The difference between let and VaR

Here is an explanation of a previous blog

Blog address ：let and var The difference between , Several small questions

The first question is ：

console.log(dog);   //undefined
var dog = " puppy "
console.log(cat);  //Error:   Cannot access 'cat' before initialization
let cat = " kitten "

explain ：

The first output , Because var Declared variables will result in variable Promotion , In short, it is the process of pre parsing variables , Will be assigned to undefined.
The second report is wrong , Because let The declared variable does not have a variable promotion

The second question is ：

The first 1 Group

var dad = ' I'm daddy ！'
console.log(dad);   // I'm daddy ！
var dad = ' I'm the father ！'
console.log(dad);   // I'm the father ！

The first 2 Group

let son = ' I'm a son ！'
console.log(son);    // I'm a son ！
let son = ' I'm the son ！'
console.log(son);    //Error:  'son' has already been declared

explain ：

The first group can output normally , Because var Declared variables can be declared repeatedly
The second report is wrong , Because let Declared variables cannot be declared repeatedly

Third question

{
	let a = 1;
	{
		//   Block level scope 
    	let b = 2;
    	console.log(a);    //   1
	}
	console.log(b);   // b is not defined
}

explain ：

The first output , Because the current block level scope does not have a This variable , It will automatically look up the scope one level up , In the parent scope a It's assigned a value 1
The second report is wrong , Because let Declared variables automatically generate block level scopes ,b Is a variable declared at the block level scope , The outer scope cannot be accessed

Fourth question

for (let i = 0; i < 3; i++) {
	console.log(i);  //   1   2
}
console.log(i);   //Error:  i is not defined

explain ：

The first output , Because for In circulation let Declared variables automatically generate block level scopes ,i Is a variable declared at the block level scope , Within this block level scope, you can normally access
The second report is wrong , Because it's beyond i Block level scope

Fifth question

The first 1 Group

var monkey = ' I'm the monkey king ！';
{
	console.log(monkey);   //  I'm the monkey king ！
	var monkey = ' I'm a six eared macaque ';
}
console.log(monkey);    // I'm a six eared macaque 

The first 2 Group

let monkey = ' I'm the monkey king ！';
{
	console.log(monkey);    //  Error: Cannot access 'monkey' before initialization
	let monkey = ' I'm a six eared macaque ';
}
console.log(monkey);   // I'm the monkey king ！

explain ：

The first group can output normally , Because var The declared variable does not have a block level scope
The second report is wrong , Because ES6 Specifies when we use... In a block level scope let and const To declare variables , This variable will initially form a closed scope , In other words, even if the variable with the same name exists in the upward scope, we cannot access

Here, we should pay attention to distinguish between functions var And let The difference between , The code is as follows

<script> let a = 1; (function () {
       console.log(a); // There will be an error  let a = 2; })(); </script>`