babel-test.js

class Listy extends Array {
  constructor() {
    super(arguments);
  }
  
  customContains(item) {
    return this.indexOf(item) > -1;
  }
}

const list = new Listy('hello', 'world');
console.log(list, list.customContains('hello'), list.customContains('world'));
alert(list);