Iterative

var fib = function(n) {

 // base case (doesn't change for n <= 2)
 var currentNum = 1;
 var prevNum = 1;
 var prevPrevNum = 0;

 // iterate through all numbers from 1-n
 for (var i = 1; i <= n; i += 1) {
   if (i > 2) {
     prevPrevNum = prevNum;
     prevNum = currentNum;
     currentNum = prevNum + prevPrevNum;
   }
 }

 return currentNum;

};

Recursive

var fib = function(n) {

 // base case
 if (n <= 2) {
   return 1;
 }

 // recursive case
 else {
   return fib(n - 1) + fib(n - 2);
 }
};