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