Using a let statement to declare the usage of scopes in JavaScript _ basics

Grammar

Let variable1 = value1

Parameters
Variable1
The name of the variable to declare.
Value1
The initial value assigned to the variable.

Note
use the LET statement to declare a variable whose scope is limited to the block in which it is declared. You can assign a value to a variable when you declare it, or you can assign a value to a variable in a script later.
Variables that use let declarations cannot be used before the declaration, or they will result in an error.
If you do not initialize your variable in a let statement, the JavaScript value undefined is automatically assigned to it.

Example:

var L = ten;
{Let
  L = 2;
  At this point, L = 2.
}
At this point, L = ten.

Additional ways to declare a is variable using let.
Let index;
Let name = "Thomas Jefferson";
Let answer =, counter, numpages = ten;
Let myarray = new Array ();

Block-level scopes

for (var i = 0; i < i++) {}
Console.log (i),//10 for

(Let j = 0; J < Ten; j) {}
Console.log (j);//"Refe RENCEERROR:J is not defined

There is no variable elevation

Console.log (a); Output undefined
console.log (b);//Error Referenceerror
Console.log (c);//Error Referenceerror
var a = 2;
Let B = 2;

Notice the difference between undefined and referenceerror.

Temporary Dead Zone (TDZ)
as long as you enter the current block-level scope, the variable you are using already exists, but before declaring it belongs to the dead zone and cannot be manipulated.
Note: typeof is no longer a 100% safe operation

typeof X; Referenceerror
typeof y//undefined let
x;

Do not allow duplicate declarations

Let x = 1;
Let X; "Syntaxerror:identifier ' x ' has already been declared

var y = 2;
var y = 3; y = 3

Block-level scopes

anonymous function notation
(function () {
 var tmp = ...;
 ...
} ());

Block-level scope notation
{let
 tmp = ...;
 ...
}

The strict mode of ES5 stipulates that functions can only be declared within the top-level scope and function, and other cases (such as if code blocks, circular blocks) will be declared with an error.

ES5
' use strict ';
if (true) {
 function f () {}//Error
}

ES6 because of the introduction of block-level scopes, this situation can be interpreted as a function declared within a block-level scope, so there is no error, but the curly braces that make up the block are not less

Error
' use strict ';
if (true)
 function f () {}

Declared global variable is no longer a window's property

"Use strict";
var a = 1;
Console.log (WINDOW.A)//1 let

b = 1;
Console.log (window.b)//undefined