js 的普通函数

function factorial(n){
  var product = 1;
  while(n>1){
    product *= n;
    n--;
  }
  return product;
}
factorial(4)

function factorial2(n) {
  var i,
  product = 1;
  for (i = 2; i <= n; i++) {
    product *= i;
  }
  return product;
}
factorial2(2)

function abs(x) {
  if (x >= 0) {
    return x;
  } else {
    return - x;
  }
}
abs(-2)