[valenterry]

That one is rather simple to break.

    console.log(add({a:"1", b:2})) //12
    console.log(_add({a:"1", b:2})) //21

So to make the code break with the change to _add, I can just do:

    if(add({a:"1", b:2}) != 12) boom()

[benburton]

Ah of course!