AJAX, Fetch, and Async/Await

Learning Objectives

Students Will Be Able To:

• Describe the Use Case for AJAX

• Use ES2017's async/await to handle promises synchronously

Roadmap

1. Setup

2. AJAX - What & Why

3. Make an HTTP Request Using the Fetch API

4. Use ES2017's async/await to Handle Promises

Setup

• We'll be using Repl.it during this lesson to learn about AJAX and async/await.
• Create a new HTML, CSS, JS repl and name it something like AJAX with Fetch.

AJAX - What & Why

• AJAX is short for Asynchronous JavaScript And XML.

• In case you're wondering what the XML is about... It's the granddaddy of all markup languages, including HTML.
• Once upon a time, XML was the de facto format for transferring data between two computers - that's why it's in the name AJAX. However, JSON has since become the data transfer format of choice.

AJAX - What & Why

• Clients (browsers) use AJAX to make HTTP requests using JavaScript.

• The browser can send AJAX requests to any API server, as long as the server is CORS compliant.
• Using AJAX, we can send an HTTP request that uses any HTTP method including, GET, POST, PUT & DELETE - no need for method-override!

AJAX - What & Why

• But, here's the best part - unlike when we click a link or submit a form on a web page, AJAX does not trigger a page reload!

• We can use AJAX to communicate with servers to do lots of things, including to read, create, update & delete data without the user seeing a page refresh.

• AJAX has made possible the modern-day Single Page Application (SPA) like what you're going to build during this unit!

AJAX - What & Why

• AJAX was originally made possible back in 1998 when IE5 introduced the XMLHttpRequest (XHR) object and today it's in all browsers. However, it's a bit clunky to use.

• One of the reasons jQuery became popular was because it made making AJAX requests easier.

• However, we no longer have to use the XHR object or load jQuery to make AJAX calls thanks to the Fetch API which is part of the collection of Web APIs included in modern browsers.

Make an HTTP Request Using the Fetch API

• So, the A in AJAX stands for asynchronous.

• Indeed, making an AJAX request is an asynchronous operation. So far, we've seen two approaches that enable us to run code after an asynchronous operation has completed. ❓ What are they?

Make an HTTP Request Using the Fetch API

• The Fetch API, like any new Web API asynchronous method, uses promises instead of callbacks.

• Let's make a GET request to the /users endpoint of JSONPlaceholder, a fake RESTful API for developers:

    fetch('https://jsonplaceholder.typicode.com/users')
    .then(response => console.log(response))

  When ran, we'll see that the fetch method returns a promise that resolves to a Fetch Response object, which has properties such as status, etc.

Make an HTTP Request Using the Fetch API

• To obtain the data in the body of the response, we need to call either the text or json method which returns yet another promise:

    // fetch defaults to making a GET request
    fetch('https://jsonplaceholder.typicode.com/users')
    .then(response => response.json())
    .then(users => console.log(users))

  As you can see, the json() method returns a promise that resolves to the data returned by the server, as JSON.

Use ES2017's async/await to Handle Promises

• Before we continue to use fetch any further, let's see how to use a fantastic new way of working with promises: async & await
• The purpose of async/await is to allow us to work with asynchronous code almost as if it were synchronous!

Use ES2017's async/await to Handle Promises

• We use the async declaration to mark a function as asynchronous when promises are going to be handled using await within it.

• We can re-write our code to use async/await like this:

    async function printUsers() {
      const endpoint = 'https://jsonplaceholder.typicode.com/users';
      let users = await fetch(endpoint).then(res => res.json());
      console.log(users);
    }
  
    printUsers();

  The await operator causes the line of code with fetch to "pause" until the promise resolves with its value - in this case an array of users.

Use ES2017's async/await to Handle Promises

• When using async/await, we cannot use a .catch() to handle a promise's rejection, instead we use JavaScripts's try/catch block:

    async function printUsers() {
      const endpoint = 'https://jsonplaceholder.typicode.com/users';
      let users;
      try {
        users = await fetch(endpoint).then(res => res.json());
        console.log(users);
      } catch(err) {
        console.log(err);
      } 
    }

  The catch block would run if the fetch failed.

Use ES2017's async/await to Handle Promises

• So basically, we've seen that async/await replaces the .then(<function>) method for when a promise resolves.

• In addition, JavaScript try/catch blocks replace the .catch(<function>) for error handling when a promise is rejected.

💪 Practice Exercise (2 MIN)

• After the console.log(users), add another AJAX request using fetch to JSONPlaceholder's /posts endpoint.

• Log out the returned posts.

References

• MDN - Async Functions
• MDN - Await Operator