Programming ColdFusion MX, 2nd Edition

Dealing with Nonexistent Parameters

The final thing to consider when passing data between templates is how to check for and handle missing parameters. When dealing with data passed via URL or form field, it is entirely possible that an application template won’t receive a parameter it is expecting. You should build mechanisms into your application templates to account for this possibility. As always, there are a few ways you can go about this.

The first method involves using the cfparam tag to assign a default value to any parameters expected by your application template. That way, if an expected parameter isn’t passed, a default value is automatically assigned, and the application can continue processing. If you remember, the cfparam tag was covered earlier in Chapter 2. Additional information on the cfparam tag can be found in Appendix A.

The cfparam tag is great for assigning a default value if one doesn’t exist; however, it doesn’t allow you to display an error message or perform an alternate action if an expected variable doesn’t exist. The next method uses the IsDefined( ) function to check for the existence of a variable before allowing processing of the template to continue. If the expected variable exists, the template is processed. If not, an error message is written to the browser. For example, if you have a template that is expecting a URL parameter named ArticleID to be passed in, you can use the following code to output an error message if the parameter isn’t present: ...