How can I copy the form field values from one set of fields to another using javascript.

The idea here is to have a 'use shipping/billing address' type of button that copies the user information from one block of fields to another identical set of fields.

Right now, I call an action upon click of a button to execute the following javascript:

this.field1.value = this.field2.value;

However that action yields an 'undefined' error in the debugger.