javascript - call a function every time i return a variable?

Question

I've noticed that in the Google text adventure game easter egg they have a function call everytime you enter in a variable on the javascript console.

How is this done?

Answer 1

It doesn't work for arbitrary variables. It doesn't work for arbitrary expressions. It only works for those commands that are given (yes, no, north, south, east, west, up, down, grab, why, inventory, use, help, exits, map, and friends).

It doesn't really work for variables anyway. What you get when you enter yes is the global window.yes property - and they have defined a getter for it. This getter will be evaluated, return the string "yes", and asynchronously log the next output to the console.