Problem calling a javascript function from an Action/Value field?

Hi

I am hoping someone can help. I have an on-screen keyboard to allow a user to enter their employee number as part of a sign on process. The end product is captured in a variable called 'Driver' and displayed via a text box. This part is working fine. The problem I am having is allowing the user to press a backspace, variable 'bksp', key to delete the employee number 1 character at a time if they make a mistake. I have found the solution using javascript, substring or slice, and this works fine as a standalone piece of code but I cant seem to get it to work inside lectora. It only needs to work on the one page.

I have the code in an external HTML object, are the settings are correct?

object name: backspace_key

object type: header scripting

code:var y = VarDriver.getValue();

var x = substring(0, y.length - 1);

Varbksp.set(x);

Have I missed something in the code?

Action:

On: mouse click

Action: Modify Variable

Target: BkSp

Value: javascript:backspace_key

Modification type: Set Variable contents

What do you put into the Value field within the action to call the javascript?

I am trying to make the elearning as real as possible as the crews will be using touchscreen technology within the vehicles so it is better for me to simulate the entry from an on-screen keyboard rather than have the user use a standard keyboard which would be input into an Entry field, which is my backup if all else fails :(

Any help is appreciated.

Cheers

Colin