浅拷贝

复杂数据类型的数据复制的是地址，修改会被“共享”
方式一，使用copyObj[key] = obj[key]

var obj = {name: 'David', age: 18, pets: ['阿猫', '阿狗']};
var copyObj = {};
for(var key in obj){
    copyObj[key] = obj[key];
}
obj.name = '周杰伦';
obj.pets.push('小黑');
console.log(obj);  // name:'周杰伦', age:18, pets:["阿猫", "阿狗", "小黑"]
console.log(copyObj);  // name:'David', age:18, pets:["阿猫", "阿狗", "小黑"]

方式二，使用Object.assign()

var obj = {name: 'David', age: 18, pets: ['阿猫', '阿狗']};
var copyObj = {};

Object.assign(copyObj, obj, {address: '广州'});

obj.name = '周杰伦';
obj.pets.push('小黑');
console.log(obj);  // name:'周杰伦', age:18, pets:["阿猫", "阿狗", "小黑"]
console.log(copyObj);  // name:'David', age:18, address:'广州', pets:["阿猫", "阿狗", "小黑"]

深拷贝

复杂数据类型的数据复制到新的地址，修改不会被“共享”

var obj = {
    name: 'David',
    age: 18,
    friends: ['小明', '小红', '小黑'],
    bestFriend: {
        name: '小黑',
        age: 19,
        address: '广州',
        pets: [{name: '土豆'}, {name: '马铃薯'}]
    },
    birthday: new Date()
};

function deepCopy2NewObj(fromObj, toObj) {
    for(var key in fromObj){
        var fromValue = fromObj[key];
        if(!isObj(fromValue)){
            toObj[key] = fromValue;
        }else{
            var tempObj = new fromValue.__proto__.constructor;
            deepCopy2NewObj(fromValue, tempObj);  // 递归
            toObj[key] = tempObj;
        }
    }
}

function isObj(obj) {
    return obj instanceof Object;
}

function isArr(obj) {
    return Array.isArray(obj);
    // return Object.prototype.toString.call(obj) === '[object Array]';  // 方式2
}

var newObj = {};
deepCopy2NewObj(obj, newObj);
console.log(obj);
console.log(newObj);