```
const MODULUS = 1000000007;

/** The nth Fibonacci number, modulo 1000000007. */
export function fibMod(n: number): number {
  if (!Number.isInteger(n) || n < 0) {
    throw new Error('n must be a non-negative integer');
  }

  let a = 0;
  let b = 1;

  for (let i = 2; i <= n; ++i) {
    [a, b] = [b, (a + b) % MODULUS];
  }

  return n < 2 ? n : b;
}
```