```typescript
/** The factorial of n. */
export function factorial(n: number): number {
  /**
   * Calculates the factorial of a given integer.
   *
   * @param n - A non-negative integer.
   * @throws {Error} If n is negative or not an integer.
   * @returns The factorial of n.
   */
  if (typeof n !== 'number' || !Number.isInteger(n) || n < 0) {
    throw new Error('Input must be a non-negative integer.');
  }
  if (n === 0 || n === 1) {
    return 1;
  }
  return n * factorial(n - 1);
}
```