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Learn about two general concepts in computing, middleware and thunks, and how they relate to Redux.

Redux Middleware and Thunks

In the last exercise, we promised to elaborate on the two arguments that the payload creator (the asynchronous function we pass to `createAsyncThunk`) receives: `arg` and `thunkAPI`. The first argument, `arg`, will be equal to the first argument passed to the thunk action creator itself. For example, if we call `fetchUserById(7)`, then inside the payload creator, `arg` will be equal to 7. 

But what if you need to pass multiple arguments to your thunk? Since the payload creator only receives the first argument passed to the thunk action creator, you'll want to bundle multiple arguments into a single object. For example, say we want to search our app's users by first and last name. If the thunk action creator is called `searchUsers`, we would call it like this: `searchUsers({firstName: 'Ada', lastName: 'Lovelace'})`.

If you need to access these variables individually, you can use ES6 destructuring assignment to unpack the object when you declare the payload creator and pass it to `createAsyncThunk`, like this :

```js
const searchUsers = createAsyncThunk(
	'users/searchUsers',
	async ({ firstName, lastName}, thunkAPI) => {
		// perform the asynchronous search request here	
	}
)
```

If your thunk requires no arguments, you can just call the action creator without it, and the `arg` argument will be undefined.

In the event the thunk requires only one param (for example, fetching a specific resource by `id`) you should name that first param semantically. Here's the `fetchUserById` example from the last exercise, with the `arg` parameter semantically renamed to `userId`. 

```js
import { createAsyncThunk } from '@reduxjs/toolkit'
import { fetchUser } from './api'
const fetchUserById = createAsyncThunk(
    'users/fetchUserById', // action type
    async (userId, thunkAPI) => { // payload creator
        const response = await fetchUser(userId)
        return response.data
    }
)
```

The payload creator's second argument, `thunkAPI`, is an object containing several useful methods, including the store's `dispatch` and `getState`. For an exhaustive list of methods available in the `thunkAPI` object, you can read [the documentation](https://redux-toolkit.js.org/api/createAsyncThunk#payloadcreator).

Passing Arguments to Thunks

Recall that our overarching goal in this lesson is to give you the tools you need to add asynchronous functionality to your Redux apps. One of the most flexible and popular ways to add asynchronous functionality to Redux involves using thunks. A thunk is a higher-order function that wraps the computation we want to perform later. For example, this `add()` function returns a thunk that will perform `x+y` when called.

```js
const add = (x,y) => {
  return () => {
    return x + y; 
  } 
}
```

Thunks are helpful because they allow us to bundle up bits of computation we want to delay into packages that can be passed around in code. Consider these two function calls, which rely on the `add()` function above:

```js
const delayedAddition = add(2,2)
delayedAddition() // => 4
```

Note that calling `add()` does not cause the addition to happen – it merely returns a function that will perform the addition when called. To perform the addition, we must call `delayedAddition()`.

Introduction to Thunks

A promise begins in the pending state. If success, it moves to the fulfilled state and returns data. If failure, it moves to the rejected state and returns an error.

In a perfect world, every network request we make would yield an immediate and successful response. But network requests can be slow, and sometimes fail. As developers, we need to account for these realities in order to create the best possible experience for our users. If we know a request is pending, we can make our application more user-friendly by displaying a loading state. Similarly, if we know a request has failed, we can display an appropriate error state. 

In order to create these satisfying user experiences, we need to keep track of the state our async requests are in at any given moment so that we can reflect those states for the user. It is common to dispatch a "pending" action right before performing an asynchronous operation, and "fulfilled" or "rejected" actions depending on the results of the completed operation. Take this illustrative example `fetchUserById`: 

```js
import { fetchUser } from './api';

const fetchUserById = (id) => {
  const payload = await fetchUser(id);
  // update user data in store with `payload`
}
```

Rewritten to include pending and rejected actions, it might look like this:

```js
import { fetchUser } from './api'
const fetchUserById = (id) => {
  // update store to show user data is being requested -> "pending" state
  try {
    const payload = await fetchUser(id)
   // update user data in store with `payload` -> "fulfilled" state
  } catch(err) {
   // notify store to show user data failed to be fetched -> "rejected" state
  }
}
```

We call these pending/fulfilled/rejected actions _promise lifecycle actions_. This pattern is so common that Redux Toolkit provides a neat abstraction, `createAsyncThunk`, for including promise lifecycle actions in your Redux apps. We'll explore that method in the following exercises.

Promise Lifecycle Actions

Out of the box, Redux may meet most of your app’s state management needs. But every project is different, and so Redux provides some ways to customize its behavior. One of the ways we can customize Redux is by adding _middleware_.

You may be familiar with middleware from experiences working with other frameworks. As the name suggests, middleware is the code that runs in the middle—usually between a framework receiving a request and producing a response. Middleware is a powerful tool for extending, modifying, or customizing a framework or library’s default behavior to meet an application’s specific needs.

In Redux, middleware runs between when an action is dispatched and when that action is passed along to the reducer. By this point, you’re familiar with the way data flows through Redux: actions are dispatched to the store, where they are processed by reducers that produce a new state; that new state becomes accessible to all the components that reference it, causing those components to update. We’ve rendered that flow in the included diagram and added middleware to help you see where and how it comes into play. 

Middleware intercepts actions after they are dispatched and before they are passed along to the reducer. Some common tasks that middleware performs include logging, caching, adding auth tokens to request headers, crash reporting, routing, and making asynchronous requests for data. You can add any of these functionalities to your apps by using popular open-source middleware. Of course, you can also write your own middleware to solve problems that are specific to your application and its architecture. 

To make asynchronous requests in our recipe app, we're using a Redux Toolkit utility function called `createAsyncThunk()` and the `extraReducers` option you can pass to the `createSlice` function. In subsequent exercises, we will walk through how `createAsyncThunk()` uses middleware and **thunks** (we'll discuss these more in the next exercise) to make asynchronous requests possible; for now, you should just understand where middleware sits in Redux’s data flow.

Middleware in Redux

By now, we can create an app with Redux state management. But we haven’t covered one of the most common challenges in app development: making asynchronous requests. With a basic Redux store, we can only do synchronous updates. When an action is dispatched, it is immediately processed by a reducer, which updates the store accordingly. But when developing applications, we often want to perform asynchronous operations (such as making API calls) and update the state based on the results. 

In this lesson, you will gain the tools necessary to write asynchronous logic that interacts with your Redux store. 
1. First, you will learn about two general concepts in computing: _middleware_ and _thunks_, and the ways they relate to Redux. 
2. Next, you will learn about the promise lifecycle and how you can use it to provide a satisfying user experience. 
3. Finally, you will practice adding asynchronous logic to your Redux apps using tools provided by `@reduxjs/redux-toolkit`.

> This lesson uses [Mock Service Worker](https://mswjs.io/) to replicate the functionality of an external API. To use MSW, you'll want to use Google Chrome and [enable third-party cookies](https://support.google.com/chrome/answer/95647?co=GENIE.Platform%3DDesktop&hl=en).

Introduction

Well done! Let's summarize what you've learned in this lesson. In this lesson, you:
* Learned about Redux middleware and wrote your own simple logging middleware
* Encountered thunks and learned about how valuable thunks are for deferring computation
* Learned the three _promise lifecycle actions_: pending, fulfilled, and rejected
* Learned how to use `createAsyncThunk()`, which abstracts the process of handling promise lifecycle states according to best practices/common design paradigms
* Imported `createAsyncThunk` from the Redux Toolkit:


```js
import { createAsyncThunk } from '@reduxjs/toolkit';
```

* Created action creators using `createAsyncThunk()`.
* Made your reducers respond to pending/fulfilled/rejected promise lifecycle actions by supplying the `extraReducers` property to `createSlice()`.

Review

As you know, `createAsyncThunk` takes care of dispatching actions for each of the promise lifecycle states: pending, fulfilled, and rejected. But what exactly do these actions look like? 

Building off the action type string you pass to it, `createAsyncThunk` produces an action type for each promise lifecycle states. If you pass the action type string `'resourceType/actionType'` to `createAsyncThunk`, it will produce these three action types: 
* `'resourceType/actionType/pending'`
* `'resourceType/actionType/fulfilled'`
* `'resourceType/actionType/rejected'`

To use our earlier example:

```js
import { createAsyncThunk } from '@reduxjs/toolkit'
import { fetchUser } from './api'

const fetchUserById = createAsyncThunk(
  'users/fetchUserById', // action type
  async (userId, thunkAPI) => { // payload creator
    const response = await fetchUser(userId)
    return response.data
  }
)
```

When you pass `createAsyncThunk` the action type string `'users/fetchUserById'`, `createAsyncThunk` produces these three action types: 
* `'users/fetchUserById/pending'`
* `'users/fetchUserById/fulfilled'`
* `'users/fetchUserById/rejected'` 

If you need to access the individual pending/fulfilled/rejected action creators, you can reference them like this: 
* `fetchUserById.pending`
* `fetchUserById.fulfilled`
* `fetchUserById.rejected`

You will have to handle these action types in your reducers if you want to reflect these promise lifecycle states in your app. In the next exercise, we will show you how to do that.

Actions Generated by createAsyncThunk()

Let's solidify our understanding of how middleware fits into Redux’s data flow. We'll explore how middleware actually gets invoked in Redux, so that we know how a middleware should be structured. After that, we’re going to write a simple middleware from scratch. 

But first, you’ll recall from the previous exercise that middleware runs after an action is dispatched and before that action is passed along to the reducer. How does this actually work? 

To add a middleware to our project, we use Redux’s `applyMiddleware` function like so. 

```js
import { createStore, applyMiddleware } from 'redux';
import { middleware1, middleware2, middleware3 } from './exampleMiddlewares';
import { exampleReducer } from './exampleReducer';
import { initialState} from './initialState';

const store = createStore(
  exampleReducer, 
  initialState, 
  applyMiddleware(
    middleware1, 
    middleware2, 
    middleware3
  )
);
```

The specifics of how `applyMiddleware` works are outside the scope of this lesson.  All you need to know is that once middleware has been added to a Redux project, calls to `dispatch` are actually calls to the middleware pipeline (the chain of all added middlewares). This means that any actions we dispatch will be passed from middleware to middleware before they hit an app’s reducers.

Middlewares must conform to a specific, nested function structure in order to work as part of the pipeline (this nested structure is also called a [higher-order function](https://eloquentjavascript.net/05_higher_order.html), if you'd like to read more). That structure looks like this:

```js
const exampleMiddleware = storeAPI => next => action => {
  // do stuff here
  return next(action);  // pass the action on to the next middleware in the pipeline
}
```

Each middleware has access to the `storeAPI` (which consists of the `dispatch` and `getState` functions), as well as the `next` middleware in the pipeline, and the `action` that is to be dispatched. The body of the middleware function performs the middleware’s specific task before calling the next middleware in the pipeline with the current action (note that if the middleware is the last in the pipeline, then `next` is `storeAPI.dispatch` so calling `next(action)` is the same as dispatching the action to the store).

Now let’s write a custom middleware that logs the contents of our store to the console.



Write Your Own Middleware

In a previous lesson, you learned about `createSlice()`. In this lesson, you will learn about `extraReducers`, a property you can optionally pass to `createSlice` that allows `createSlice` to respond to action types it did not generate. 

To refresh your memory, `createSlice()` accepts a single argument, `options`, which is an object containing configuration parameters including a name, some initial state, and reducers. `createSlice()` then uses these configuration parameters to generate a slice of the store, including action creators and action types for updating the state contained in that slice. Consider the following example:

```js
const usersSlice = createSlice({
  name: 'users',
  initialState: { users:  [] },
  reducers: {
    addUser: (state, action) => { 
      state.users.push(action.payload) 
    }        
  },
})
```

This call to `createSlice()`, generates a slice of the store that responds to the action creator `usersSlice.actions.addUser`. But what if we've generated our action creators via calls to `createAsyncThunk()`? Consider `fetchUserById`, the asynchronous action creator from earlier in this lesson:

```js
const fetchUserById = createAsyncThunk(
  'users/fetchUserById', // action type
  async (userId, thunkAPI) => { // payload creator
    const response = await fetchUser(userId)
    return response.data
  }
)
```

This asynchronous action creator will generate three action types: `'users/fetchUserById/pending'`, `'users/fetchUserById/fulfilled'`, and `'users/fetchUserById/rejected'`. Currently, these action types have no effect on our user's slice, which only responds to the `users/addUser` action type generated by `createSlice()`. 

How can we account for these promise lifecycle action types in our user slice? This is exactly the problem that `extraReducers`, an optional property on the configuration object passed to `createSlice()`, was designed to solve. `extraReducers` allows `createSlice()` to respond to action types generated elsewhere. To make the user's slice respond to promise lifecycle action types, we pass them to `createSlice()` in the `extraReducers` property. 

Open **usersSlice.js** in your code editor to see an example of the `extraReducers` property in context.

Note that in addition to using the `extraReducers` property, we also added some extra fields to our state object: a boolean, `isLoading`, which will be true when a request is pending, and otherwise false, and a boolean `hasError`, which we will set to `true` if our request to fetch a user is rejected. These additions allow us to track promise lifecycle states so that we can create satisfying and informative user interfaces when the promise is either `pending` or `rejected`. When the promise is `fulfilled` these are set to `false`, and the user data is added to the state.

Using createSlice() with Async Action Creators

`createAsyncThunk()` is a function with two parameters, an action type string, and an asynchronous callback, that generates a thunk action creator that will run the provided callback and automatically dispatch promise lifecycle actions as appropriate so that you don't have to dispatch pending/fulfilled/rejected actions by hand. 

To use `createAsyncThunk()`, you'll first need to import it from Redux Toolkit like so:

```js
import { createAsyncThunk } from '@reduxjs/toolkit';
```

Next, you'll need to call `createAsyncThunk()`, passing two arguments. The first is a string representing the asynchronous action's type. Conventionally, type strings take the form `"resourceType/actionName"`. In this case, since we are getting an individual user by their `id`, our action type will be `users/fetchUserById`. The second argument to `createAsyncThunk` is the payload creator: an asynchronous function that returns a promise resolving to the result of an asynchronous operation. Here is `fetchUserById` rewritten using `createAsyncThunk`:

```js
import { createAsyncThunk } from '@reduxjs/toolkit'
import { fetchUser } from './api'
const fetchUserById = createAsyncThunk(
  'users/fetchUserById', // action type
  async (arg, thunkAPI) => { // payload creator
    const response = await fetchUser(arg);
    return response.json();
  }
)
```

There are a few things worth highlighting here. First, observe that the payload creator receives two arguments—`arg` and `thunkAPI`. We will elaborate on those in the next exercise. Second, note that the payload creator we provided doesn't dispatch any actions at all. It just returns the result of an asynchronous operation.

As you can see, `createAsyncThunk()` makes defining thunk action creators concise. All you have to write is an asynchronous thunk function; `createAsyncThunk()` takes care of the rest, returning an action creator that will dispatch pending/fulfilled/rejected actions as appropriate. 