Creating a Backspace Button

I have created a keyboard on Taco HTML via a table of buttons and within this keyboard I need a functioning Backspace button using javascript. I currently have a 'reset' button which clears all of the values in the text box.

If anyone could help on the coding for this backspace button it would be greatly appreciated :)

No idea what a Taco is other than a wonderful food source, but...

Can't you just do
fld.value = fld.value.substring( 0, fld.value.length-1 );

???

fld.value = fld.value.substring( 0, fld.value.length-2 );

???? - 2 ??? Why? Instead of -1 ???