title: How to add an method to a class in Javascript ES6
body: <p>I need do add a method to a Javascript class using the new syntax. I tried this way:</p>
  
  <pre><code>class X{
  
      constructor() {
          this.a = 'b'
      }
  
      x(){
  
      }
  }
  
  X.prototype.y = function (){
      console.log('y')
  }
  
  var x = new X()
  
  x.y()
  
  console.log(X) // print the  the class but not the new method.
  </code></pre>
  
  <p>It just prints:</p>
  
  <pre><code>class X{
  
      constructor() {
          this.a = 'b'
      }
  
      x(){}
  }
  </code></pre>
  
  <p>But I expected</p>
  
  <pre><code>class X{
  
          constructor() {
              this.a = 'b'
          }
  
          x(){}
  
          y(){
              console.log('y');
          }
      }
  </code></pre>
  
  <p>How could I add a new method to a Javascript class?</p>