```ts
/**
 * a divided by b, safely.
 * Throws an Error if the divisor is zero or either argument is not a finite number.
 */
export function safeDivide(a: number, b: number): number {
  if (Number.isNaN(a) || Number.isNaN(b)) throw new Error('Both arguments must be numbers.');
  if (b === 0 || b === -0) throw new Error('Divisor cannot be zero.');
  return a / b;
}
```