fibonacci.js

var fPrev, fCurr;

function fib (n) {
  var result;
  
  if (n < 0) throw Error('invalid given number');
  
  if (n <= 1) {
    return n;
  }
  
  fPrev = 0;
  fCurr = 1;
  
  for (var i = 2; i <= n; i++) {
    var newCurr = fPrev + fCurr;
    
    fPrev = fCurr;
    fCurr = newCurr;
  }

  return fCurr;
}