learn-co-students/react-async-readme-recoded-sanaa-2019

Asynchronous React

Problem Statement

We've seen that React components come with some neat-o bells and whistles. They can be nested within each other. They can pass information and logic between them with props and they can keep track of their own information in state.

So far though, we've been restricted to displaying information organized by the React app itself. In this lesson, we're going to go a step further and incorporate remote data into our React apps. Using fetch requests to APIs, we can build dynamic, responsive apps around data that is provided to us remotely.

Objectives

• Introduce the use of fetch within components
• Consider some of the best places to include fetch in our React app

Using fetch Within React

For a minute, consider how a site like Instagram works. If you've got an account on Instagram, when you visit the site, you'll see an endless scroll of photos from people you follow. Everyone sees the same Instagram website, but the images displayed are unique to the user.

Similarly, consider IMDb, the Internet Movie Database. When you click to look at a movie's information, the page is always the same. The data, the images, the reviews, the cast... this information changes.

Both of these websites are built with React. When you go to one of these sites, React doesn't have the specific movie or image content. If you're on a slow connection (or want to mimic one using the Chrome Dev Tools), you can see what is happening more clearly. React shows up first and renders something. Sometimes it is just the background or the skeleton of a website, or maybe navigation and CSS. On Instagram, a photo 'card' might appear but without an image or username attached.

React is mounting its basic components first. Once these are mounted, remote data is then requested. When that data has been received, React runs through an update of the necessary components and fills in the info it received. Text content will appear, user information, etc... This first set of data is likely just a JSON object specific to the user or content requested. This object might contain image URLs, so right after the component update, images will be able to load.

So, since the data is being requested after React has mounted its components, is there a component lifecycle method that might be useful here?

Why yes there is! componentDidMount happens to be a great place for making fetch requests. By putting a fetch() within componentDidMount, when the data is received, we can use setState to store the received data. This causes an update with that remote data stored in state. A very simple implementation of the App component with fetch might look like this:

import React, { Component } from 'react'

class App extends Component {

  state = {
    peopleInSpace: []
  }

  render() {
    return (
      <div>
        {this.state.peopleInSpace.map(person => person.name)}
      </div>
    )
  }

  componentDidMount() {
    fetch('http://api.open-notify.org/astros.json')
      .then(response => response.json())
      .then(data => {
        this.setState({
          peopleInSpace: data.people
        })
      })
  }
}

export default App

In the code above, once App mounts, a fetch is called to an API. Once data is returned from the API, the simplest way to store some or all of it is to put it in state.

If you have JSX content reliant on that state information, when setState is called and the component re-renders, the content will appear.

Placing fetch in componentDidMount is ideal for data that you need immediately when a user visits your website or uses your app. Since componentDidMount is also commonly used to initialize intervals, it is ideal to set up any repeating fetch requests here as well.

Using fetch With Events

We aren't limited to sending fetch requests when a component is mounted. We can also tie them into events:

handleClick = event => {
  fetch('your API url')
    .then(res => res.json())
    .then(json => this.setState({data: json}))
}

render() {
  return (
    <button onClick={this.handleClick}>Click to Fetch!</button>
  )
}

This lets us send requests on demand. Submitting form data would be handled this way, using a POST request instead of GET.

A slightly more complicated example would be the infinite scroll of sites like Instagram. An event listener tied to changes in the scroll position of a page could fire off a handleScroll method that requests data before a user reaches the bottom of a page.

Using State with POST Requests

One of the beautiful features of state is that we can organize it however we need. If we were building a form to submit to a server, we can structure state to work nicely with what the server is expecting in a POST request.

Say we were building a user sign up form. When we send the data, our server is expecting two values within the body of the POST, username and password.

Setting up a React controlled form, we can structure our state in the same way:

state = {
  username: "",
  password: ""
}

//since the id values are the same as the keys in state, we can write an abstract setState here
handleChange = event => {
  this.setState({
    [event.target.id]: event.target.value
  })
}

render() {
  return (
    <form onSubmit={this.handleSubmit}>
      <input type="text" id="username" value={this.state.username} onChange={this.handleChange}/>
      <input type="text" id="password" value={this.state.password} onChange={this.handleChange}/>
    </form>
  )
}

Then, when setting up the fetch request, we can just pass the entire state within the body, as there are no other values:

handleSubmit = event => {
  event.preventDefault()
  fetch('the server URL', {
    method: "POST",
    headers: {
      "Content-Type": "application/json"
    },
    body: JSON.stringify(this.state)
  })
}

Notice how we're not bothering to worry about event.target when posting the data. Since the form is controlled, state contains the most up-to-date form data, and it is already in the right format!

Conclusion

There are no hard and fast rules for how to include fetch requests, and a lot of structure will depend on the data you're working with. As a general practice for writing simpler component code, include fetch calls in the same component as your top level state. You can always pass down methods as props that contain fetch.

Resources

• fetch