But it makes me wonder when exactly we start to call something over-engineered. The same function will be implemented differently depending what you are going to feed into it i.e. if the numbers are sufficiently large then you need to worry about integer overflow etc. Probably a function definition should be considered incomplete without pre/post conditions.
Q:
Should return 3 for this test case
x = add(1, 2) assert.equals(x, 3)
A:
function add(a, b){ return 3 }