Default Vaalue in ComboBox

First, the code you provided above

DropDownList1.DefaultValue = "a";

is incorrect in both AcroForms and LiveCycle forms JavaScript. Let's assume that you are using AcroForms. Then the default is set with:

this.getField("DropDownList1").defaultValue = "a";

Where "a" is an existing entry on the Combox list. This code sets the default value for the field. It does not display it. Default values for all fields, in any forms technology, are displayed when the form is reset. In AcroForms, a form is reset with.

this.resetForm();

Which resets the entire form. If you just want to reset the combobox then use this

this.resetForm("DropDownList1");