JS 对象属性相关--检查属性、枚举属性等

 delete person.age //即person不再有属性age
 delete person['age'] //或者这样

var a = {p:{x:1}};
var b = a.p;
console.log(a.p.x); //1
delete a.p;
console.log(a.p.x); //TypeError a.p is undefined
console.log(a.p); //undefined
console.log(b.x); //1

function inherit(p){ 
    if(p == null){   // 不能从null中继承
        throw TypeError();
    }
    if(Object.create){   //如果有这个方法就直接使用
        return Object.create(p);
    }
    var t = typeof p;
    if(t !== "object" || t !== "function"){   //要继承的对象  类型要符合
        throw TypeError();
    }
    function f(){ };  //定义一个空的构造函数
    f.prototype = p;  //原型指向要继承的对象p
    return new f();   //创建f对象，此对象继承自p
}

var obj = {x:1};
var obj1 = inherit(obj);
obj1.y = 2;
console.log("x = "+obj1.x+" y = "+obj1.y); //x = 1 y = 2
delete obj1.x;
delete obj1.y;
console.log("x = "+obj1.x+" y = "+obj1.y); //x = 1 y = undefined

delete Object.prototype; // 不能删除  不可配置

var x = 1;
delete this.x; //不能删除
this.y = 1;
delete y; //这样可以删除

function f(){ }
delete this.f; //不能删除

var data = [5,6,7];
console.log("0" in data);  //有下标0
console.log(1 in data);    //1可以转换成"1"
console.log("4" in data);  //下标只有 1  2  3
 
var obj = {x:1};
console.log("x" in obj); //true
console.log("y" in obj); //false
console.log("toString" in obj); //true  因为obj继承了这个方法

var obj = {x:1};
console.log(obj.hasOwnProperty("x")); //true
console.log(obj.hasOwnProperty("y")); //false
console.log(obj.hasOwnProperty("toString")); //false  因为obj继承了这个方法,但不是它自己的

var obj = {x:1};
console.log(obj.propertyIsEnumerable("x")); //true
console.log(obj.propertyIsEnumerable("y")); //false
console.log(obj.propertyIsEnumerable("toString")); //false  因为obj继承了这个方法,但不是它自己的
console.log(Object.prototype.propertyIsEnumerable("toString")); //false  因为最原始的的 toString就是不可枚举的

var obj = {x:1};
console.log(obj.x !== undefined);//true
console.log(obj.y !== undefined);//false
console.log(obj.toString !== undefined); //true

var obj = {x:1,y:2};
for(p in obj){
  console.log(p);//x y
  console.log(obj.p);// undefined undefined
  console.log(obj[p]);//1 2
}

var p = {x:1}; //p原型对象继承自Object.prototype
var o = Object.create(p); //o对象继承自p

console.log(p.isPrototypeOf(o));//true
console.log(Object.prototype.isPrototypeOf(o));//true
console.log(Object.prototype.isPrototypeOf(p));//true

var p = {x:1}; 
console.log(p instanceof Object);//true

var d = new Date();
console.log(d instanceof Date);//true
console.log(d instanceof Object);//true
console.log(d instanceof Number);//false

var obj = {x:1,y:2};
console.log(obj.toString());//[object Object]

function classOf(obj){  // 找出类名
  if(obj === null){
    return "Null";
  }
  if(obj === undefined){
    return "Undefined";
  }
  return Object.prototype.toString.call(obj).slice(8,-1);
}

console.log(classOf(1)); //Number
//注意，实际上是这些类型的变量调用toString()方法，而不是通过他们自身直接调用
//console.log(1.toString()); //会报错
var a = 1;
console.log(Number(1).toString()); //1
console.log(a.toString()); //1
console.log({x:1}.toString()); //[object Object]

console.log(classOf(1)); //Number
console.log(classOf("")); //String
console.log(classOf("str")); //String
console.log(classOf(null)); //Null
console.log(classOf(false)); //Boolean
console.log(classOf({})); //Object
console.log(classOf([])); //Array
console.log(classOf(new Date())); //Date
function f(){}
console.log(classOf(new f())); //Object