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JS Es5 can also create constants?

2022-07-07 01:39:00 【Director of Moral Education Department】

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 .