How to Develop a Web Application with Ajax

See Ajax: Tips and Tricks for similar articles.

Ajax web applications use JavaScript to interact with a server and update web pages without reloading them. The following eight steps show how to develop a simple Ajax web application.

Create or find your data source URL. An Ajax web application needs a server or other source of data that can be accessed using a URL in order to update the page. For this example, we'll use the following text, saved in a file named data.txt.
```
Hello, World! This text is loaded using Ajax.
```
Create the HTML for your application. In the following HTML, I've used a basic HTML5 template containing an h1 element with an id attribute. I've also created a button that will trigger the Ajax functionality on the page when clicked.
```
<html>
<head>
	<title>An Ajax Web Application</title>
</head>
<body>
<h1 id="page-title"></h1>
<button id="load-data">Click Here to Load the Data</button>
</body>
</html>
```
Create a script block in your HTML (just above the closing body tag) and write an event listener to detect click events on the button.
```
<script>
document.getElementById("load-data").addEventListener("click",function(){

});
</script>
```
Use an XMLHttpRequest object inside the event listener's callback function to request the data file.
```
var xmlhttp = new XMLHttpRequest();
xmlhttp.open("GET", "data.txt");
```
To detect the response, look for a change in the state of the xmlhttp response, using the onreadystatechange event handler.
```
xmlhttp.onreadystatechange = function() {
	//Do something here
}
```
Write a callback function to insert the data from the text file into the h1 element when the state of the response changes.
```
xmlhttp.onreadystatechange = function() {
	document.getElementById("page-title").innerHTML = this.responseText;
}
```

Use the send() method to send the request.

xmlhttp.send();

The finished HTML page should look like this:

<html>
<head>
	<title>An Ajax Web Application</title>
</head>
<body>
<h1 id="page-title"></h1>
<button id="load-data">Click Here to Load the Data</button>
<script>
	document.getElementById("load-data").addEventListener("click",function(){
		var xmlhttp = new XMLHttpRequest();
		xmlhttp.open("GET", "data.txt");
		xmlhttp.onreadystatechange = function() {
			document.getElementById("page-title").innerHTML = this.responseText;
		};
		xmlhttp.send();
	});
</script>
</body>
</html>

Open the HTML page in a browser and click the button. The script will produce the following output in a browser: