简单写一下Map对象的底层实现原来，包括常用的几个方法：

size，set()，get()，delete()，clear()

Map底层数据结构是，hashMap，即哈希表或者散列表
哈希表冲突解决方案采用拉链法

function MyMap(){
    this.init();
}
MyMap.prototype.init = function(){
    this.size = 0;
    this.bucket = new Array(8);
    for(let i = 0; i < 8; i++){
        this.bucket[i] = {};
        this.bucket[i].next = null;
    }
}
MyMap.prototype.hash = function(key){
    let index = 0;
    // 根据key的不同的数据类型，返回不同的index,其实就是决定放到bucket的那个链表上
    if(typeof key === 'object'){
        index = 0;
    }else if(typeof key === 'number'){
        index = key % this.bucket.length;
    }else if(typeof key === 'undefined'){
        index = 1;
    }else if(typeof key === 'null'){
        index = 2;
    }else if(typeof key === 'string'){
        for(let i = 0; i < 10; i++){
            index += isNaN(key.charCodeAt(i)) ?  0 : key.charCodeAt(i);
        }
    }
    index = index % this.bucket.length;
    return index;
}
MyMap.prototype.get = function(key){
    const i = this.hash(key);
    let target = this.bucket[i];
    // 找到在链表上哪个位置
    while(target.next){
        if(target.next.key === key){
            return target.next.value;
        }
        target = target.next;
    }
    return undefined;
}
MyMap.prototype.set = function(key, value){
    // 1、决定key,value放到哪个链表上
    const i = this.hash(key);
    // 2、获取当前要存放的链表
    let target = this.bucket[i];
    // 放到链表上哪个位置
    while(target.next){
        // key已经存在，直接修改
        if(target.next.key === key){
            target.next.value = value;
            return this;
        }
        target = target.next;
    }
    target.next = {key, value, next : null}
    this.size++;
    return this;
}
MyMap.prototype.has = function(key){
    const i = this.hash(key);
    let target = this.bucket[i];
    while(target.next){
        if(target.next.key === key){
            return true;
        }
        target = target.next;
    }
    return false;
}
MyMap.prototype.clear = function(){
    this.init();
}
MyMap.prototype.delete = function(key){
    const i = this.hash(key);
    let target = this.bucket[i];
    // 防止删除第一个，设置虚拟头
    let front = {
        next: target
    };
    let cur = front;
    while(cur.next){
        if(cur.next.key === key){
            cur.next = cur.next.next;
            return true;
        }
        cur = cur.next;
    }
    return false
}