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17.1 AJAX Intro

AJAX is a developer's dream, because you can:
- Read data from a web server - after the page has loaded
- Update a web page without reloading the page
- Send data to a web server - in the background

AJAX Example

Let AJAX change this text
Change Content
AJAX
AJAX is not a programming language.
AJAX is a technique for accessing web servers from a web page.
AJAX stands for Asynchronous JavaScript And XML.

Try it yourself

AJAX Example Explained

HTML Page

<!DOCTYPE html>
<html>
<body>

<div id="demo">
  <h2>Let AJAX change this text</h2>
  <button type="button" onclick="loadDoc()">Change Content</button>
</div>

</body>
</html>

The HTML page contains a <div> section and a <button>.
The <div> section is used to display information from a server.
The <button> calls a function (if it is clicked).
The function requests data from a web server and displays it:

Function loadDoc()

function loadDoc() {
  var xhttp = new XMLHttpRequest();
  xhttp.onreadystatechange = function() {
    if (this.readyState == 4 && this.status == 200) {
     document.getElementById("demo").innerHTML = this.responseText;
    }
  };
  xhttp.open("GET", "ajax_info.txt", true);
  xhttp.send();
}

What is AJAX?

AJAX = Asynchronous JavaScript And XML.
AJAX is not a programming language.
AJAX just uses a combination of:
- A browser built-in XMLHttpRequest object (to request data from a web server)
- JavaScript and HTML DOM (to display or use the data)
AJAX is a misleading name. AJAX applications might use XML to transport data, but it is equally common to transport data as plain text or JSON text.
AJAX allows web pages to be updated asynchronously by exchanging data with a web server behind the scenes. This means that it is possible to update parts of a web page, without reloading the whole page.

How AJAX Works

Image courtesy of W3Schools.com

An event occurs in a web page (the page is loaded, a button is clicked)
An XMLHttpRequest object is created by JavaScript
The XMLHttpRequest object sends a request to a web server
The server processes the request
The server sends a response back to the web page
The response is read by JavaScript
Proper action (like page update) is performed by JavaScript

JavaScript Fundamentals course home
Module 17. JS AJAX

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