function popper(a) {
  return a.pop();
}

function foo(x) {
  for (var i = 0; i < 10; i++) {
    var q = popper(x);
    if (i < 5)
      assertEq(q, 5 - i);
    else
      assertEq(q, undefined);
  }
  return q;
}

var q = foo([1,2,3,4,5]);
assertEq(q, undefined);