使用递归

function factorial(num) {
  if (num < 0) {
    return -1;
  }
  if (num === 0 || num === 1) {
    return 1;
  } else {
    return (num * factorial(num - 1));
  }
}
console.log(factorial(5));
// 120

使用for循环

 function factorial(num) {

  if (num < 0) {
    return -1;
  }
  if (num === 0 || num === 1) {
    return 0;
  } else {
    for (var i = num - 1; i >= 1; i--) {
      num *= i;
    }
  }
  return num;
}

console.log(factorial(5));

// 120

使用while循环

function factorial(num) {
  var result = num;
  if (num < 0) {
    return -1;
  }
  if (num === 0 || num === 1) {
    return 0;
  } else {
    while (num > 1) {
      num--;
      result *= num;
    }
  }
  return result;
}
console.log(factorial(5));

// 120