With the advent of the XMLHttpRequest (XHR) object, the Asynchronous JavaScript And XML (AJAX) era began. Figure 1.4 illustrates the AJAX web application architecture.
After the client's initial request, the server sends back a web page response containing HTML, CSS, and JavaScript. Once the web page has finished loading, the JavaScript application on the client side may initiate asynchronous requests back to the web server over HTTP, using the XHR object.
Some observers have characterized the advent of AJAX as the Web 2.0 era, where websites became more interactive with more rich user experiences and the use of JavaScript libraries started to gain traction.
Figure 1.4: The AJAX web application architecture
Because the XHR calls are asynchronous in nature, they don't block the single threaded JavaScript application running in the web browser. Once a response is received from the server for a given XHR request, an action can be taken with the data that was returned from the server.