What else do you not know about new map()

1、Map Characteristics

• Map  Object to save key value pairs , And remember the original insertion order of the keys , Any value can be a key or a value
• Map  Objects will be iterated according to the insertion order of elements in the object
• Map Object use for ... of  After each iteration, the loop returns a value of the form  [key, value]  Array of
• Map  The comparison of object keys is based on Zero equal The algorithm of , At present ECMAScript Specification ,-0  and  +0  Considered equal , Use Object.is(-0,+0) return false
•  Map Object setting properties , The following code works correctly , But not recommended , Because this way of setting attributes will not change Map Data structure of , Cannot be queried

        const map = new Map();
        map['bla'] = 'blaa';
        console.log(map); // {bla: 'blaa', size: 0}
        console.log(map.has('bla')); //false
        console.log(map.get('bla')); //undefined
        console.log(map.delete('bla')); //false

• NaN  It can also be used as  Map  Object's key

        const map = new Map();
        map.set(NaN, 'not a number');
        console.log(map); //{NaN => 'not a number'}

• Store data correctly to Map The way in is to use  set(key, value)  Method

        const map = new Map();
        map.set('bla', 'blaa');
        console.log(map); //{'bla' => 'blaa'}
        console.log(map.has('bla')); //true
        console.log(map.get('bla')); //blaa
        console.log(map.delete('bla')); //true

2、Map Instance properties for  

size: return Map Number of key value pairs in the object

      const map = new Map();
      map.set('bla', 'blaa');
      console.log(map.size); //1

3、Map Instance method of :

1. set ( key, value ) :  stay  Map  The key set and specified in the object  key  The value of the Association  value, And back to  Map  object
2. has( key ) :  Returns a Boolean value , Used to indicate  Map  Whether there is a connection with  key  The value of the Association
3. get ( key ) : Return and  key  The value of the Association , If there is no associated value , Then return to  undefined
4. delete(key) :  remove  Map  Specifies the key value in the object pair , If the key value pair exists and is successfully removed , return  true, Otherwise return to  false. call  delete  Then call  Map.prototype.has(key)  Will return  false
5. clear():  remove  Map  All key value pairs in the object

      const map = new Map();
      map.set('name', 'Andy'); // {'name' => 'Andy'}
      map.set('age', 18); // {'name' => 'Andy'}
      map.has('name'); //true
      map.get('name'); //Andy
      map.delete('name'); //true
      map.set('name', 'Andy'); // {'name' => 'Andy'}
      map.clear();

 4、 Iterative method

1. keys( ) :  Returns a new iteration object , It includes  Map  All of the objects key , And insert  Map  The order of objects
2. values( ) :  Returns a new iteration object , It includes  Map  All of the objects value , And insert  Map  The order of objects
3. entries( ) :  Returns a new iteration object , It is a containing  Map  Of all key value pairs in the object  [key, value]  Array , And insert  Map  The order of objects
4. forEach( callback ) : 

      const map = new Map();
      map.set('name', 'Andy'); // {'name' => 'Andy'}
      map.set('age', 18); // {'name' => 'Andy'}
      map.keys(); // {'name', 'age'}
      map.values(); // {'Andy', 18}
      map.entries(); // {'name' => 'Andy', 'age' => 18}
      map.forEach((item) => {
        console.log(item); //Andy 18
      });

5、 Use for ... of iteration

      const map = new Map();
      map.set('name', 'Andy');
      map.set('age', 14);
      for (const [key, value] of map) {
        console.log(`key: ${key},value: ${value}`);
        // key: name,value: Andy
        // key: age,value: 14
      }
      for (const [key, value] of map.entries()) {
        console.log(`key: ${key},value: ${value}`);
        // key: name,value: Andy
        // key: age,value: 14
      }
      for (const key of map.keys()) {
        console.log(`key: ${key}`);
        // key: name
        // key: age
      }
      for (const value of map.values()) {
        console.log(`value: ${value}`);
        // value: Andy
        // value: 14
      }

6、Map  Relationships to arrays

1. Use regular Map Constructor can convert a two-dimensional array of key value pairs into a Map object

         const arr = [['name', 'Andy'], ['age', 14]];
         const map = new Map(arr);
         console.log(map); // {'name' => 'Andy', 'age' => 14}

2. Use Array.from Or the expansion operator can put a Map Object into a two-dimensional array

         const map = new Map();
         map.set('name', 'Andy');
         map.set('age', 14);
         console.log([...map]);//[['name','Andy'],['age',14]]

7、 Copy or merge Map 

1. Copy

         const map1 = new Map([['name', 'zs']]);
         const map2 = new Map(map1);
         console.log(map2 === map1);//false  Shallow comparison   Not a reference to the same object 
   
   
   
   
   
   
         const map1 = new Map([
           ['name', 'zs'],
           ['hobby', { sing: 'xxxx' }],
         ]);
         const map2 = new Map(map1);
         for (const [key, value] of map1) {
           if (key === 'hobby') {
             value.sing = 'sub';
           }
         }
         console.log(map1, map2);// hobby  Reference the same address 

2. Map Merge between objects , But it will keep the uniqueness of the key , The same key , The latter will overwrite the previous value

         const map1 = new Map([
           ['name', 'zs'],
           ['age', 18],
         ]);
         const map2 = new Map([
           ['name', 'ls'],
           ['age', 20],
         ]);
         const map3 = new Map([...map1, ...map2]);
         console.log(map3); // {'name' => 'ls', 'age' => 20}

3. Map Object and array merge , But it will keep the uniqueness of the key , The same key , The latter will overwrite the previous value

         const map1 = new Map([
           ['name', 'zs'],
           ['age', 18],
         ]);
         const arr = ['name', 'ls'];
         const newMap = new Map([...map1, arr]); //{'name' => 'ls', 'age' => 18}