I've never written a line of Javascript in my life.

    a.filter(x=>1)

guessing that arrow looking thing is syntax sugar for a lambda that always returns a 1, then it's implicitly cast to boolean true in the filter, so nothing gets filtered out and b ends up just the same as a.

    !!1

not not 1 -> true

    b.some(e=>e>6)

at least one element of b greater than 6, clearly true

so both arms of the && are true, so foo() gets called every time.

I don't think this is a very good test if someone like me can pass it just by making educated guesses.