Javascript for auto populate field based on checkboxes

I guess the title of the discussion is a bit misleading because I have already figured out how to fill out a text field based on the value entered on another text field if a certain box is checked, however: 

If the second checkbox is checked (var b2), I would like the user to be able to free text in that field. So essentially, I am asking how do I write Javascript for "Free Text" in an if statement. 

I tried not having a second variable at all, however, if the user accidentally selects the first checkbox, the value remains in the text field even if the checkbox is unchecked afterwards. 

var b1 = this.getField("RCB1").isBoxChecked(0);
var b2 = this.getField("RCB1").isBoxChecked(1);

if(b1)
event.value = this.getField("DLAST").value

if(b2)
event.value = (????????????????)