define variable command

where

　　There is variable promotion

 

 function

　　There is variable promotion. In es6 browsers, it is best not to use function declarations in block-level scopes, and use expressions

 

let

　　Temporary dead zone, no variable promotion, no longer a property of the top-level object, the global object is decoupled from the top-level object

 

const

　　Temporary dead zone, no variable promotion, constant definition, once declared, must be assigned immediately, no longer a property of the top-level object, the global object is decoupled from the top-level object

 

import

 

class