JavaScript でのスコープ セーフ コンストラクターのサンプル コードの詳細な説明

function Person(name,age,job){
  this.name=name;
  this.age=age;
  this.job=job;
}
var person=new Person('match',28,'Software Engineer');
console.log(person.name);//match

function Person(name,age,job){
  this.name=name;
  this.age=age;
  this.job=job;
}     
var person=Person('match',28,'Software Engineer');
console.log(person);//undefined
console.log(window.name);//match

function Person(name,age,job){
  if(this instanceof Person){
    this.name=name;
    this.age=age;
    this.job=job;
  }else{
    return new Person(name,age,job);
  }
}
var person=Person('match',28,'Software Engineer');
console.log(window.name); // ""
console.log(person.name); //'match'
var person= new Person('match',28,'Software Engineer');
console.log(window.name); // ""
console.log(person.name); //'match'

function Polygon(sides){
  if(this instanceof Polygon){
    this.sides=sides;
    this.getArea=function(){
      return 0;
    }
  }else{
    return new Polygon(sides);
  }
}
function Rectangle(wifth,height){
  Polygon.call(this,2);
  this.width=this.width;
  this.height=height;
  this.getArea=function(){
    return this.width * this.height;
  };
}
var rect= new Rectangle(5,10);
console.log(rect.sides); //undefined

function Polygon(sides){
  if(this instanceof Polygon){
    this.sides=sides;
    this.getArea=function(){
      return 0;
    }
  }else{
    return new Polygon(sides);
  }
}
function Rectangle(wifth,height){
  Polygon.call(this,2);
  this.width=this.width;
  this.height=height;
  this.getArea=function(){
    return this.width * this.height;
  };
}
Rectangle.prototype= new Polygon();
var rect= new Rectangle(5,10);
console.log(rect.sides); //2