This variable stores a string of the last key pressed. This variable is not read only and you can change it, for example to set it to "" (an empty string) if you handled it already.
if keyboard_lastkey != -1
str += keyboard_lastchar;
keyboard_lastkey = -1;
The above code checks to see if the lastkey variable is greater than -1, and if it is it adds whatever the last key was as a string to the variable "str".