Javascript Basics Part 10

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Originally published at Internet.com

You've probably heard the term "AJAX" thrown around recently. Basically it's a fancy name for a technology that's been around for years. Over the past year or so, however, AJAX-style JavaScript has become increasingly popular among many developers and we're starting to see some amazing things come from it. Google Maps and GMail are two of the most well known AJAX applications, but companies all over the web are starting to incorporate it into their sites. AJAX stands for Asynchronous JavaScript And XML. All it really consists of is JavaScript, just like you would have in any other application, and an XMLHTTP connection to your web server. That's it! The general idea is that when you need to pull data from a server you can just go and get the specific data you need instead of refreshing the entire page. To start with, AJAX almost always relies on a server-side language such as PHP or ASP. When your users need to get new data, the JavaScript requests it, and the server will probably query a database and then return the data. This data can be returned in several forms. If it is structured, then you will generally have XML or JSON data. If it is very simple data (such as getting a description for an item), you will often see people just writing that data directly to the AJAX response. Creating an XMLHttp Object The first thing we need to do when creating an AJAX request is to create an XMLHTTP object. Netscape/Firefox, Opera and other browsers have this object built-in. Internet Explorer uses an ActiveXObject. So we create one function that works for all of these browsers: if(typeof(XMLHttpRequest)!='undefined'){ var getXMLHttpObj = function(){ return new XMLHttpRequest(); } } else { var getXMLHttpObj = function(){ var activeXObjects = ['Msxml2.XMLHTTP.6.0', 'Msxml2.XMLHTTP.5.0', 'Msxml2.XMLHTTP.4.0', 'Msxml2.XMLHTTP.3.0', 'Msxml2.XMLHTTP', 'Microsoft.XMLHTTP']; for(var i=0; i JSON: {contacts:[ {"firstname":"Joe", "lastname":"Smith", "phone":"555-1212"}, {"firstname":"Sam", "lastname":"Stevens", "phone":"123-4567"} ]} You may notice that the XML notation looks suspiciously like HTML. Well for the most part, it is. HTML and XML are both tag-based languages and they can even be parsed the same way (see the 6th article in this series for more information). Further, the JSON notation looks suspiciously like plain ordinary JavaScript. JSON stands for JavaScript Object Notation and as such it is just plain ordinary JavaScript. Either notation can just be sent as plain text from your web server. None of the spacing included in these examples is necessary except for the single spaces