ES6 and let the const

In the js, to use the definition of the variable var operator, but var has many disadvantages, such as: can be repeated a variable declaration, not block-level scope, modifications and the like can not be restricted.

@ Disadvantage: declare variable can be repeated 
var A = 1 ;
 var A = 2 ; 
the console.log (A); // 2 
// 2 disadvantage: it can not limit the modified 
@ disadvantages 3: no block-level scope 
{
     var B 12 is = ; 
} 
the console.log (B); // 12 is

ES6 new const and let you solve these problems. Let's take a look and let it const.

1. let const and can not be repeated statement

   let a=1;
   let a=2;
   console.log(b); //SyntaxError: Identifier 'a' has already been declared

2. let statement is variable value can be modified; declared const is a constant and can not be modified

   const a=1;
   a=2;
   console.log(a); //TypeError: Assignment to constant variable.
   

3. There are block-level scope

   var a=[];
   for(var i=0; i<5;i++){
       a[i] = function (){
           console.log(i);
       }
   }
   a[2](); //5

   In the above example, the variable var i is defined, the global scope is, only a global i, each cycle, the value of the variable i + 1 will, after the end of the cycle, i is 5. All members of the array a, a [0] ... a [4] i are the same inside, so that the final output is 5.

   let a=[];
   for(let i=0; i<5;i++){
       a[i] = function (){
           console.log(i);
       }
   }
   a[2](); //2

   In this example, i is declared using let only effective block-level scope. Each member of a array has its own i.

　　　  Further, it is noted that, for that part of the circulating loop variable is set parent scope, the loop body is a separate sub-scope.

for(let i=0; i<2; i++){
    let i=12;
    console.log(i);
}
//12
//12