Need javascript for button to set all fields to read only with pop up prompt

Hi, 

As a newbie, I'm stuck.  

This is as far as I could get in setting up a "Done" button to lock all the fields as read only.

for (var i = 0; i < this.numFields; i++)

{

if (app.alert({nType:2,nIcon:2,cMsg:"This will lock the document for printing and email submission. Are you sure you want to continue?"})==4)

{

this.getField(getNthFieldName(i)).readonly=true;

}

else

{

}}

The pop up works, but the script hangs up at the pop up window and I have to Force Quit the app.  I can get the fields to set to read only without the pop up, however the folks using this form will need to understand the consquences. of selecting the "Done" button.