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Store

A store holds the whole state tree of your application. The only way to change the state inside it is to dispatch an action on it, which triggers the root reducer function to calculate the new state.

A store is not a class. It's just an object with a few methods on it.

To create a store, pass your root reducer function to Redux Toolkit's configureStore method, which will set up a Redux store with a good default configuration. (Alternately, if you're not yet using Redux Toolkit, you can use the original createStore method, but we encourage you to migrate your code to use Redux Toolkit as soon as possible)

Store Methods​

getState()​

Returns the current state tree of your application. It is equal to the last value returned by the store's reducer.

Returns​

(any): The current state tree of your application.

dispatch(action)​

Dispatches an action. This is the only way to trigger a state change.

The store's reducer 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.

caution

If you attempt to call dispatch from inside the reducer, it will throw with an error saying "Reducers may not dispatch actions." Reducers are pure functions - they can only return a new state value and must not have side effects (and dispatching is a side effect).

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.

Arguments​

  1. 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 type field 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 type than 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.

Returns​

(Object†): The dispatched action (see notes).

Notes​

† The “vanilla” store implementation you get by calling [`createStore`](/api/createstore) only supports plain object actions and hands them immediately to the reducer.

However, if you wrap createStore with 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 applyMiddleware.

Example​

import { createStore } from 'redux'
const store = createStore(todos, ['Use Redux'])

function addTodo(text) {
  return {
    type: 'ADD_TODO',
    text
  }
}

store.dispatch(addTodo('Read the docs'))
store.dispatch(addTodo('Read about the middleware'))

subscribe(listener)​

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:

  1. 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.

  2. 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.

  3. 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 subscribe.

Arguments​

  1. 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.
Returns​

(Function): A function that unsubscribes the change listener.

Example​
function select(state) {
  return state.some.deep.property
}

let currentValue
function handleChange() {
  let previousValue = currentValue
  currentValue = select(store.getState())

  if (previousValue !== currentValue) {
    console.log(
      'Some deep nested property changed from',
      previousValue,
      'to',
      currentValue
    )
  }
}

const unsubscribe = store.subscribe(handleChange)
unsubscribe()

replaceReducer(nextReducer)​

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.

Arguments​

  1. nextReducer (Function) The next reducer for the store to use.