var    obj = { key1: "hello" , key2: "world" };
function modifyObj(obj){
    obj = { key3: "world1", key4: "hello1" };  //将不会改变外部的obj
}
function changeObj(obj){
    obj.key3 = "world2";
    obj.key4 = "hello2";  //将改变外部的obj对象的属性
}
obj  //函数调用前obj属性
Object
key1: "hello"
key2: "world"
__proto__: Object

modifyObj(obj)
obj  //函数调用后obj属性没有变化
Object
key1: "hello"
key2: "world"
__proto__: Object

changeObj(obj)
obj   //函数调用后,属性发生了变化
Object
key1: "hello"
key2: "world"
key3: "world2"
key4: "hello2"
__proto__: Object