fibonacci.js

function memoize(fn) {
  let cache = {}
  return function (...args) {
    if (cache[args]) {
      return cache[args]
    }
    const res = fn.apply(this, args)
    cache[args] = res
    return res
  }
}

const fib = memoize((n) => (n < 2) ? n : fib(n - 1) + fib(n - 2))

function assertion(condition, message = 'Assertion failed') {
  if (!condition) {
    throw message
  }
}

console.time('fibonacci time'); 

assertion(fib(0) + fib(1) === fib(2))

console.timeEnd('fibonacci time');