JS three kinds of definition variable var let const

• let is only valid within the block of code in which it is located, and will be an error on the outside
• Let is a block-level scope that has no effect on external functions after using let definitions inside the function
• Let/const does not exist variable promotion, must be declared before use, in use, otherwise will error, can not repeat the declaration
• Once a const variable is declared, the value of the constant cannot be changed, it must be initialized immediately and cannot be left to the last assignment

JS three kinds of definition variable var let const