JS之对象

每个对象的属性有两种，每种属性有4中特征描述符

1.数据属性

1.1 [[configurable]]:表示不能通过delete删除属性，不能修改属性的特性，不能将数据属性改为访问器属性,默认值false

<script>
        var person = {
            name:"pmx"
        }
        Object.defineProperty(person,"name",{
            configurable:false
        });

        console.log(person.name); //pmx
        delete person.name;
        console.log(person.name); //pmx
</script>

打印出:

pmx

pmx

<script>
        var person = {
            name:"pmx"
        }
        Object.defineProperty(person,"name",{
            configurable:false
        });
        Object.defineProperty(person,"name",{
            configurable:true
        });
</script>

报错:can not redefine property :name

<script>
        var person = {
            name:"pmx"
        }
        Object.defineProperty(person,"name",{
            configurable:false
        });
        Object.defineProperty(person,"name",{
            enumerable:false
        });
 </script>

报错:can not redefine property :name

<script>
        var person = {
            name:"pmx"
        }
        Object.defineProperty(person,"name",{
            configurable:false
        });
        Object.defineProperty(person,"name",{
            enumerable:true
        });

        for(var key in person){
            console.log(key+":"+person[key]);
        }
 </script>

打印出:name:pmx

<script>
        var person = {
            name:"pmx"
        }
        Object.defineProperty(person,"name",{
            configurable:false,
            enumerable:false
        });
        for(var key in person){
            console.log(key+":"+person[key]);
        }
</script>

没有打印，但也没有报错

<script>
        var person = {
            name:"pmx"
        }
        Object.defineProperty(person,"name",{
            configurable:false
        });
        Object.defineProperty(person,"name",{
            writable:false
        });
        person.name = "pan";
        console.log(person.name);
 </script>

打印出:pan

<script>
        var person = {
            name:"pmx"
        }
        Object.defineProperty(person,"name",{
            configurable:false
        });
        Object.defineProperty(person,"name",{
            value:"panpan"
        });
        console.log(person.name);
 </script>

打印出:panpan

<script>
        var person = {
            name:"pan",
            city:"shanghai"
        }
        Object.defineProperty(person,"name",{
            configurable:true
        });
        Object.defineProperty(person,"name",{
            get:function(){
                return this.city;
            }
        });
        console.log(person.name);
</script>

打印出:shanghai

<script>
        var person = {
            name:"pan",
            city:"shanghai"
        }
        Object.defineProperty(person,"name",{
            configurable:false
        });
        Object.defineProperty(person,"name",{
            get:function(){
                return this.city;
            }
        });
        console.log(person.name);
</script>

报错:TypeError: Cannot redefine property: name

综上:

　　a.configurable设置为false后，那么当前属性就是不可删除滴,delete无效果

　　b.有两种方式创建对象。

　　　　1.对象字面量法:var obj = {name:"pmx"};

　　　　2.构造函数定义法:var obj = new Object();obj.name = "pmx";

　　　　这两种方式创建的对象的属性的特征描述分别是:configurable:true,enumerable:true,writable:true,value:pmx

　　c.configurable设置为false之后，不能再更改configurable和enumerable的值，但是对writable和value没有丝毫影响

　　d.configurable设置为false之后，不能将数据属性更改为访问器属性

1.2 [[enumerable]]:表示能否通过for-in循环遍历对象属性,默认值false

<script>
        var person = {
            name:"pan",
            city:"shanghai"
        }
        Object.defineProperty(person,"name",{
            enumerable:false
        });

        for(var key in person){
            console.log(key+":"+person[key]);
        }
</script>

打印出:city=shanghai

没有name=pan

1.3 [[writable]]:表示能否修改属性的值,默认值false

<script>
        var person = {
            name:"pan"
        }
        Object.defineProperty(person,"name",{
            writable:false
        });
        person.name = 'pmx';
        console.log(person.name);
</script>

打印出:pan

1.4 [[value]]:包含这个属性的数据值，取数据时，从这里取，写数据时，将值写到这里，默认值undefined

<script>
        var person = {
            name:"pan"
        }
        Object.defineProperty(person,"name",{
            value:"pmx"
        });
        console.log(person.name);
 </script>

打印出:pmx

2.访问器属性

2.1 [[configurable]]:表示能否通过delete删除属性，更改属性的特性，将访问器属性更改为数据属性,默认值false

<script>
        var person = {
            _name:"pan"
        }
        Object.defineProperty(person,"name",{
            configurable:false,
            get:function(){
                return this._name;
            }
        });
        console.log(person.name);//pan
        delete person.name;
        console.log(person.name);//pan
</script>

打印出:

pmx

pmx

<script>
        var person = {
            _name:"pan"
        }
　　　　//使用defineProperty创建的对象属性，默认的configurable:false,enmuerable:false,get:undefined,set:undefined
        Object.defineProperty(person,"name",{
            configurable:false,
            enumerable:true,
            get:function(){
                return this._name;
            }
        });
       for(var i in person){
            console.log(i+":"+person[i]);
       }
    </script>

打印出:

_name:pan

name:pan

<script>
        var person = {
            _name:"pan"
        }
        Object.defineProperty(person,"name",{
            configurable:false,
            enumerable:true,
            get:function(){
                return this._name;
            }
        });
        Object.defineProperty(person,"name",{
            enumerable:false
        });

       for(var i in person){
            console.log(i+":"+person[i]);
       }
</script>

报错:TypeError: Cannot redefine property: name

<script>
        var person = {
            _name:"pan"
        }
        Object.defineProperty(person,"name",{
            configurable:false,
            enumerable:true,
            get:function(){
                return this._name;
            }
        });
        Object.defineProperty(person,"name",{
            set:function(value){
                this._name = value;
            }
        });

        person.name = "pmx";
       for(var i in person){
            console.log(i+":"+person[i]);
       }
</script>

报错:TypeError: Cannot redefine property: name

<script>
        var person = {
            _name:"pan"
        }
        Object.defineProperty(person,"name",{
            configurable:false,
            enumerable:true,
            get:function(){
                return this._name;
            }
        });
        Object.defineProperty(person,"name",{
            get:function(){
                return "get";
            }
        });

        person.name = "pmx";
       for(var i in person){
            console.log(i+":"+person[i]);
       }
</script>

报错:TypeError: Cannot redefine property: name

<script>
        var person = {
            _name:"pan"
        }
        Object.defineProperty(person,"name",{
            configurable:false,
            enumerable:true,

        });
        Object.defineProperty(person,"name",{
            get:function(){
                return this._name;
            }
        });
</script>

报错:TypeError: Cannot redefine property: name

<script>
        var person = {
            _name:"pan"
        }
        Object.defineProperty(person,"name",{
            configurable:true,
            enumerable:true,
            get:function(){
                return this._name;
            }
        });
        Object.defineProperty(person,"name",{
            enumerable:true,
            writable:true,
            value:"pmx"
        });

       for(var i in person){
            console.log(i+":"+person[i]);
       }
</script>

打印出:

_name:pmx

name:pmx

综上:

　　a.使用Object.defineProperty()创建的属性的默认特 性,configurable:false,enumerable:false,writable:false,value:undefined,get:undefined,set:undefined

　　b.configurable:false时，不能更改enumerable,get,set;即使get,set为undefined,也不能重新定义get函数

　　c.给访问器属性添加writable,value特性，就能将其转换为数据属性

2.2[[enumerable]]:表示能否通过for-in循环遍历对象属性,默认值false

2.3[[get]]:读取属性时调用的函数，默认值undefined

2.4[[set]]:写入属性时调用的函数，默认值undefined

3.为对象定义多个属性

Object.defineProperties()

<script>
        var person = {}
        Object.defineProperties(person,{
            name:{
                value:'pmx',
                writable:false,
                enumerable:true
            },
            city:{
                value:"shanghai",
                enumerable:false
            },
            _year:{
                value:2016,
                writable:false,
                enumerable:false
            },
            year:{
                enumerable:true,
                get:function(){
                    return this._year;
                }
            }
        });

        console.log(person.name); //pmx
        person.name = "pan";
        console.log(person.name); //pmx
        for(var key in person){
            console.log(key+":"+person[key]);  //name:pmx;year:2016
        }
</script>

4.读取属性的特性描述

Object.getOwnPropertyDescriptor()

<script>
        var person = {}
        person.name = "pmx";

        var des = Object.getOwnPropertyDescriptor(person,"name");
        for(var key in des){
            console.log(key+":"+des[key]);
        }
</script>

value:pmx
writable:true
enumerable:true
configurable:true