thisOne.js

/*
 * Comparing arrow functions' lexical this binding to ordinary functions
 */
function Dog() {
   // The arrow function binds this to the new Dog instance
   // It is the same as writing it as 
   // const doSomething = function() { this.bark(); }.bind(this);
   const doSomething = () => { this.bark() };
   doSomething();
}

Dog.prototype.bark = function() {
  console.log("woof");
}

new Dog(); // woof