数据结构：map (不是数组的map方法)

注：与对象不同的是，键和值可以是任意类型的。

应用场景1: 创建map数据结构

const map1 = new Map();
//设置key键
const key1 = "name",
  key2 = {},
  key3 = function() {};
//设置value
map1.set(key1, "key1‘s value");
map1.set(key2, "key2’s value");
map1.set(key3, "key3‘s value");
console.log(map1.get("name"));  //key1‘s value
console.log(map1.get(key2));   //key2’s value
console.log(map1.get(key3));  //key3‘s value

应用场景2：获取map数据结构的长度

console.log(map1.size); //3

应用场景3：遍历map数据结构的key和value

for(var [key,value] of map1){
    console.log(`${key} = ${value}`);
}
//结果：name = key1‘s value
        [object Object] = key2’s value
        function () {} = key3‘s value

for (let item of map1.keys()) {
  console.log(item);
}
//结果：name         {}           ƒ () {}

for (let item of map1.values()) {
  console.log(item);
}
//结果：key1’s value         key2’s value          key3‘s value

应用场景4：将map数据结构的key和value转为数组　

const keyArr = Array.from(map1.keys());
console.log(keyArr);
//结果：["name", {…}, ƒ]
const valueArr = Array.from(map1.values());
console.log(valueArr);
//结果：["key1‘s value", "key2’s value", "key3‘s value"]