es6 var, let, const commands

1. let and var

　　<1> Variables declared by let are only valid within the block-level scope;

　　　　Variables declared by var are globally valid;

 

　　<2> var variables are happy to be used before declaration, output undefined;

　　　　let can't, throw an error directly;

 

E.g:

　　//var 声明

　　console.log(a);   //undefined
　　var a = 55;

　　// let statement 

　　console.log(b);    // Error ReferenceError 

　　let b = 67;

 

Note: The part of the loop variable of the for loop is a parent scope, and the inside of the loop body is a separate child scope.

ES6 clearly stipulates that if there are letand constcommands in a block, the variables declared by this block for these commands form an enclosing scope from the beginning. Anytime these variables are used before they are declared, an error will be reported.

　　<3> letIt is not allowed to declare the same variable repeatedly in the same scope.

　　E.g:

　　　　

function func(){ 

　   // Error 
　　var a;

　　let a;

}

function func(arg) {
  　　let arg; // Error report 
}

 

 

2.const

　　constDeclare a read-only constant. Once declared, the value of the constant cannot be changed.

　　 constThe scope of the letcommand is the same as that of the command: it is only valid within the block-level scope in which it is declared.