fib.js

function fast_double_fib(n) {
  var a, b, c, d
  if (n == 0) {
    return [0, 1]
  } else {
    l = fast_double_fib(Math.floor(n / 2))
    a = l[0]
    b = l[1]
    c = a * (b * 2 - a)
    d = a * a + b * b
    d = a * a + b * b
    if (n % 2 == 0) {
      return [c, d]
    } else {
      return [d, c + d]
    }
  }
}

console.log(fast_double_fib(1000))