Doing the same Fibonacci operation, with very different performance

fibbonacci.js

//easy to visualize and code, but...
function fib(n) {
   if (n <= 1) return 1;
   return fib(n - 1) + fib(n - 2);
}
//very time-consuming.

//SOLUTION
//using memoization
function fib(n) {
   if (n in memo) return memo[n];
   if (n <= 1) return 1;
   memo[n] = fib(n - 1) + fib(n - 2);
   return memo[n];
}