title: private function return the last instance
body: <p>this is my code</p>
  <pre><code>const myFunction = function(number){
  
      this.number = number;
      
      const foo = () =&gt; {
          return this.number;
      }
   
      myFunction.prototype.log = function () {
          console.log(foo());
      }
   
      
  }
  
  let firstInstance = new myFunction(12);
  let secondInstance = new myFunction(13);
  
  firstInstance.log() // 13
  secondInstance.log() // 13
  </code></pre>
  <p>when I'm using the foo function, returns the last instance;</p>
  <p>is there a way to using a private function and log function in the prototype of the function</p>