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CHAPTER 2 THE NUTS AND BOLTS OF AJAX Consider the following source code that seems correct, but will work incorrectly: function AsyncUpdateEvent() { window.alert( “Who’s calling (” + this.myState + “)”); } function GetIt(xmlhttp, url) { if( xmlhttp) { xmlhttp.open(’GET’, url, true); xmlhttp.onreadystatechange = AsyncUpdateEvent; xmlhttp.send(null); } } var xmlhttp1 = FactoryXMLHttpRequest(); xmlhttp1.myState = “xmlhttp1″; var xmlhttp2 = FactoryXMLHttpRequest(); xmlhttp2.myState = “xmlhttp2″; GetIt(xmlhttp1, ‘/chap02/serverhang.aspx’); GetIt(xmlhttp2, ‘/books/cgross’); The functions GetIt and AsyncUpdateEvent are like previous examples in which asynchronous function calls were made. New to the function GetIt is the additional parameter xmlhttp. This was added so that multiple XMLHttpRequest instances could be used with GetIt. The variables xmlhttp1 and xmlhttp2 represent two different instances of XMLHttpRequest, and assigned to each instance is the data member myState. To make two separate HTTP requests, GetIt is called twice with different XMLHttpRequest instances and different URLs. When the asynchronous XMLHttpRequest returns, the function AsyncUpdateEvent is called. The function AsyncUpdateEvent is assigned to the instance of either xmlhttp1 or xmlhttp2, and therefore in the implementation of the function, the thiskeyword should work. What happens is that the this.myState reference in the function is undefined, and therefore AsyncUpdateEvent has no idea to which XMLHttpRequest instance it is assigned. A solution would be to create two callback functions, AsyncUpdateEvent and AsyncUpdateEvent2, and assign them individually to the instances xmlhttp1 and xmlhttp2. The function GetIt would be updated to include an additional parameter that represents the callback where the request results are processed. Creating two callback functions would work but is not elegant because for three independent requests you would need three callbacks. The real context of this problem is that JavaScript in this instance has lost its object-oriented features. What needs to be solved is the association of an XMLHttpRequest instance with a callback, and that is solved in the next section.
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