AJAX enables

  • loading data into your web page from a web server...
  • ...**after** the page initially loads!

AJAX Examples

  • Load the comments on an article later, so the rest of the page is usable sooner
  • Once a minute, check for any new comments and display them too
  • Load today's weather forecast from, and update it every hour
  • Load an ad from a different web server and switch ads every few minutes
  • Dynamically display search results as the user types their query

AJAX Definition

Asynchronous JavaScript And XML

  1. Request data from an external source
  2. Parse the data returned by the request
  3. Load that data into the page without a refresh
  4. Data can be any formats, most common:
    • JSON
    • HTML

AJAX Advantages/Disadvantages


  • Page components can be loaded individually
  • New data can be loaded asynchronously
  • User interacts with the page and sees results immediately


  • JavaScript must be enabled
  • Adds complexity to JavaScript applications
  • Without refreshes the page state can get bloated
  • Screen-readers cannot read the whole page at once

AJAX History

  • (1998) First implemented by Microsoft for use in Outlook email web client
  • (1999-2003) Several other browsers copied Microsoft for compatibility
  • (2004) Google used XMLHttpRequest for loading data in Gmail and Google Maps
  • (2005) AJAX Name coined by Jesse James Garrett [0]


Jesse James Garrett Quote

Ajax isn’t a technology. It’s really several technologies, each flourishing in its own right, coming together in powerful new ways. Ajax incorporates:

  • standards-based presentation using XHTML HTML and CSS

  • dynamic display and interaction using the Document Object Model

  • data interchange and manipulation using XML and XSLT JSON

  • asynchronous data retrieval using XMLHttpRequest

  • and JavaScript binding everything together.

This was the moment that people realized web applications could be a competitor to desktop applications, and could run on every computer in the world.

XMLHttpRequest (Old Way)


<button id="fire-away" type="button">Fire the request</button>

(function() {
  var httpRequest;
  document.getElementById("fire-away").addEventListener('click', runAjax);

  function runAjax() {
    httpRequest = new XMLHttpRequest();

    if (!httpRequest) {
      alert('Giving up :( Cannot create an XMLHTTP instance');
      return false;
    httpRequest.onreadystatechange = alertContents;'GET', response.json);

  function alertResponse() {
    if (httpRequest.readyState === XMLHttpRequest.DONE &&
        httpRequest.status === 200) {
    } else {
        alert('There was a problem with the request.');

Browser Fetch API - Remote

  • Please type this URL into the address bar of your browser

let postNumber = 1;
fetch('' + postNumber)
  .then(function(response) {
    return response.json();
  .then(function(myJson) {
  • fetch() takes at least one argument, the URL of the resource to fetch
  • fetch then calls the server, just like you did in the address bar above
  • The server passes the Response from the server into the first callback function
  • response.json parses the body of the response as JSON
  • The body of the response is then logged to the console

Browser Fetch API - Local

If you want to request data from a local webserver

  .then(function(response) {
    return response.text();
  .then(function(myText) {
  • This happens to be response.text because it is a Markdown file
  • If it were JSON then response.json would be used

Browser Fetch API - Errors

  • The system will raise errors as exceptions by default
  • If you want to handle errors catch them like shown below
  • Use .catch(function(error) { do_something_here })

  .then(function(response) {
    return response.json();
  .then(function(myJson) {
  .catch(function(error) {
    console.error('Yikes! I should handle this better:\n', error);

JSON (JavaScript Object Notation)

See a more comprehensive JSON lesson here: JSON lesson


{"userId": 1,
  "id": 1,
  "title": "My most amazing post",
  "body": "This is my first post, isn't it great. Maybe I'll write some more."


  • No comments
  • No functions
  • Only data is allowed
    • Objects (Hashes), Arrays, Numbers, Booleans, Strings

Parsing & Producing JSON

The Fetch API converts text into JSON for you if you call response.json()

but if you want to do it yourself...

  • Assume textual JSON is in a string named text:
let text = '{ "name": "Ada Lovelace", "id": 1, "title": "The Queen of Numbers" }'

This converts the String data into a JavaScript object:

let data = JSON.parse(text)

And this converts the JavaScript object back into a String:

let newText = JSON.stringify(data)


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