关于修改js对象的一个疑问

var p1 = {name:"john",age:20};

var p2 = {name:"lucy",age:10};

function model1(obj){

    obj={name:'0000'};

}

function model2(obj){

    obj.name = 'back';

}

model1(p1);

model2(p2);

console.log(p1);

console.log(p2);

疑问：为什么p1没有变？