function_class.js

function Demo() {
  this.x = 42;
}

Demo.prototype = {
  logX: function() {
    console.log(this.x);
  },
};

var demo = new Demo();

demo.logX();

//////////////////////////

class Demo2 {
  constructor() {
    this.x = 42;
  }

  logX() {
    console.log(this.x);
  }
}

const demo2 = new Demo2();

demo2.logX();