Object特性

var obj = {};
var tmp = undefined;
Object.defineProperty(obj,'a',{
    //该属性是否可枚举(for in,Object.keys)，默认为false
    enumerable: false,
    //该属性的特性是否可修改(除writable特性外的其他特性是否可以被修改,
    //如重新定义enumerable，configurable等属性)，默认为false
    configurable: false,
    //该属性是否可读写，默认为false，不能与set或get同时存在
    // writable: true
    //属性的值，不能与set或get同时存在
    // value: 1,
    //setter
    set: function(value){
        console.log('set:' + value);
        tmp = value;
    },
    //getter
    get: function(value){
        return 'get:' + tmp;
    }
});
obj.b = 2;
obj.a = 3;//set:3
// Object.defineProperty(obj, "a", {enumerable : true});//TypeError
console.log(Object.keys(obj));//["b"]
console.log(obj.a);//get:3

var p = {a:1};
var obj = Object.create(p, {//可以在obj上追加属性
    b: {
        value: 2, 
        writable: true,
        enumerable: true,
        configurable: true 
    } 
});
console.log(obj.__proto__==p);//true
console.log(obj.a);//1

//Object.create的简单实现
function create(p){
    var F = function(){};
    F.prototype = p;
    return new F();
}
var obj = create(p);
console.log(obj.__proto__==p);//true
console.log(obj.a);//1

var s1 = {a:'a1',b:2};
var s2 = {a:'a2',c:3};
var s3 = {a:'a3',d:4};
var tmp = {};
var obj = Object.assign(tmp,s1,s2,s3);
console.log(obj==tmp);//true
console.log(obj);//{a: "a3", b: 2, c: 3, d: 4}

//assgin的简单实现
function assign(){
    var arg = arguments;
    var target = arg[0];
    for(var i=1; i<arg.length; i++){
        copy(arg[i]);
    }
    function copy(obj){
        for(var key in obj){
            if(obj.hasOwnProperty(key)){
                target[key]=obj[key];
            }
        }
    }
    return target;
}
var obj = assign(tmp,s1,s2,s3);
console.log(obj==tmp);//true
console.log(obj);//{a: "a3", b: 2, c: 3, d: 4}

var tmp = {a:1};
var obj = Object.create(tmp,{
    'b':{
        enumerable:true,
        value:2
    },
    'c':{
        enumerable:false,
        value:3
    }
})
console.log(Object.keys(obj));//["b"]
tmp = [];
for(var key in obj){
    tmp.push(key);
}
console.log(tmp);//["a","b"]