```ts
/** Factorial function that accepts a single parameter n (non-negative integer). */
export function factorial(n: number): number {
  if (!Number.isInteger(n) || n < 0) {
    throw new Error("Input must be a non-negative integer");
  }
  if (n === 1) {
    return 1;
  } else {
    return n * factorial(n - 1);
  }
}
```