ES6, the distinction var, let the const and

var features

• There is no concept of code blocks, globally valid
  
  
• There is "variable lift" phenomenon, that can be used before variable declarations, value is undefined

let's features

• Variables declared valid only within the block-level action, the presence of the concept of block
  
  
• There is no "variable lift" phenomenon
  
  
• There is a temporary dead zone, before the tmp variable that is declared, all belong tmp "dead zone"
  
  
• Not allowed to repeat a statement

const features

• Once the modified identifier to be assigned const, can not be modified
  const name = 'Why';
  name = 'ABC'; // will complain

• Use const definition identifier must be assigned
  const name; // will complain

• The meaning of the object pointed constant is not modified, but the attribute of an object can change
  const obj = {name: 'XXX', Age: 18 is}
  obj.name = 'AAA';
  obj.age = 20 is; // this is not being given