memoize_fibonacci.js

function fibonacci(n) {
  return n < 2 ? 1 : fibonacci(n-1)+fibonacci(n-2)
}

function memoize(fn) {
  var cache = {}
  return function(x) {
    if (x in cache) {
      console.log('retrieved value from cache for', x)
    }
    return x in cache ? cache[x] :cache[x] = fn.apply(this,arguments)
  }
  
}

fibonacci = memoize(fibonacci)


console.log(fibonacci(1))
console.log(fibonacci(4))
console.log(fibonacci(5))

// → node memoize_fibonacci.js
// 1
// retrieved value from cache for 1
// retrieved value from cache for 1
// retrieved value from cache for 2
// 5
// retrieved value from cache for 4
// retrieved value from cache for 3
// 8