Redux Thunk

Thunk middleware for Redux. It allows writing functions with logic inside that can interact with a Redux store's dispatch and getState methods.

For complete usage instructions and useful patterns, see the Redux docs Writing Logic with Thunks page.

Installation and Setup

Redux Toolkit

If you're using our official Redux Toolkit package as recommended, there's nothing to install - RTK's configureStore API already adds the thunk middleware by default:

1import { configureStore } from '@reduxjs/toolkit'
2
3import todosReducer from './features/todos/todosSlice'
4import filtersReducer from './features/filters/filtersSlice'
5
6const store = configureStore({
7  reducer: {
8    todos: todosReducer,
9    filters: filtersReducer
10  }
11})
12
13// The thunk middleware was automatically added

Manual Setup

If you're using the basic Redux createStore API and need to set this up manually, first add the redux-thunk package:

1npm install redux-thunk
2
3yarn add redux-thunk

The thunk middleware is the default export.

1import thunk from 'redux-thunk' // no changes here 😀

1- const thunk = require('redux-thunk')
2+ const thunk = require('redux-thunk').default

1const ReduxThunk = window.ReduxThunk

Then, to enable Redux Thunk, use applyMiddleware():

1import { createStore, applyMiddleware } from 'redux'
2import thunk from 'redux-thunk'
3import rootReducer from './reducers/index'
4
5const store = createStore(rootReducer, applyMiddleware(thunk))

Injecting a Custom Argument

Since 2.1.0, Redux Thunk supports injecting a custom argument into the thunk middleware. This is typically useful for cases like using an API service layer that could be swapped out for a mock service in tests.

For Redux Toolkit, the getDefaultMiddleware callback inside of configureStore lets you pass in a custom extraArgument:

1import { configureStore } from '@reduxjs/toolkit'
2import rootReducer from './reducer'
3import { myCustomApiService } from './api'
4
5const store = configureStore({
6  reducer: rootReducer,
7  middleware: getDefaultMiddleware =>
8    getDefaultMiddleware({
9      thunk: {
10        extraArgument: myCustomApiService
11      }
12    })
13})
14
15// later
16function fetchUser(id) {
17  // The `extraArgument` is the third arg for thunk functions
18  return (dispatch, getState, api) => {
19    // you can use api here
20  }
21}

If you need to pass in multiple values, combine them into a single object:

1const store = configureStore({
2  reducer: rootReducer,
3  middleware: getDefaultMiddleware =>
4    getDefaultMiddleware({
5      thunk: {
6        extraArgument: {
7          api: myCustomApiService,
8          otherValue: 42
9        }
10      }
11    })
12})
13
14// later
15function fetchUser(id) {
16  return (dispatch, getState, { api, otherValue }) => {
17    // you can use api and something else here
18  }
19}

If you're setting up the store by hand, the named export withExtraArgument() function should be used to generate the correct thunk middleware:

1const store = createStore(reducer, applyMiddleware(withExtraArgument(api)))

Why Do I Need This?

With a plain basic Redux store, you can only do simple synchronous updates by dispatching an action. Middleware extends the store's abilities, and lets you write async logic that interacts with the store.

Thunks are the recommended middleware for basic Redux side effects logic, including complex synchronous logic that needs access to the store, and simple async logic like AJAX requests.

For more details on why thunks are useful, see:

• Redux docs: Writing Logic with Thunks
  https://redux.js.org/usage/writing-logic-thunks
  The official usage guide page on thunks. Covers why they exist, how the thunk middleware works, and useful patterns for using thunks.

• Stack Overflow: Dispatching Redux Actions with a Timeout
  http://stackoverflow.com/questions/35411423/how-to-dispatch-a-redux-action-with-a-timeout/35415559#35415559
  Dan Abramov explains the basics of managing async behavior in Redux, walking through a progressive series of approaches (inline async calls, async action creators, thunk middleware).

• Stack Overflow: Why do we need middleware for async flow in Redux?
  http://stackoverflow.com/questions/34570758/why-do-we-need-middleware-for-async-flow-in-redux/34599594#34599594
  Dan Abramov gives reasons for using thunks and async middleware, and some useful patterns for using thunks.

• What the heck is a "thunk"?
  https://daveceddia.com/what-is-a-thunk/
  A quick explanation for what the word "thunk" means in general, and for Redux specifically.

• Thunks in Redux: The Basics
  https://medium.com/fullstack-academy/thunks-in-redux-the-basics-85e538a3fe60
  A detailed look at what thunks are, what they solve, and how to use them.

You may also want to read the Redux FAQ entry on choosing which async middleware to use.

While the thunk middleware is not directly included with the Redux core library, it is used by default in our @reduxjs/toolkit package.

Motivation

Redux Thunk middleware allows you to write action creators that return a function instead of an action. The thunk can be used to delay the dispatch of an action, or to dispatch only if a certain condition is met. The inner function receives the store methods dispatch and getState as parameters.

An action creator that returns a function to perform asynchronous dispatch:

1const INCREMENT_COUNTER = 'INCREMENT_COUNTER'
2
3function increment() {
4  return {
5    type: INCREMENT_COUNTER
6  }
7}
8
9function incrementAsync() {
10  return dispatch => {
11    setTimeout(() => {
12      // Yay! Can invoke sync or async actions with `dispatch`
13      dispatch(increment())
14    }, 1000)
15  }
16}

An action creator that returns a function to perform conditional dispatch:

1function incrementIfOdd() {
2  return (dispatch, getState) => {
3    const { counter } = getState()
4
5    if (counter % 2 === 0) {
6      return
7    }
8
9    dispatch(increment())
10  }
11}

What’s a thunk?!

A thunk is a function that wraps an expression to delay its evaluation.

1// calculation of 1 + 2 is immediate
2// x === 3
3let x = 1 + 2
4
5// calculation of 1 + 2 is delayed
6// foo can be called later to perform the calculation
7// foo is a thunk!
8let foo = () => 1 + 2

The term originated as a humorous past-tense version of "think".

Composition

Any return value from the inner function will be available as the return value of dispatch itself. This is convenient for orchestrating an asynchronous control flow with thunk action creators dispatching each other and returning Promises to wait for each other’s completion:

1import { createStore, applyMiddleware } from 'redux'
2import thunk from 'redux-thunk'
3import rootReducer from './reducers'
4
5// Note: this API requires redux@>=3.1.0
6const store = createStore(rootReducer, applyMiddleware(thunk))
7
8function fetchSecretSauce() {
9  return fetch('https://www.google.com/search?q=secret+sauce')
10}
11
12// These are the normal action creators you have seen so far.
13// The actions they return can be dispatched without any middleware.
14// However, they only express “facts” and not the “async flow”.
15
16function makeASandwich(forPerson, secretSauce) {
17  return {
18    type: 'MAKE_SANDWICH',
19    forPerson,
20    secretSauce
21  }
22}
23
24function apologize(fromPerson, toPerson, error) {
25  return {
26    type: 'APOLOGIZE',
27    fromPerson,
28    toPerson,
29    error
30  }
31}
32
33function withdrawMoney(amount) {
34  return {
35    type: 'WITHDRAW',
36    amount
37  }
38}
39
40// Even without middleware, you can dispatch an action:
41store.dispatch(withdrawMoney(100))
42
43// But what do you do when you need to start an asynchronous action,
44// such as an API call, or a router transition?
45
46// Meet thunks.
47// A thunk in this context is a function that can be dispatched to perform async
48// activity and can dispatch actions and read state.
49// This is an action creator that returns a thunk:
50function makeASandwichWithSecretSauce(forPerson) {
51  // We can invert control here by returning a function - the "thunk".
52  // When this function is passed to `dispatch`, the thunk middleware will intercept it,
53  // and call it with `dispatch` and `getState` as arguments.
54  // This gives the thunk function the ability to run some logic, and still interact with the store.
55  return function (dispatch) {
56    return fetchSecretSauce().then(
57      sauce => dispatch(makeASandwich(forPerson, sauce)),
58      error => dispatch(apologize('The Sandwich Shop', forPerson, error))
59    )
60  }
61}
62
63// Thunk middleware lets me dispatch thunk async actions
64// as if they were actions!
65
66store.dispatch(makeASandwichWithSecretSauce('Me'))
67
68// It even takes care to return the thunk’s return value
69// from the dispatch, so I can chain Promises as long as I return them.
70
71store.dispatch(makeASandwichWithSecretSauce('My partner')).then(() => {
72  console.log('Done!')
73})
74
75// In fact I can write action creators that dispatch
76// actions and async actions from other action creators,
77// and I can build my control flow with Promises.
78
79function makeSandwichesForEverybody() {
80  return function (dispatch, getState) {
81    if (!getState().sandwiches.isShopOpen) {
82      // You don’t have to return Promises, but it’s a handy convention
83      // so the caller can always call .then() on async dispatch result.
84
85      return Promise.resolve()
86    }
87
88    // We can dispatch both plain object actions and other thunks,
89    // which lets us compose the asynchronous actions in a single flow.
90
91    return dispatch(makeASandwichWithSecretSauce('My Grandma'))
92      .then(() =>
93        Promise.all([
94          dispatch(makeASandwichWithSecretSauce('Me')),
95          dispatch(makeASandwichWithSecretSauce('My wife'))
96        ])
97      )
98      .then(() => dispatch(makeASandwichWithSecretSauce('Our kids')))
99      .then(() =>
100        dispatch(
101          getState().myMoney > 42
102            ? withdrawMoney(42)
103            : apologize('Me', 'The Sandwich Shop')
104        )
105      )
106  }
107}
108
109// This is very useful for server side rendering, because I can wait
110// until data is available, then synchronously render the app.
111
112store
113  .dispatch(makeSandwichesForEverybody())
114  .then(() =>
115    response.send(ReactDOMServer.renderToString(<MyApp store={store} />))
116  )
117
118// I can also dispatch a thunk async action from a component
119// any time its props change to load the missing data.
120
121import { connect } from 'react-redux'
122import { Component } from 'react'
123
124class SandwichShop extends Component {
125  componentDidMount() {
126    this.props.dispatch(makeASandwichWithSecretSauce(this.props.forPerson))
127  }
128
129  componentDidUpdate(prevProps) {
130    if (prevProps.forPerson !== this.props.forPerson) {
131      this.props.dispatch(makeASandwichWithSecretSauce(this.props.forPerson))
132    }
133  }
134
135  render() {
136    return <p>{this.props.sandwiches.join('mustard')}</p>
137  }
138}
139
140export default connect(state => ({
141  sandwiches: state.sandwiches
142}))(SandwichShop)

License

MIT