title: ES6 Javascript Classes - define class method to an existing function
body: <p>In es5 you can do it by setting a prototype to an existing function but how would you go about this with ES6 classes</p>
  
  <pre><code>//// ES5
  function existingFn = function () {
      // does something
  };
  
  function MyClass () {
      // constructor stuff
  };
  
  MyClass.prototype.newFn = function () {
      // …
  };
  
  MyClass.prototype.existingFn = existingFn;
  
  //// ES6
  class MyClass {
      constructor() {}
  
      newFn() {
          // …
      }
  
      // ???????
      // existingFn = existingFn
      // ???????
  }
  </code></pre>