ES6新增map映射表数据类型

map存储的是一个个的键值对。主要特性介绍如下：

1 获取map

• 构造函数+set方法

let mapList = new Map();
mapList.set('name','zhangsan');
console.log(mapList);//{"name" => "zhangsan"}

• 构造函数初始化方法 传入一个特殊的二维数组

let mapList = new Map(
[['name','zhangsan'],
['age',18]]
);
console.log(mapList );//{"name" => "zhangsan", "age" => 18}

需要注意的是内层数组元素个数都为2，映射为一个键值对，超过2个会被忽略，1个会认为该键值对值为undefiend "name" => undefined，0个会认为该键值对键与值均为undefiend undefined => undefined

2 map的长度size

let mapList = new Map(
[['name','zhangsan'],
['age',18]]
);
console.log(mapList.size);//2

3 map的增删改查操作

• 增 set方法设置不存在的键时即为增

let mapList = new Map(
[['name','zhangsan'],
['age',18]]
);
mapList.set('count',90);
console.log(mapList);
//{"name" => "zhangsan", "age" => 18, "count" => 90}

• 删 mapList.delete(key),mapList.clear()

let mapList = new Map(
[['name','zhangsan'],
['age',18]]
);
mapList.delete('count');
console.log(mapList);
//{"name" => "zhangsan", "age" => 18}
mapList.clear();
console.log(mapList);
//{}

• 改 mapList.set(key)设置已有键key时即为更改

let mapList = new Map(
[['name','zhangsan'],
['age',18]]
);
mapList.set('name','lisi');
console.log(mapList);
//{"name" => "lisi", "age" => 18}

• 查 mapList.get(key)即获取对应键的值，mapList.has(key)即判断有无该键key

let mapList = new Map(
[['name','zhangsan'],
['age',18]]
);
console.log(mapList.has('name'));//true
console.log(mapList.get('name'));//zhangsan

4 遍历

• let .. of

let mapList = new Map(
[['name','zhangsan'],
['age',18]]
);
for(let key of mapList.keys()){
  console.log(key);
}
//name age
for(let value of mapList.values()){
  console.log(value);
}
//zhangsan 18
for(let [key,value ]of mapList.entries()){
  console.log(key,value);
}
//name zhangsan
//age 18

• forEach

let mapList = new Map(
[['name','zhangsan'],
['age',18]]
);
mapList.forEach(function(value,key){
  console.log(value,key);
})
//zhangsan name
//18 "age"

需要注意的是map的key完全可以是引用类型

let mapList = new Map();
let obj = {};
mapList.set(obj,{name:'zhangsan'});
console.log(mapList.get(obj));//{name:'zhangsan'}

4 WeakMap

map的阉割版，键只能为引用类型，没有size属性，没有clear方法，不能遍历
ES6总篇--目录