob/master/src/createStore.js#L149

function dispatch(action) {
  // ...
  if (isDispatching) {
    throw new Error('Reducers may not dispatch actions.')
  }

  try {
    isDispatching = true
    currentState = currentReducer(currentState, action)
  } finally {
    isDispatching = false
  }
  // ...
}

[brigand]

It catches this case, dispatching from within a reducer.

var store = createStore((state={}, action) => {
  if (something) {
    store.dispatch({type: 'ANOTHER_ACTION'})
  }
  return state
})

[anler]

Oh I see, now is clear, is dispatching from within a reducer forbidden to avoid things such as loops (reducer -> dispatch -> reducer ->...) am I right?

Thanks!

[gaearon]

Yep.