《JavaScript高级程序设计》——笔记(2)

原型（prototype）

　　1.1访问原型的方法

function human () {}
human.prototype.name = "people"
human.prototype.sayname = function(){console.log(this.name);} 

var jack = new human
console.log(human.prototype)                         //{ name: ‘people‘, sayname: [Function] }
console.log(jack.__proto__)                          //{ name: ‘people‘, sayname: [Function] }
console.log(Object.getPrototypeOf(jack))             //{ name: ‘people‘, sayname: [Function] }

　　前两种方法都能修改原型，最后一种只能访问。

创建对象

　　1.1使用构造函数问题

        function Person(name, age, job){
            this.name = name;
            this.age = age;
            this.job = job;
            this.sayName = function(){
                console.log(this.name);
            };
        }

        var person1 = new Person
        var person2 = new Person

        console.log(person1.sayName == person2.sayName);  //false

　　每次实例化一个对象，构造函数中的方法就重新创建一遍。

　　1.2原型模式的缺点

        function Person(){
        }

        Person.prototype = {
            constructor: Person,
            name : "Nicholas",
            age : 29,
            job : "Software Engineer",
            friends : ["Shelby", "Court"],
            sayName : function () {
                alert(this.name);
            }
        };

        var person1 = new Person();
        var person2 = new Person();

        person1.friends.push("Van");

        alert(person1.friends);    //"Shelby,Court,Van"
        alert(person2.friends);    //"Shelby,Court,Van"
        alert(person1.friends === person2.friends);  //true

　　原型模式通过原型共享属性和方法，但是也是这样引出了问题。person1.friends和person2.friends都是指向原型中friends的引用，所以当有一个实例修改friends后，其他的实例也会一起改变。

　　1.3常用创建对象方法（构造函数模式与原型模式的组合使用）

        function Person(name, age, job){
            this.name = name;
            this.age = age;
            this.job = job;
            this.friends = ["Shelby", "Court"];
        }

        Person.prototype = {
            constructor: Person,
            sayName : function () {
                alert(this.name);
            }
        };

        var person1 = new Person("Nicholas", 29, "Software Engineer");
        var person2 = new Person("Greg", 27, "Doctor");

        person1.friends.push("Van");

        alert(person1.friends);    //"Shelby,Court,Van"
        alert(person2.friends);    //"Shelby,Court"
        alert(person1.friends === person2.friends);  //false
        alert(person1.sayName === person2.sayName);  //true

　　构造函数模式创建属性，原型模式创建方法

继承的基本原理-原型链

function human (jobName) {
    this.jobName = jobName
}
human.prototype.sayjobName = function(){console.log(this.jobName);} 

function student (learn){}
student.prototype = new human("student")
student.prototype.learn = function(learn){
    console.log(learn);
};

jack = new student("jack")
console.log(jack.learn("math"));                //math
console.log(jack.jobName);                      //student
console.log(jack.sayjobName());                 //student

console.log(jack.__proto__);                    //{ jobName: ‘student‘, learn: [Function] }
console.log(jack.__proto__.__proto__);          //{ sayjobName: [Function] }

student继承与human有其jobName属性和sayjobName方法。通过对student原型内利用创建一个human实例的方法，添加了一个指向human原型的引用。