factorial method

int factorial()

Returns the factorial of this number (must be a non-negative integer).

5.factorial() // 120

Implementation

int factorial() {
  final n = toInt();
  if (n < 0) throw ArgumentError('factorial requires a non-negative integer');
  if (n == 0 || n == 1) return 1;
  var result = 1;
  for (var i = 2; i <= n; i++) {
    result *= i;
  }
  return result;
}