Scoped Security Constructor _ function

Problem Introduction:

Before see "JS language Essence", introduce the dross of JS language, one of them is the global object . When you use a constructor, you may forget to write new, and the object is added to the Global object window, causing an unexpected increase in the Error object properties.

For example, a constructor:

function Person (gender,age) {
    this.gender = gender;
    This.age = age;
}

The correct way to open it should be var p = new Person (), which points to the newly created object. But when you forget to use new, such as var p1 = person (), the equivalent of calling the function directly, this points to the global object.

var p = new Person (' female ',);
Console.log (P.gender); Wei
console.log (window.gender);//undefined

var p1 = person (' male ', n);
Console.log (P1.gender); Error,p1 is undefined
console.log (window.gender);//xiao

scoped Security Constructor : Returns a new instance of person regardless of whether or not to use new.

function Polygon (sides) {if (this instanceof Polygon) {this.sides = sides;
      This.getarea = function () {return 0;
    } else {return new Polygon (name);
    } function Rectangle (width, height) {Polygon.call (this, 2);//Borrow constructor this.width = width;
    This.height = height;
    This.getarea = function () {return this.width * this.height; } Rectangle.prototype = new Polygon ();

  Prototype chain inheritance, which implements the rectangle instance is also a Polygon instance, thereby passing the checksum of ' if (this instanceof Polygon) ' in the Polygon constructor.
  var rect = new Rectangle (5,10); alert (rect.sides);