Considering the input 0 in let p = Number, I don't understand how this code still works if p = 1 is outside the if and else of the countdown and count up.

function count() {
  let i = Number
  let f = Number
  let p = Number
  if (p <= 0) {
    window.alert('Impossible to count if 0. Considering 1.')
    p = 1
  }
  if (i < f) {
    // Count up
    for (let c = i; c <= f; c += p) {}
  } else {
    // Count down
    for (let c = i; c >= f; c -= p) {}
  }
}
}

Why is it not necessary to create another for for if (p <=0)?