js 创建空对象的3种方法 object.create(null) 和 {}创建对象的区别

    var obj1 = {};
    var obj2 = Object.create(null);
    var obj3 = new Object();

    var con1 = obj1.constructor;
    var con2 = obj2.constructor;
    var con3 = obj3.constructor;

    console.log(con1);//ƒ Object() { [native code] }
    console.log(con2);//undefined
    console.log(con3);// Object() { [native code] }

这两个可以用来判断js变量是数组还是对象
console.log(Array.prototype.isPrototypeOf(book[0]));
console.log(Object.prototype.toString.call(book[0]));