BottomUp.js

function fib(n) {
   // edge cases:
   if (n === 0 || n === 1)
   return n
   /**
   * we’ll be building the fibonacci series from the bottom up
   * so we’ll need to track the previous 2 numbers at each step
   **/
   let prevPrev = 0 // 0th fibonacci
   let prev = 1 // 1st fibonacci
   let current // Declare current
   for (let i = 1; i < n; i++) {
     // Iteration 1: current = 2nd fibonacci
     // Iteration 2: current = 3rd fibonacci
     // Iteration 3: current = 4th fibonacci
     // To get nth fibonacci … do n-1 iterations.
     current = prev + prevPrev
     prevPrev = prev
     prev = current
   }  
   return current
}