A Note for Flux Users
If you're coming from Flux, there is a single important difference you need to understand. Redux doesn't have a Dispatcher or support many stores. Instead, there is just a single store with a single root reducing function. As your app grows, instead of adding stores, you split the root reducer into smaller reducers independently operating on the different parts of the state tree. You can use a helper like
combineReducersto combine them. This is similar to how there is just one root component in a React app, but it is composed out of many small components.
Returns the current state tree of your application. It is equal to the last value returned by the store's reducer.
(any): The current state tree of your application.
Dispatches an action. This is the only way to trigger a state change.
The store's reducing function will be called with the current
getState() result and the given
action synchronously. Its return value will be considered the next state. It will be returned from
getState() from now on, and the change listeners will immediately be notified.
A Note for Flux Users
If you attempt to call
dispatchfrom inside the reducer, it will throw with an error saying “Reducers may not dispatch actions.” This is similar to “Cannot dispatch in a middle of dispatch” error in Flux, but doesn't cause the problems associated with it. In Flux, a dispatch is forbidden while Stores are handling the action and emitting updates. This is unfortunate because it makes it impossible to dispatch actions from component lifecycle hooks or other benign places.
In Redux, subscriptions are called after the root reducer has returned the new state, so you may dispatch in the subscription listeners. You are only disallowed to dispatch inside the reducers because they must have no side effects. If you want to cause a side effect in response to an action, the right place to do this is in the potentially async action creator.
action(Object†): A plain object describing the change that makes sense for your application. Actions are the only way to get data into the store, so any data, whether from the UI events, network callbacks, or other sources such as WebSockets needs to eventually be dispatched as actions. Actions must have a
typefield that indicates the type of action being performed. Types can be defined as constants and imported from another module. It's better to use strings for
typethan Symbols because strings are serializable. Other than
type, the structure of an action object is really up to you. If you're interested, check out Flux Standard Action for recommendations on how actions could be constructed.
(Object†): The dispatched action (see notes).
† The “vanilla” store implementation you get by calling [`createStore`](/api/createstore) only supports plain object actions and hands them immediately to the reducer. Notes
However, if you wrap
applyMiddleware, the middleware can interpret actions differently, and provide support for dispatching async actions. Async actions are usually asynchronous primitives like Promises, Observables, or thunks.
Middleware is created by the community and does not ship with Redux by default. You need to explicitly install packages like redux-thunk or redux-promise to use it. You may also create your own middleware.
To learn how to describe asynchronous API calls, read the current state inside action creators, perform side effects, or chain them to execute in a sequence, see the examples for
Adds a change listener. It will be called any time an action is dispatched, and some part of the state tree may potentially have changed. You may then call
getState() to read the current state tree inside the callback.
You may call
dispatch() from a change listener, with the following caveats:
The listener should only call
dispatch()either in response to user actions or under specific conditions (e. g. dispatching an action when the store has a specific field). Calling
dispatch()without any conditions is technically possible, however it leads to an infinite loop as every
dispatch()call usually triggers the listener again.
The subscriptions are snapshotted just before every
dispatch()call. If you subscribe or unsubscribe while the listeners are being invoked, this will not have any effect on the
dispatch()that is currently in progress. However, the next
dispatch()call, whether nested or not, will use a more recent snapshot of the subscription list.
The listener should not expect to see all state changes, as the state might have been updated multiple times during a nested
dispatch()before the listener is called. It is, however, guaranteed that all subscribers registered before the
dispatch()started will be called with the latest state by the time it exits.
It is a low-level API. Most likely, instead of using it directly, you'll use React (or other) bindings. If you commonly use the callback as a hook to react to state changes, you might want to write a custom
observeStore utility. The
Store is also an
Observable, so you can
subscribe to changes with libraries like RxJS.
To unsubscribe the change listener, invoke the function returned by
listener(Function): The callback to be invoked any time an action has been dispatched, and the state tree might have changed. You may call
getState()inside this callback to read the current state tree. It is reasonable to expect that the store's reducer is a pure function, so you may compare references to some deep path in the state tree to learn whether its value has changed.
(Function): A function that unsubscribes the change listener.
Replaces the reducer currently used by the store to calculate the state.
It is an advanced API. You might need this if your app implements code splitting, and you want to load some of the reducers dynamically. You might also need this if you implement a hot reloading mechanism for Redux.
nextReducer(Function) The next reducer for the store to use.