Ty, this was a great walk through TDDing a problem for me, a recent bootcamp graduate.

suggestions -

1. after

it("pushes digits to the stack",()=>{...})

add:

it('ends a program correctly', () => {
  expect(interpreter('1.@2.@')).to.equal('1')
})

2. missing definition from level 5:
   , Pop top of stack and output as ASCII character

3. Perhaps extend test time limit for random movement to 15. When I had console logs in my solution, I would reliably fail this at 10ms execution time.