Grammar definition ：  There are two main forms of property descriptors in objects ： Data descriptor and Access descriptor .

Object.defineProperty() Method defines a new property directly on an object , Or modify an object's existing

attribute , And return this object .

grammar

//obj: The object to define the property 
//prop: The name of the property to be defined or modified 
//descriptor:{}
Object.defineProperty(obj, prop, descriptor)

The first part ： Data descriptor It's a property with value , The value can be writable , It can also be unwritten .

// Three default characteristics of data descriptor ：
1. Do not modify the ：
2. Non rewritable ：
3. countless ：
let obj = {}
Object.defineProperty(obj,'a',{
    value:1,
})
console.log(obj) //{a:1}

Key values that data descriptors can have ：

configurable： If and only if the  configurable  The key value is  true  when , This attribute can also be

Delete . The default is  false

/*
 If and only if the  configurable  The key value is  true  when ,
 This attribute can be deleted from the corresponding object . The default is  false.
*/ 
let obj = {}
Object.defineProperty(obj,'a',{
    value:1,
    configurable:true,

})
//  configurable:true  Deleting 
delete obj.a
console.log(obj) //{}   Print an empty string 

enumerable： If and only if the  enumerable  The key value is  true  when , This property will appear in the enumeration property of the object . The default is  false.

/*
 If and only if the  enumerable  The key value is  true  when , This attribute will appear in the object 
 In the enumeration properties of . The default is  false.
*/
let obj = {}
Object.defineProperty(obj,'a',{
    value:1,
    enumerable:true,
})
for (const key in obj) {
    console.log(key)//enumerable：false countless ： Can't print out 
}

value： The value corresponding to this property . It can be anything that works JavaScript value （ The number , object , Functions, etc ）. The default is  

undefined.

 The given property name is undefined ： Default undefined

writable： If and only if the  writable  The key value is  true  when , The value of the property , That's the top  value, Can be changed by the assignment operator . The default is  false.

/*
 If and only if the  writable  The key value is  true  when , The value of the property , That's up there 
 Of  value, Can be assigned to the operator  (en-US) change . The default is  false.
*/ 
let obj = {}
Object.defineProperty(obj,'a',{
    value:1,
    writable:true

})
obj.a = 2
console.log(obj) //{a:2}

//writable:false It can't be changed a Value 

The second part ： Access descriptor ： Access descriptors are created by getter Functions and setter The properties described by the function .

/*
getter
 Attribute  getter  function , without  getter, Then for  undefined.
 When accessing this property , This function will be called . No parameters are passed in during execution , But it will pass in  this  object 
 The return value of this function will be used as the value of the property .
*/ 
/*
set
 Attribute  setter  function , without  setter, Then for  undefined.
 When the property value is modified , This function will be called . This method takes a parameter （ That is, the new value given ）,
 The  this  object .
*/ 
let obj = {}
Object.defineProperty(obj,'a',{
  get(){
     //  visit a Time call 
     console.log('get a:',1) // to get Inside a assignment 1
     return 1
    },
   set(newValue){
      //  modify a Start when the value of : Note that there is no return
      // newValue: For the modified attribute value 
      console.log('set a:',newValue)
    }
})

The third part ：Object.defineProperties

<script>
   let obj = {}
   let num = 1
   Object.defineProperties(obj,{
      a:{
         get(){
             return num++
         },
         set(){
         }
      },
      b:{
         value:1
      }
   })
   if(obj.a===1&&obj.a===2&&obj.a===3&&obj.a===4){
       console.log(' Hello ')
   }
   console.log(obj)
</script>