The ServerVariables Collection

Another useful built-in ASP object is the Request Object. This object deals with server input, with information transmitted from a Web page to the server. This information is packaged within collections, which are arrays of values collected from a user's URL request for a Web page. Among other features, the Request Object contains the ServerVariables collection. This set of items contains information taken from HTTP headers that are transmitted from the browser to the server when the user makes a URL request. The collection contains information about the browser being used along with other information about the page being accessed.

Among the 50 or so items in the ServerVariables collection, the following ones are particularly useful:

Values in this collection are accessed by using the reference

This is a reference to the Request object, the ServerVariables collection, and the specific name of the variable in the collection. We can display the value of any variable by using this reference within a Response.Write() statement. Thus, if we code

we produce the output

which is the value of the server variable HTTP_USER_AGENT identifying the browser being used when a request was made for this page. This reference is, of course, to the browser you are currently using.

Determining Browsers

For certain ASP applications it may be necessary to know which browser the user is running. Because of incompatibilities between Internet Explorer and Navigator and because of differences in their versions, you may need to provide different coding for different browsers and versions. Therefore, you can include an ASP script on a page to determine a user's browser and to provide alternate coding or alternate pages for different browsers.

It is a matter, then, of parsing these string values to determine which browser is being used.

Among other differences in the values, note that Internet Explorer is identified by the substring "MSIE" which appears for all versions of that browser. Navigator does not produce this substring. So, one way to identify Internet Explorer is to see if "MSIE" appears in the value returned by the HTTP_USER_AGENT variable. Otherwise, this is Netscape Navigator or a compatible browser.

VBScript has an InStr function that checks for the presence of a substring within a text string. The general format for this function is:

The function checks for the presence of substring within string. If it locates the substring of characters, it returns the starting postion of the substring within the string. If the substring of characters is not present within string, then the value 0 is returned. So, the following script

produces the following output for the browser you are currently using:

You are using Netscape Navigator.

Thus, at the top of your ASP pages you can code a routine such as the following:

Then, throughout your page you can apply different processing or formatting by checking the value of variable Browser:

Determining The Browser Version

You can also determine the browser version by referencing the HTTP_USER_AGENT substring containing the version number. In modern browsers these values are at postion 31 (for three characters) in Internet Explorer and position 9 (for four characters) in Navigator:

You can pick out these version numbers using the VBScript Mid function to extract a substring from a string. Its general format is

where string is the string containing the substring of characters, start is the starting position of the substring, and length is the number of characters to extract as the substring. The following script first checks to see whether the visitor's browser is Internet Explorer or Navigator and then extracts the version number substring, assigning it to variable Version:

Your browser is Navigator 5.0 .

Determining Other Header Information

There is additional useful information in the URL headers transmitted to the server. The following table shows this information pertinent to your current visit to this page. The values displayed in bold are the real-time server values displayed with Response.Write() statements.