this.message = 'hello world';
var that = this;

process.nextTick(function() {
  console.log(that.message);
});

process.nextTick(() => {
  console.log(this.message);
});

for (var i = 0; i < 3; i++) {
   let j = i * i;
   console.log(j);
}
console.log(j); // => error, j is undefined

const PI = 3.14159265359;
PI = 0; // => error: PI is read-only

class Monster extends Entity {
  constructor(name) {
    super();
    this.name = name;
  }

  get scariness() {
    return 'mild';
  }

  speak() {
    super.speak();
  }

  static create() {
    return new Monster();
  }
}