Simple caching algorithm for javascript functions that can be optionally added as middleware

cache.js

function sleep(seconds) 
{
  var e = new Date().getTime() + (seconds * 1000);
  while (new Date().getTime() <= e) {}
}

var Obj = function() {
    this.fn = function(arg) {
        console.log('Long operation zzzz...');
        
        sleep(8);

        return arg;
    }
}

Obj.prototype.cache = function() {
  var self = this;

  if(!this._cache) {
    this._cache = {}
    console.log('Creating cache!');
  }

  var orig = {
    fn: this.fn
  };

  return {
    fn: function() {
      var arg = arguments[0];

      if(self._cache[arg] !== undefined) {
        console.log('From cache!');
        return self._cache[arg];
      } else {
        self._cache[arg] = orig.fn.apply(self, arguments);

        return self._cache[arg]
      }
    }
  }
}

var obj = new Obj();

console.log(obj.cache().fn(1));
console.log(obj.cache().fn(1));