fast_fibonacci.js

function fibonacci(n) {
	if (n < 0) throw RangeError("Negative arguments not implemented")
	return fib(n)[0]
}

function fib(n) {
	if (n) {
		const [a, b] = fib(~~(n / 2)),
		  c = a * (b * 2n - a),
		  d = a * a + b * b
    return n % 2 ? [d, c+d] : [c,d]
  }
  return [0n, 1n]
}