Javascript code to undo Javascript read only code when signature cleared

++Adding to the topic,

The signature field object won't work with as a listener event the way you have it run from the target signature field.

As suggested earlier by @Bernd Alheit , you need to incorporate another button with a Mouse-Up  javascript action or you may simply opt to use a hidden read-only field with a Validating script or Custom Calculation Script to make the desired action appear as if it is tied up with the signature as soon as the user interacts with it to clear it.

You may achieve this in different ways with Acrobat JavaScript. As a result, in my examples below I used a couple of script methods in the absence of the feature that you are asking about.

For instance, you may try a custom calculation script executing from the hidden text field like so:

var f = this.getField("mySigningField");

if (f.value =="") this.resetForm();

You may also do the same with a custom validating script as shown in my next example. 

I am using a longer script based on what you've shared and I am running it as a custom validating script ( also executed in the hidden field which  produces the same result):

var checkSignature = this.getField("mySigningField");

 for(var i=0; i<this.numFields; i++) 
 {
  var fname = this.getNthFieldName(i);
  var f = this.getField(fname);

   if(f.type !="button" && f.readonly == true) 
   {
    if(checkSignature.value ="") 
    {
     this.resetForm();
    }
   }
 }
 

However, if you need this workflow to be exclusively tied up with the signature field (no additional buttons or hidden fields), then the single line of script shown below can do the trick as a "Mouse Exit" action:

if(event.target.valueAsString =="") this.resetForm();

This last method assumes that only one signature field object exists in that PDF.

If you have more than one signature fields and you wish to keep them as read-only soon after the target signature field is cleared, then you must define  through the "this.resetForm()" function just  the field objects that you want to clear (keeping the additional signature fields untouched as read-only).

I think this last method is more in-tune with your desired workflow. So, when the user clears the signature field, and as soon as the mouse pointer is moved outside of that signature field, it triggers the reset form action.