Lesson 3: AJAX

Michelle Colón
May 17, 2018 · 5 min read

For the sake of completeness, I’m going to start with what AJAX is an acronym for, which is: Asynchronous JavaScript and XML. I’ll get into what AJAX does in a sec, but first, I want to explain what the heck ‘Asynchronous’ even means.

An Asynchronous call is when you can move from one task to another, before the current one finishes. Conversely, there is a Synchronous call, which is when you have to wait for one thing to execute before moving onto the next. A good way of remembering it is to think of it as a line at the bank. An example of an Asynchronous set-up at the bank would be if there were multiple lines and multiple tellers- in this way, there are many deposits, withdrawals, etc. that are being executed all at once. A Synchronous set-up would be if there were only one line and one teller- in this instance, one person must be taken care of before the next can be attended to- only one thing can be done at a time.

Okay, so now that that’s out there- why do we use AJAX? We use AJAX when we want to update a page without navigating away from it. And, in order to use AJAX, we have to make HTTP Requests.

To make HTTP Requests, we need to know what the HTTP Methods are as well as what the HTTP Status Codes are.

HTTP Methods:

  • GET- Requests data from a specified resource; it pulls from the resource
  • POST- Submits data to be processed to a specified resource; it creates the resource
  • DELETE- Deletes the specified resource; it removes the resource

HTTP Status Codes:

  • 100-level: indicates that the request was received and understood, but nothing has happened yet. It is essentially saying: “We got the request. Hold on just a sec.”
  • 200-level: indicates that the action requested by the client was received, understood, accepted, and processed successfully. “Here you go, the request was successful!”
  • 300-level: indicates the client must take additional action to complete the request, often used for redirection. “Go away, you’re being redirected.”
  • 400-level: the error seems to have been caused by the client. “Yikes, you messed up…”
  • 500-level: the server failed to fulfill a valid request. “The server is messed up.”

To make an HTTP Request with AJAX, you need to use a method to call a function, or file, and handle the response upon a successful status code; and handle errors with an unsuccessful one.

Here is an example of an AJAX request:

In plain English, here is what each line is doing:

Line 1: Creates a new XMLHttpRequest object and assigns it to the variable a

Line 3: Adds an event listener to a (the object), that listens for readystatechange and that listens to the response of an API call made with a ; function (r) is a callback function, r being the captured event

Line 4: Gets the status of the event (r); we put in a status of 200 because the desired response is a successful one

Line 5: If we get a successful response, it will be printed via the .response method

Line 9: This is where we start to make the actual AJAX request! a opens up a GET call to the GitHub users API. When a.open runs, the event listener is triggered because the state change of a is going to change, which is when we’ll get r (the actual event object). Note: whenever you get an event object captured in an event listener, you do `whateveryoureventiscalled.target` to get the actual object that was captured by that event

Line 10: Sends the API call

*phew* That was a lot. But! There is a different way to make an AJAX request. There are libraries and APIs that can make an AJAX Request a wee bit cleaner. Fetch API (only in modern browsers) is a newer browser API that provides a more powerful and flexible feature set than XMLHttpRequests.

Here’s what using the fetch API looks like!

And here’s what the code is doing:

Line 1: Calls the fetch function on the URL

Line 2: After we get the response (from calling the fetch function on the URL), and the promise (more on promises in a bit) is fulfilled, we say ‘Hey, after these things are done, THEN …’

Line 3: This has the .json method in it, which takes the response we got and converts it to JSON (JavaScript Object Notation); this JSON is what is returned from the API.

Line 5: ‘After this is done, THEN…’ Whatever is returned in the first .then (Line 3) is passed in as the parameter to the next .then on this line.

Line 6: Printsj (in this case, in the console), j is the result of the previous function

This way of doing it is so much shorter! The .thens are a bit confusing, so let me provide a bit more information. First, it’s good to know that the then calls are very similar to callbacks, but they’re used in something called promises.

Promises are JavaScript objects which can be returned synchronously from an asynchronous function. For example, when you make an AJAX request, the response is not synchronous because you want a resource that takes some time to come, and it may never come if you have some kind of 400 or 500-level error. To handle these types of situations, we have promises.

Promises have 3 different states:

  • Pending: Incomplete; its still being worked on
  • Fulfilled: Complete
  • Rejected: Failed

Typically, you construct a promise object with functions built in that resolve it (a.k.a. make it hit the fulfilled state); or reject it (a.k.a. make it hit the failed state). Also, you can daisy-chain fulfilled promises with the .thenfunction. In our example, the fetch API handles the construction phase of promises.

NB: Fetch defaults to making GET requests (like we’re doing), but you can set it up to make POST requests, too.

Back to the example for a second: once we construct our GET request, this creates a promise that is pending. While this promise is pending, we call the first then, and when that promise is resolved, the function inside the thenwill be called. Then, the result of the first function is passed in as the variable r, which is a response object.

A response object is very specific to fetch and had specific features, such as:

  • Status- which will give an HTTP status code
  • A URL property, and several others…check out more on response objects here.

Here, at long last, is the result in the console:

That was a lot to digest, but it’s pretty cool! I’m going to be re-reading this post and going through this part of Cassidy Williams’ Udemy course a bit more so I can get a better handle on it.

Next up, JSON!