[map set]

 new An instance Map aggregate ：

let m = new Map();

1、map Properties of ：size

          return map Number of elements of

2、map Methods :

 <script>
        // new  An instance Map  aggregate 
        let m = new Map();
        // set- Set key sum value , Returns the added map（ Assigning a value to an existing key will overwrite the previous value 
        m.set("name", " Zhang San ");
        m.set(5, " bachelor ")
        m.set("5", " golden bachelor ")
        m.set("", "111")
        console.log(m);

        // get- Get a key Value , return key Corresponding value , No return undefined
        console.log(m.get(5));

        // has- Detect if there is a key, Returns a Boolean value 
        console.log(m.has(6));

        // delete- Delete some key And the corresponding value, Returns a Boolean value , If it is successfully deleted, it will be true
        console.log(m.delete(5));
        console.log(m);

        // clear return undefind, The original array is emptied 
        console.log(m.clear());

    </script>

3、 Traverse the way ：

     for of Only arrays can be traversed , Traversal is map The iterator ;for in Only objects can be traversed ;

    //  Use for of  Can be used to cycle map iterator 
        for (var key of m.keys()) {
            console.log(key);  // Return key 
        }
        for (var v of m.values()) {
            console.log(v); // Return value 
        }

        for (var e of m.entries()) {
            console.log(e); // Returns an array of , The first item of the array is the key , The second item is the value 
        }

        // for of  You can only traverse arrays , And traverse all elements , for in You can only traverse objects 
        var arr = [{ name: " Zhang San ", age: 30 }, { name: " Zhang San ", age: 30 }]
        for (var item of arr) {
            console.log(item);
        }