簡單寫一下Map對象的底層實(shí)現(xiàn)原來庵朝，包括常用的幾個(gè)方法：

size吗冤，set()，get()九府，delete()椎瘟，clear()

Map底層數(shù)據(jù)結(jié)構(gòu)是，hashMap侄旬，即哈希表或者散列表
哈希表沖突解決方案采用拉鏈法

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;
    // 根據(jù)key的不同的數(shù)據(jù)類型肺蔚，返回不同的index,其實(shí)就是決定放到bucket的那個(gè)鏈表上
    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];
    // 找到在鏈表上哪個(gè)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、決定key,value放到哪個(gè)鏈表上
    const i = this.hash(key);
    // 2儡羔、獲取當(dāng)前要存放的鏈表
    let target = this.bucket[i];
    // 放到鏈表上哪個(gè)位置
    while(target.next){
        // key已經(jīng)存在宣羊，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];
    // 防止刪除第一個(gè)，設(shè)置虛擬頭
    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
}