# Testing for a variable's existence

Before relying on a variable's existence in an application page, you can test to see if it exists using the IsDefined function. For example, the following code checks to see if a Form variable named Order_ID exists:

    <CFIF Not IsDefined("FORM.Order_ID")>
        <CFLOCATION URL="previous_page.cfm">
    </CFIF>

The argument passed to the IsDefined function must always be enclosed in double quotes. See the CFML Language Reference for more information on the IsDefined function.

If you attempt to evaluate a variable that has not been defined, ColdFusion will not be able to process the page. To help diagnose such problems, use the interactive debugger in ColdFusion Studio or turn debugging on in the ColdFusion Administrator. The Administrator debugging information shows which variables are being passed to your application pages.