function extend(parent, child) {
var i;
//如果不传入第二参数child
//那么就创建一个新的对象
child = child || {}; 
//遍历parent对象的所有属性
//并且过滤原型上的属性
//然后将自身属性复制到child对象上
for(i in parent) {
if(parent.hasOwnProperty(i)) {
child[i] = parent[i];
}
}
//返回目标对象child
return child;
} 

var dad = {name: "Adam"};
var kid = extend(dad);
console.log(kid.name); //Adam 

var dad = {
counts: [1, 2, 3],
reads: {paper: true}
};
var kid = extend(dad) //调用extend()方法将dad的属性复制到kid上面
kid.counts.push(4); //把4追加到kid.counts数组里面
console.log(dad.counts); //[1, 2, 3, 4] 

function extendDeep(parent, child) {
child = child || {};
for(var i in parent) {
if(parent.hasOwnProperty(i)) {
//检测当前属性是否为对象
if(typeof parent[i] === "object") {
//如果当前属性为对象，还要检测它是否为数组
//这是因为数组的字面量表示和对象的字面量表示不同
//前者是[],而后者是{}
child[i] = (Object.prototype.toString.call(parent[i]) === "[object Array]") ? [] : {};
//递归调用extend
extendDeep(parent[i], child[i]);
} else {
child[i] = parent[i];
}
}
}
return child;
} 

var dad = {
counts: [1, 2, 3],
reads: {paper: true}
};
var kid = extendDeep(dad);
//修改kid的counts属性和reads属性
kid.counts.push(4);
kid.reads.paper = false;
console.log(kid.counts); //[1, 2, 3, 4]
console.log(kid.reads.paper); //false
console.log(dad.counts); //[1, 2, 3]
console.log(dad.reads.paper); //true 

function extendDeep(parent, child) {
var i,
proxy;
proxy = JSON.stringify(parent); //把parent对象转换成字符串
proxy = JSON.parse(proxy) //把字符串转换成对象，这是parent的一个副本
child = child || {};
for(i in proxy) {
if(proxy.hasOwnProperty(i)) {
child[i] = proxy[i];
}
}
proxy = null; //因为proxy是中间对象，可以将它回收掉
return child;
} 

var dad = {
counts: [1, 2, 3],
reads: {paper: true}
};
var kid = extendDeep(dad);
//修改kid的counts属性和reads属性
kid.counts.push(4);
kid.reads.paper = false;
console.log(kid.counts); //[1, 2, 3, 4]
console.log(kid.reads.paper); //false
console.log(dad.counts); //[1, 2, 3]
console.log(dad.reads.paper); //true