On let variable promotion

let Whether the declared variable has variable promotion ？

We all know var Declared variables , There will be variable Promotion , Just before the variable declaration , Can be accessed .
The code is as follows

(function(){
    
	console.log(x) //undefind
	var x = "1111"
}())

there x What's printed out is undefind, It's because the variable increases , Everyone knows this, no problem

We're looking at a piece of code

var x = y = "global"
(function(){
    
	console.log(x) //global
	console.log(y) //global
}())

We use it var Variable declared x y by global, In the self executing function , We print x and y, All show global, No problem

Look again

var x = y = "global"
(function(){
    
	console.log(x)//undifind
	console.log(y)// Report errors   The picture below shows 
	var x = "1111"
	let y = "1111"
}())

It is also stated here x y by global , Print in the self executing function x and y, Just two more lines of code , Here the print x The result is undifind, Print y Will report a mistake ,x Print out undifind Because we defined it later var x=“1111”, The variable goes up , Print y Does the error report prove let Declared variables also exist, and variables are promoted , Otherwise, print y, Should be displayed global That's right .

After reading the above , I think you should know ,let There are also variable improvements , Let's take a look at the contents of the error report , Cannot access before initialization y, That's what's called let Of " Temporary dead zone ",let Declared variables , Cannot be accessed before declaration .

Let's see ,var To declare a variable establish initialization The process of assignment , Suppose you have the following code

function fn(){
    
	var x = 1;
	var y = 1;
}
fn()

• First, create variables in memory x and y
• Then initialize these variables to undifind
• Then execute the code in the function , Perform assignment operation

Look again. let Of

{
    
	let x = 1
	x = 2
}

• find let Declared variables , Create these variables in memory
• Then start executing the code , Note that there is no initialization operation here
• let x = 1, This step is the initialization operation , take x Initialize to 1
• x = 2 , This step is assignment

To sum up
var Declared variables , The creation and initialization phases have been upgraded
let Declared variables , The creation phase has been upgraded , There is no prompt during initialization