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The first method uses the ActiveXObject to create a reference to the Microsoft.XMLHTTP ActiveX control. The second method simply creates a new instance of the XMLHttpRequest object itself. Once this object has been defined, your client script has a set of properties and methods that can be used to retrieve data from a defined server. The table below contains a list of the properties supported by the XMLHttpRequest object.

The onreadystatechange property defines a function to be called when the value of the readyState property changes. The readyState property returns a value identifying the state of the XML request. Values returned are shown below:

• 0 = uninitialized
• 1 = loading
• 2 = loaded
• 3 = interactive
• 4 = complete

The responseText property reads the response from the server as a text string. The responseXML property also reads the server response, but this property returns the data in an XML document object. The status property retrieves the numeric status of the XML request. Common codes returned by this property include 404 to indicate a document is "Not Found" or 200 for an "OK" status. The text associated with the status property can be retrieved via the statusText property. In addition to these properties, the XMLHttpRequest object also supports a set of methods, as shown in the following table:

The open() method allows us to define information related to the server and document from which the data is being retrieved. The parameters supplied on this method are as shown here:

The method parameter identifies the method used to send data to the server using a value of "GET" for retrieving data from the server or "POST" for sending data to the server. The URL value supplies the uniform resource locator (URL) location of the document to be read. The asnyc parameter represents a true/false value that identifies whether or not the connection is to be made asynchronously. If the connection is made asynchronously, the script continues execution without waiting for results from the server. If the ansyc value is false, the script will wait for a response from the server before continuing execution. Because many factors come into play when connecting to a remote system, it's not always the best idea to make connections that are not asynchronous. If, for example, a server is down or unreachable, you wouldn't want your script to hang indefinitely.

The open() method defines the connection to the server, but the connection is not actually initiated until the send() method is executed. The optional parameter for this method is used to supply data to be posted to the server as a string. As mentioned earlier, when a connection is made asynchronously, the script continues execution immediately after this statement is executed; otherwise, the script will wait for a response from the server before continuing. Below is a sample of the code required to make a connection using these two methods and the XMLHttpRequest object.

In this example, the variable parms is created to contain data to be posted to the server. The XMLHttpRequest object is then defined and used with the open() method to connect to the defined Web server using the POST method. The send() method is then used to open the connection using the parameters defined in the first part of the script.

The setRequestHeader() method identifies HTTP header values. The two string parameters supplied to this method represent the HTTP header name and the value to be supplied for that header. The example below illustrates the use of this method to define the HTTP header Content-Type value:

Note that the first parameter identifies the header name, while the second parameter identifies the new value for the header. This example identifies that the MIME content type for the document being accessed is a Microsoft Excel file.

These basic properties and methods of the XMLHttpRequest object give us the ability to retrieve a plethora of information from readily available Web services. Next time, we'll explore a practical example of utilizing AJAX within an HTML Web page.