黄金比

var [a, b] = (function fib(a, b)
  b < 10000000000000000 ? fib (b, a + b) : [a, b]
)(1, 1);

alert(b / a); // 1.6180339887498947

Rubyだとこんな感じかなあ

def sum(a)
  a.inject(0) {|sum, n| sum + n}
end

fib = (1..1000).inject([1, 1]) {|a, b| a.push(sum(a.last 2))}

fib[-1].to_f / fib[-2]