source: node_modules/redux-thunk/README.md@ a762898

# Redux Thunk

Thunk [middleware](https://redux.js.org/tutorials/fundamentals/part-4-store#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](https://redux.js.org/usage/writing-logic-thunks).

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## Installation and Setup

### Redux Toolkit

If you're using [our official Redux Toolkit package](https://redux-toolkit.js.org) as recommended, there's nothing to install - RTK's `configureStore` API already adds the thunk middleware by default:

```js
import { configureStore } from '@reduxjs/toolkit'

import todosReducer from './features/todos/todosSlice'
import filtersReducer from './features/filters/filtersSlice'

const store = configureStore({
  reducer: {
    todos: todosReducer,
    filters: filtersReducer
  }
})

// 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:

```sh
npm install redux-thunk

yarn add redux-thunk
```

The thunk middleware is the default export.

<details>
<summary><b>More Details: Importing the thunk middleware</b></summary>

If you're using ES modules:

```js
import thunk from 'redux-thunk' // no changes here 😀
```

If you use Redux Thunk 2.x in a CommonJS environment,
[don’t forget to add `.default` to your import](https://github.com/reduxjs/redux-thunk/releases/tag/v2.0.0):

```diff
- const thunk = require('redux-thunk')
+ const thunk = require('redux-thunk').default
```

Additionally, since 2.x, we also support a
[UMD build](https://unpkg.com/redux-thunk/dist/redux-thunk.min.js) for use as a global script tag:

```js
const ReduxThunk = window.ReduxThunk
```

</details>

Then, to enable Redux Thunk, use
[`applyMiddleware()`](https://redux.js.org/api/applymiddleware):

```js
import { createStore, applyMiddleware } from 'redux'
import thunk from 'redux-thunk'
import rootReducer from './reducers/index'

const 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`:

```js
import { configureStore } from '@reduxjs/toolkit'
import rootReducer from './reducer'
import { myCustomApiService } from './api'

const store = configureStore({
  reducer: rootReducer,
  middleware: getDefaultMiddleware =>
    getDefaultMiddleware({
      thunk: {
        extraArgument: myCustomApiService
      }
    })
})

// later
function fetchUser(id) {
  // The `extraArgument` is the third arg for thunk functions
  return (dispatch, getState, api) => {
    // you can use api here
  }
}
```

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

```js
const store = configureStore({
  reducer: rootReducer,
  middleware: getDefaultMiddleware =>
    getDefaultMiddleware({
      thunk: {
        extraArgument: {
          api: myCustomApiService,
          otherValue: 42
        }
      }
    })
})

// later
function fetchUser(id) {
  return (dispatch, getState, { api, otherValue }) => {
    // you can use api and something else here
  }
}
```

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

```js
const 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](https://redux.js.org/faq/actions#what-async-middleware-should-i-use-how-do-you-decide-between-thunks-sagas-observables-or-something-else)**.

While the thunk middleware is not directly included with the Redux core library,
it is used by default in our
**[`@reduxjs/toolkit` package](https://github.com/reduxjs/redux-toolkit)**.

## Motivation

Redux Thunk [middleware](https://redux.js.org/tutorials/fundamentals/part-4-store#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:

```js
const INCREMENT_COUNTER = 'INCREMENT_COUNTER'

function increment() {
  return {
    type: INCREMENT_COUNTER
  }
}

function incrementAsync() {
  return dispatch => {
    setTimeout(() => {
      // Yay! Can invoke sync or async actions with `dispatch`
      dispatch(increment())
    }, 1000)
  }
}
```

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

```js
function incrementIfOdd() {
  return (dispatch, getState) => {
    const { counter } = getState()

    if (counter % 2 === 0) {
      return
    }

    dispatch(increment())
  }
}
```

## What’s a thunk?!

A [thunk](https://en.wikipedia.org/wiki/Thunk) is a function that wraps an
expression to delay its evaluation.

```js
// calculation of 1 + 2 is immediate
// x === 3
let x = 1 + 2

// calculation of 1 + 2 is delayed
// foo can be called later to perform the calculation
// foo is a thunk!
let foo = () => 1 + 2
```

The term [originated](https://en.wikipedia.org/wiki/Thunk#cite_note-1) 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:

```js
import { createStore, applyMiddleware } from 'redux'
import thunk from 'redux-thunk'
import rootReducer from './reducers'

// Note: this API requires redux@>=3.1.0
const store = createStore(rootReducer, applyMiddleware(thunk))

function fetchSecretSauce() {
  return fetch('https://www.google.com/search?q=secret+sauce')
}

// These are the normal action creators you have seen so far.
// The actions they return can be dispatched without any middleware.
// However, they only express “facts” and not the “async flow”.

function makeASandwich(forPerson, secretSauce) {
  return {
    type: 'MAKE_SANDWICH',
    forPerson,
    secretSauce
  }
}

function apologize(fromPerson, toPerson, error) {
  return {
    type: 'APOLOGIZE',
    fromPerson,
    toPerson,
    error
  }
}

function withdrawMoney(amount) {
  return {
    type: 'WITHDRAW',
    amount
  }
}

// Even without middleware, you can dispatch an action:
store.dispatch(withdrawMoney(100))

// But what do you do when you need to start an asynchronous action,
// such as an API call, or a router transition?

// Meet thunks.
// A thunk in this context is a function that can be dispatched to perform async
// activity and can dispatch actions and read state.
// This is an action creator that returns a thunk:
function makeASandwichWithSecretSauce(forPerson) {
  // We can invert control here by returning a function - the "thunk".
  // When this function is passed to `dispatch`, the thunk middleware will intercept it,
  // and call it with `dispatch` and `getState` as arguments.
  // This gives the thunk function the ability to run some logic, and still interact with the store.
  return function (dispatch) {
    return fetchSecretSauce().then(
      sauce => dispatch(makeASandwich(forPerson, sauce)),
      error => dispatch(apologize('The Sandwich Shop', forPerson, error))
    )
  }
}

// Thunk middleware lets me dispatch thunk async actions
// as if they were actions!

store.dispatch(makeASandwichWithSecretSauce('Me'))

// It even takes care to return the thunk’s return value
// from the dispatch, so I can chain Promises as long as I return them.

store.dispatch(makeASandwichWithSecretSauce('My partner')).then(() => {
  console.log('Done!')
})

// In fact I can write action creators that dispatch
// actions and async actions from other action creators,
// and I can build my control flow with Promises.

function makeSandwichesForEverybody() {
  return function (dispatch, getState) {
    if (!getState().sandwiches.isShopOpen) {
      // You don’t have to return Promises, but it’s a handy convention
      // so the caller can always call .then() on async dispatch result.

      return Promise.resolve()
    }

    // We can dispatch both plain object actions and other thunks,
    // which lets us compose the asynchronous actions in a single flow.

    return dispatch(makeASandwichWithSecretSauce('My Grandma'))
      .then(() =>
        Promise.all([
          dispatch(makeASandwichWithSecretSauce('Me')),
          dispatch(makeASandwichWithSecretSauce('My wife'))
        ])
      )
      .then(() => dispatch(makeASandwichWithSecretSauce('Our kids')))
      .then(() =>
        dispatch(
          getState().myMoney > 42
            ? withdrawMoney(42)
            : apologize('Me', 'The Sandwich Shop')
        )
      )
  }
}

// This is very useful for server side rendering, because I can wait
// until data is available, then synchronously render the app.

store
  .dispatch(makeSandwichesForEverybody())
  .then(() =>
    response.send(ReactDOMServer.renderToString(<MyApp store={store} />))
  )

// I can also dispatch a thunk async action from a component
// any time its props change to load the missing data.

import { connect } from 'react-redux'
import { Component } from 'react'

class SandwichShop extends Component {
  componentDidMount() {
    this.props.dispatch(makeASandwichWithSecretSauce(this.props.forPerson))
  }

  componentDidUpdate(prevProps) {
    if (prevProps.forPerson !== this.props.forPerson) {
      this.props.dispatch(makeASandwichWithSecretSauce(this.props.forPerson))
    }
  }

  render() {
    return <p>{this.props.sandwiches.join('mustard')}</p>
  }
}

export default connect(state => ({
  sandwiches: state.sandwiches
}))(SandwichShop)
```

## License

MIT