创建对象 --以下内容来自JavaScript高级程序设计
工厂模式
用函数来封装以特定接口创建对象的细节。
function createPerson(name, age, job){ var o = new Object(); o.name = name; o.age = age; o.job = job; o.sayName = function(){ alert(this.name); }; return o; } var person1 = createPerson("Nicholas", 29, "Software Engineer"); var person2 = createPerson("Greg", 27, "Doctor"); person1.sayName(); //"Nicholas" person2.sayName(); //"Greg"
构造函数
用来创建特定类型的对象
function Person(name, age, job){ this.name = name; this.age = age; this.job = job; this.sayName = function(){ alert(this.name); }; } var person1 = new Person("Nicholas", 29, "Software Engineer"); var person2 = new Person("Greg", 27, "Doctor"); person1.sayName(); //"Nicholas" person2.sayName(); //"Greg" alert(person1 instanceof Object); //true alert(person1 instanceof Person); //true alert(person2 instanceof Object); //true alert(person2 instanceof Person); //true alert(person1.constructor == Person); //true alert(person2.constructor == Person); //true //不同实例的同名函数是不相等的,每个方法都要在每个实例上重新创建一次。 alert(person1.sayName == person2.sayName); //false
function Person(name, age, job){ this.name = name; this.age = age; this.job = job; this.sayName = sayName; } //把函数定义转移到构造函数外部,作为全局函数,没有封装性 function sayName(){ alert(this.name); } var person1 = new Person("Nicholas", 29, "Software Engineer"); var person2 = new Person("Greg", 27, "Doctor"); person1.sayName(); //"Nicholas" person2.sayName(); //"Greg" alert(person1 instanceof Object); //true alert(person1 instanceof Person); //true alert(person2 instanceof Object); //true alert(person2 instanceof Person); //true alert(person1.constructor == Person); //true alert(person2.constructor == Person); //true alert(person1.sayName == person2.sayName); //true
原型模式
每个函数都有一个原型属性,它指向一个对象,这个对象包含由特定类型实例共享的属性和方法。使用原型对象可以使所有对象实例共享它所包含的属性和方法。
function Person(){ } Person.prototype.name = "Nicholas"; Person.prototype.age = 29; Person.prototype.job = "Software Engineer"; Person.prototype.sayName = function(){ alert(this.name); }; var person1 = new Person(); person1.sayName(); //"Nicholas" var person2 = new Person(); person2.sayName(); //"Nicholas" alert(person1.sayName == person2.sayName); //true alert(Person.prototype.isPrototypeOf(person1)); //true alert(Person.prototype.isPrototypeOf(person2)); //true //only works if Object.getPrototypeOf() is available if (Object.getPrototypeOf){ alert(Object.getPrototypeOf(person1) == Person.prototype); //true alert(Object.getPrototypeOf(person1).name); //"Nicholas" }
alert(Person.prototype.constructor == Person); //true
in操作符:当对象能够访问给定的属性时,返回true,不论改属性在实例还是原型中。hasOwnProperty()只针对对象实例。自定义hasPrototypeProperty()监测原型对象中的属性。
function hasPrototypeProperty(object, name){ return !object.hasOwnProperty(name) && (name in object); }
不是所有的属性都是可枚举的 ECMAScript5中constructor和prototype都是不可枚举的。Object.keys()可以取得对象的上所有可枚举的属性。
function Person() { } Person.prototype.name = "Nicholas"; Person.prototype.age = 29; Person.prototype.job = "Software Engineer"; Person.prototype.sayName = function () { alert(this.name); }; var keys = Object.keys(Person.prototype); alert(keys); //"name,age,job,sayName" //返回所有属性 var k0 = Object.getOwnPropertyNames(Person.prototype); alert(k0); //"constructor,name,age,job,sayName" var person1 = new Person(); person1.name = "yuansong"; person1.age = 28; var k1 = Object.keys(person1); alert(k1);//"name,age"
原型的动态性
尽管我们可以随时的为原型添加属性和方法,并且修改能够立即在所有对象实例中反应出来,但是重写整个原型对象就不一样了。
function Person(){ } var friend = new Person(); Person.prototype = { constructor: Person, name : "Nicholas", age : 29, job : "Software Engineer", sayName : function () { alert(this.name); } }; friend.sayName(); //error
原型对象的问题
原型对象中的所有属性是被很多实例共享的,这对于包含原始类型值的属性没有影响,但对于包含引用类型值得属性来说,比较问题就严重了。。。
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