JS Es5 can also create constants?

Brief introduction

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ES6

  When it was first launched ,

let

  and  

const

  Most people should learn  

ES6

  My first knowledge point .

among  

const

  It can be used to define  

Constant

 , Define data that does not need to be changed as a constant .

But in fact  

ES6

  Before, we also had a way to define constants .

ES 5  Create constant

Object.defineProperty

  Basic usage

stay  

ES6

  Not before  

const

  Of , If you need to define

Constant

, have access to  

Object.defineProperty

 .

A lot of people know  

Vue2

  Use  

Object.defineProperty

  Monitor data changes , But not necessarily  

Object.defineProperty

  It can also be used to define constants .

Object.defineProperty

  Method defines a new property directly on an object , Or modify the existing properties of an object , And return this object .

Object.defineProperty(obj, prop, descriptor)

  receive 3 Parameters

• obj:  The object to define the property .

• prop:  The name of the property to be defined or modified or  
  Symbol
   .

• descriptor:  Property descriptor to define or modify .

for instance

var LH = {}
Object.defineProperty(LH, 'name', {
 value: ' Thunder monkey '
})

console.log(LH) //  Output : {name: ' Thunder monkey '}

LH.name = ' Shark chilli '

console.log(LH) //  Output : {name: ' Thunder monkey '}


You can put the above code into your browser to try .

Why modify  

LH.name

  It doesn't work ？ because  

descriptor

  except  

value

  outside , There are other properties , such as  

writable

  Can be used to define whether the object is allowed to be modified , The default is  

false

 , That is, you can't modify .

therefore  

LH.name = ' Shark chilli '

  The modification is invalid .

If you will  

writable

  Change to  

true

  It can be modified

var LH = {}
Object.defineProperty(LH, 'name', {
 value: ' Thunder monkey ',
 writable: true //  Allow modification of
})

console.log(LH) //  Output : {name: ' Thunder monkey '}

LH.name = ' Shark chilli '

console.log(LH) //  Output : {name: ' Shark chilli '}


Create constant

Follow the ideas above , If we put  

LH

  Change to  

window

 , Is it possible to  

window

  Define an attribute on , And the attribute is global , After definition, it can be called anywhere .

Object.defineProperty(window, 'NAME', {
 value: ' Thunder monkey '
})

console.log(NAME) //  Output :  Thunder monkey

NAME = ' Shark chilli '
console.log(NAME) //  Output :  Thunder monkey

window.NAME = ' Cockroach bully '
console.log(NAME) //  Output :  Thunder monkey


No matter how it is modified ,

NAME

  Are the first defined values .

Constants can actually change values ？

Constants created above ,

value

  It's a

Basic data type

Value . If replaced

Reference type

Value , That content can be modified .

Object.defineProperty(window, 'NAME', {
 value: {
 nickname: ' Thunder monkey '
 }
})

console.log(NAME)

NAME.nickname = ' Shark chilli '

console.log(NAME)


as a result of , A constant locks the memory address to which it is defined .

When defining basic data types , The memory address points directly to that value . But definition

Reference type

when , Memory address is the reference address . So the definition of a constant is

The reference address cannot be modified

.

Compatibility

Use  

Object.defineProperty

  You need to pay attention to compatibility when defining constants

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