This is the 9th day of my participation in the August Wen Challenge.More challenges in August

preface

As we all know, before ES6, we used var to declare variables.

Es6 has added two new commands to declare variables, const and let.

The let command

The let command declares variables, which can only be accessed and used in the current scope without variable promotion (variable promotion is covered below).

let name = 'the answer cp3'
console.log(name) / / the answer cp3
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Const command

The const command declares a constant that cannot be changed or promoted.

const name = 'the answer cp3'
console.log(name)  / / the answer cp3
name = 'cp3' Assignment to constant variable
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While const declares a constant to be immutable, it only guarantees that the value of the variable on the stack is immutable.

  • So if you declare variables as primitive types, that’s fine, because their values are stored on the stack and are guaranteed to be immutable
  • But if the variable is a reference type, because the value of the reference type is stored in the heap, the stack only stores the address of the value pointing to the heap, and the value of the heap can change
const obj = {}
obj = {}
console.log(obj) Assignment to constant variable

const obj = {}
obj.name = 'the answer cp3'
console.log(obj) // {name: "answer cp3"}
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Here’s how they differ from VAR.

  1. Var boosts variables, const and let do not. Variable promotion refers to declaring a variable as undefined at the beginning of the current scope.

    console.log(name) // undefined
var name = 'the answer cp3'
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    console.log(name) // name is not defined
let name = 'the answer cp3'
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    Var declares name, which will promote the variable, declare the variable as undefined, and then assign the answer cp3

    Is equivalent to

    var name = undefined
console.log(name) // undefined
name = 'the answer cp3'
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    Const and let are not variable promoted and cannot be used before declaration, which is grammatically called temporal dead zone (TDZ).

  2. Var can be declared repeatedly. Const and let cannot be declared repeatedly in the same scope.

    var name = The 'with answers'
var name = 'cp3' / / normal

let name = The 'with answers'
let name = 'cp3' // Warning Identifier 'name' has already been declared

const name = The 'with answers'
const name = 'cp3' // Warning Identifier 'name' has already been declared
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    If it is declared under a different scope, it is fine, as follows

     let name = The 'with answers'
 if(true) {
   let name = 'cp3' / / without error
 }
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    Note: Because the if statement belongs to another block-level scope, it does not affect it

  3. Var declarations in the global scope mount variables on window objects, but let and const declarations do not mount variables on any objects.

    var name = 'the answer cp3'
cosole.log(window.name) / / the answer cp3

let name1 = 'the answer cp3'
console.log(window.name1) //undefined

const name2 = 'the answer cp3'
console.log(window.name2) //undefined
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conclusion

To summarize the const and let commands:

  • Const declares a constant, which guarantees immutable values for base types but not for reference types
  • Let declares variables. The same variable cannot be declared repeatedly in the same scope, nor is const
  • Const and let do not have variable promotion
  • Declarations of const and let in global scope are not mounted towindowobject

Thank you for reading.