new关键字的作用

new关键字的作用
new关键字创建实例的过程
1)创建一个新对象
2)将构造函数的作用域赋给新对象
3)执行构造函数中的代码
4)返回新对象
对于每一个实例，都会从原型上复制出一份一模一样的内存.

var F = function(){}; 
Object.prototype.a = function(){}; 
Function.prototype.b = function(){}; 
var f = new F();

new的过程拆解：
1)var f={}; 也就是说，初始化一个对象p。
2) f.__proto__=F.prototype;
3)F.call(f);也就是说构造p，也可以称之为初始化p。
console.log(f.__proto__===F.prototype); // 返回值为true，证明第2点
console.log(F.__proto__===Function.prototype); // 返回值为true
console.log(F.prototype.__proto__===Object.prototype); // 返回值为true
重点部分：console.log(f.__proto__.__proto__===Object.prototype); // 返回值为true

    var animal = function(name){
        this.name = name;
    };
    animal.prototype.eat = function(){
        console.log('animal eat!');
    };
    var cat = new animal('cat');
    cat.eat(); // animal eat!
    cat.eat = function (){
        console.log('cat eat!');
    };
    cat.eat();//cat eat!
    var dog = new animal('cat');
    dog.eat();// animal eat!

如试例代码,对实例cat的eat方法的修改并未影响到原型以及其他实例。

    function Person(){
        this.type = 'human';
    }
    Person.prototype.say = function(){
        console.log('person say!');
    };
    var F = function(){};
    F.prototype = Person.prototype;
    function Student(name){
        this.name = name;
    }
    Student.prototype = new F();
    //Student.prototype.constructor = Student;
    var xiaoming = new Student('xiaoming');
    xiaoming.say();
    console.log(xiaoming.constructor === Student); // false
    console.log(Student.prototype.constructor === Student);// false
    console.log(F.prototype.constructor === F);// false
    console.log(xiaoming.constructor === F); // false
    console.log(Student.prototype.constructor === F); // false
    console.log(xiaoming.constructor === Person); // ture
    console.log(Student.prototype.constructor === Person); // ture
    console.log(F.prototype.constructor === Person); // ture

    // 比较区别
    function Person(){
        this.type = 'human';
    }
    Person.prototype.say = function(){
        console.log('person say!');
    };
    var F = function(){};
    F.prototype = Person.prototype;
    function Student(name){
        this.name = name;
    }
    Student.prototype = new F();
    Student.prototype.constructor = Student;
    var xiaoming = new Student('xiaoming');
    xiaoming.say();
    console.log(xiaoming.constructor === F); // false
    console.log(Student.prototype.constructor === F); // false
    console.log(F.prototype.constructor === F);// false
    console.log(xiaoming.constructor === Person);  // false
    console.log(Student.prototype.constructor === Person);  // false
    console.log(xiaoming.constructor === Student); // true
    console.log(Student.prototype.constructor === Student);// true
    console.log(F.prototype.constructor === Person); // ture

原型链的操作。整个过程的变化图如下：

默认

增加代码

F.prototype = Person.prototype;

增加代码

Student.prototype.constructor = Student;

参考文章：
1.f 能取到a，b 吗？原理是什么？