对象 实例
每个对象都含有以下属性方法
- ➤ constructor — The function that was used to create the object.
- ➤ hasOwnProperty(propertyName) — Indicates if the given property exists on the object instance (not on the prototype). The property name must be specified as a string (for example, o.hasOwnProperty(“name”)).
- ➤ isPrototypeOf(object) — Determines if the object is a prototype of another object.
- ➤ propertyIsEnumerable(propertyName) — Indicates if the given property can be enumerated using the for-in statement (discussed later in this chapter). As with hasOwnProperty(), the property name must be a string.
- ➤ toLocaleString() — Returns a string representation of the object that is appropriate for the locale of execution environment.
- ➤ toString() — Returns a string representation of the object.
- ➤ valueOf() — Returns a string, number, or Boolean equivalent of the object. It often returns the same value as toString().
但是那些宿主对象host object并不一定含有，因为它们是由host implementation定义和提供，并不受ECMA-262管理，例如Dom,BOM 对象，它们可能不是继承于Object
在使用var object =｛｝来创建对象时，并没有调用Object（）构造函数，同理用这种模式创建的数组也是没有调用构造函数
对象的属性分为数据属性和存取器属性，数据属性有四个内部属性，[[Configurable]], [[Enumerable]], [[Writable]] ， [[Value]]，value是该属性值的存储位置，想要修改这些内部属性需要使用 object.defineProperty（）
var person = {};
Object.defineProperty(person, “name”, {
writable: false,
value: “Nicholas”
});
Object.defineProperty(person, “name”, {
writable: false,
value: “Nicholas”
});
一旦configurable设为false，就无法再修改为true，调用这个函数时，参数默认为false，除非显式指定了true
-
➤ [[Configurable]] — Indicates if the property may be redefined by removing the property via delete, changing the property’s attributes, or changing the property into a data property. By default, this is true for all properties defined directly on an object.
-
➤ [[Enumerable]] — Indicates if the property will be returned in a for-in loop. By default, this is true for all properties defined directly on an object.
-
➤ [[Get]] — The function to call when the property is read from. The default value is undefined.
-
➤ [[Set]] — The function to call when the property is written to. The default value is undefined.
不可以直接定义一个存取器属性，也需要用object.defineProperty（）
var book = {
_year: 2004,
edition: 1
};
Object.defineProperty(book, “year”, {
get: function(){
return this._year;
},
set: function(newValue){
if (newValue > 2004) {
this._year = newValue; this.edition += newValue - 2004;
}
}
});
book.year = 2005; alert(book.edition); //2
并不是一定要同时指定getter和setter，如果浏览器不支持object.defineProperty（），可以使用
__defineGetter__() and __defineSetter__()来指定getter和setter函数，但是configurable和enumerable就设定不了
一次性定义多个属性使用 Object.defineProperties()方法：
var book = {};
Object.defineProperties(book, {
_year: {
value: 2004
},
edition: {
value: 1
},
year: {
get: function(){
return this._year;
},
set: function(newValue){
if (newValue > 2004) {
this._year = newValue;
this.edition += newValue - 2004;
}
}
}
});
