object

1、 All object property names must be of type string , It must be ensured that Each key name is a string type and is unique in the object

When the key name of the attribute is Number Type ：

var map=Object.create(null)
map[5]='hello'
conslole.log(map['5'])  //'hello'
conslole.log(map[5])   //'hello'

In this example, the object （map） Properties of （5） Assign as string ’hello’, The key name of this attribute is numeric 5, It will be Automatically ( mandatory ) conversion Is a string type , therefore map[5]===map[‘5’], They actually refer to the same attribute .

• When the key name of the attribute is Object Type ：

var map=Object.create(null)
var key1={
    },key2={
    }
map[key1]='hello'
console.log(map[key2]) //'hello'
console.log(map[key2]) //'hello'

In this case key1 and key2 Will be converted to the string corresponding to the object ’[Object Object]’, therefore map[key1]===map[]key2], It refers to the same attribute .

Map

1. ECMAScript6 Medium Map A type is one that stores many An ordered list of key value pairs , The key name and its corresponding value Support all data types .

2. Map Collection support methods ：

• has(key)—— return Boolean, Detect that the specified key is in Map Whether the collection already exists

• delete(key)—— from Map Collection Delete the specified key value pair

• clear()—— Delete Map Collection All key value pairs

• forEach()—— Traverse and manipulate each value in the collection

  let map=new Map([['name','dog'],['age',3]])
  map.forEach(function(value,key,ownerMap){
        //forEach The callback function of the method accepts 3 Parameters ,（ value , Key name ( In the array, the corresponding index value is numeric ),Map The assembly itself ）
      console.log(key+' '+value)
      console.log(map===ownerMap)
  })
  //output:
  //name dog
  //true
  //age 3
  //true
  

• set()—— Add any type of value to the collection

• get()—— Retrieve all values in the collection

• size—— adopt size Property can check the number of values contained in the collection

3. Map Set initialization

let map=new Map()
map.set('name','dog')
map.set('age',3)
// Empathy 
// let map=new Map([['name','dog'],['age',3]])

console.log(map.size)  //2
console.log(map.has('name'))//true
console.log(map.get('name'))//'dog'

map.delete('age')
console.log(map.has('age'))//false
console.log(map.get('age'))//undefined
console.log(map.size)  //1

map.clear()
console.log(map.has('name'))//false
console.log(map.size)//0

WeakMap

1. This set is a special kind of Map aggregate , Only key names of object types are supported .

2. Stored in the collection Keys are weak references to objects

3. Key name enumeration is not supported ： That is not supported forEach() and clear() Method

4. Use scenarios ：

• Add additional information for those objects whose actual use is separated from lifecycle management ;

• Storage DOM Elements ;

• Store private properties of object instances

  let Person=(function(){
        
  	let privateData=new WeakMap() // Store private properties 
  	function Person(name){
            //Person Instance of object （this） It can be used directly as the key of the collection 
  		privateData.set(this,{
        name:name})// call Person Constructor , New entries will be added to WeakMap Collection , The key of the entry is this, Value is the private information contained in the object （ Which contains name Object of property ）
  	}
  	Person.prototype.getName=function(){
        
  		return privateData.get(this).name// Access to private information 
  	}
  	return Person // As long as the object instance is destroyed , Relevant information will also be destroyed , So as to ensure the privacy of information 
  }())