By default, Redux’s actions are dispatched synchronously, which is a problem for any non-trivial app that needs to communicate with an external API or perform side effects. Redux also allows for middleware that sits between an action being dispatched and the action reaching the reducers.
There are two very popular middleware libraries that allow for side effects and asynchronous actions: Redux Thunk and Redux Saga. In this post, you will explore Redux Thunk.
Thunk is a programming concept where a function is used to delay the evaluation/calculation of an operation.
Redux Thunk is a middleware that lets you call action creators that return a function instead of an action object. That function receives the store’s dispatch method, which is then used to dispatch regular synchronous actions inside the function’s body once the asynchronous operations have been completed.
In this article, you will learn how to add Redux Thunk and how it can fit in a hypothetical Post List application.
.The first step:

Adding redux-thunk

First, use the terminal to navigate to the project directory and install the redux-thunk package in your project:

npm install redux-thunk@2.1.0``

Now apply the middleware when creating your app’s store using Redux’s applyMiddleware. Given a React application with redux and react-redux, your configurationStore.js file might look like this:

`
import { createStore, applyMiddleware } from 'redux';
import thunk from 'redux-thunk';
import rootReducer from '../reducers';
export default function configureStore(initialState) {
const enhancer = applyMiddleware(thunk)
return createStore(
rootReducer,
initialState,
enhancer
);
}
`
Using Redux Thunk in a Sample Application
The most common use case for Redux Thunk is for communicating asynchronously with an external API to retrieve or save data. Redux Thunk makes it easy to dispatch actions that follow the lifecycle of a request to an external API.
showing a list of posts involves first dispatching an action. Then, if the list of post is successfully showed and returned by the external server, then list of posts are shown Otherwise an error message is displayed
Let’s see how this would be accomplished using Redux Thunk.

`
import React, { Component, PropTypes } from 'react';
import { connect } from 'react-redux';
import { ListGroup, ListGroupItem } from 'react-bootstrap';
import { itemsFetchData } from '../actions/items';
class ItemList extends Component {
componentDidMount() {
this.props.fetchData('https://jsonplaceholder.typicode.com/posts');
}
render() {
if (this.props.hasError) {
return

Sorry! There was an error loading the items

Loading…

))}

Let’s have a look at the first 3 action creators:

`
export function itemsHaveError(bool) {
return {
type: 'ITEMS_HAVE_ERROR',
hasError: bool
};
}
export function itemsAreLoading(bool) {
return {
type: 'ITEMS_ARE_LOADING',
isLoading: bool
};
}
export function itemsFetchDataSuccess(items) {
return {
type: 'ITEMS_FETCH_DATA_SUCCESS',
items
};
}
`
The last one will be called after the fetching was successful and will receive the fetched items as an parameter. This action creator will return an object with a property called items that will receive as value the array of items which were passed as an argument. Instead if items: items, we can just write items, using an ES6 syntactic sugar called property shorthand.
To visualize a bit what was described earlier, this is how it looks in Redux DevTools:
Out of the box, action creators can return just actions. That’s where Redux Thunk comes in handy. Thunk allows us to have action creators that return a function instead of an action and dispatch an action only in certain cases.
If it wasn’t for Redux Thunk, we would probably end up having just one action creator, something like this:

`
export function itemsFetchData(url) {
const items = axios.get(url);
return {
type: 'ITEMS_FETCH_DATA',
items
};
}
`
Obviously, it would be a lot harder in this scenario to know if the items are still loading or checking if we have an error.
Knowing these and using Redux Thunk, our action creator will be:

`
export function itemsFetchData(url) {
return (dispatch) => {
dispatch(itemsAreLoading(true));
axios.get(url)
.then((response) => {
if (response.status !== 200) {
throw Error(response.statusText);
}
dispatch(itemsAreLoading(false));
return response;
})
.then((response) => dispatch(itemsFetchDataSuccess(response.data)))
.catch(() => dispatch(itemsHaveError(true)));
};
}
`
Reducers
Now that we have our action creators in place, let’s start writing our reducers.
All reducers will be called when an action is dispatched. Because of this, we are returning the original state in each of our reducers. When the action type matches, the reducer does what it has to do and returns a new slice of state. If not, the reducer returns the original state back.
Each reducer takes 2 parameters: the (soon to be previous) slice of state and an action object:

`
export function itemsHaveError(state = false, action) {
switch (action.type) {
case 'ITEMS_HAVE_ERROR':
return action.hasError;
default:
return state;
}
}
export function itemsAreLoading(state = false, action) {
switch (action.type) {
case 'ITEMS_ARE_LOADING':
return action.isLoading;
default:
return state;
}
}
export function items(state = [], action) {
switch (action.type) {
case 'ITEMS_FETCH_DATA_SUCCESS':
return action.items;
default:
return state;
}
}
`
Now that we have the reducers created, let’s combine them in our index.js from our reducersfolder:

`
import { combineReducers } from 'redux';
import { items, itemsHaveError, itemsAreLoading } from './items';
export default combineReducers({
items,
itemsHaveError,
itemsAreLoading
});

`