Here's a bad, bad implementation of Fibonacci sequence. And yet it's the best I'm capable of coming up with by myself.

fibonacci.js

function fibonacci( n )
{
  prev_1 = total = 0;
  prev_2 = 1;
  if ( n == 0 )
    return 0;
  if ( n < 3 )
    return 1;
  for ( var i = 0; i < n - 1; i++ )
  {
    prev_1 = prev_2;
    prev_2 = total;
    total = prev_1 + prev_2;
  }
  return total;
}