JS cache decorator function

cachable.js

/**
 * Cacheable decorator function.
 * @returns cacheable function. Results will be cached based on first argument.
 **/
let cacheable = function (f) {
  var __ = {};
  return function () {
    if (__[arguments[0]]) {
      return __[arguments[0]];
    } else {
      __[arguments[0]] = f.apply(null, arguments);
      return __[arguments[0]];
    }
  };
};

examples.js

// example 1: not cached
let fib = function (n) {
  if (n === 0) {
    return 0;
  } else if (n === 1) {
    return 1;
  } else {
	return fib(n-1) + fib(n-2);
  }
};

// example 2: cached
let fib2 = cacheable(function (n) {
  if (n === 0) {
    return 0;
  } else if (n === 1) {
    return 1;
  } else {
	return fib2(n-1) + fib2(n-2);
  }
});

// results, times using console.time('...');
var n = 45;
fib(n);  // 15689.771ms
fib2(n); //     0.090ms