JSIn thelet,const,var

1.JSHow many ways are there to define variables in?

  • let
  • const
  • var
  • class
  • import
  • function

2,let,const,varWhat’s the difference?

var let const
There is no block-level scope There are block-level scopes There are block-level scopes
Declare global variables inwindow 下 Global variables are not under global attributes Global variables are not under global attributes
Redefining a variable does not report an error complains complains
Declare a variable Declare a variable Declare a constant
There is variable promotion There is no variable promotion There is no variable promotion
Assign at any time after the declaration Assign at any time after the declaration Assign immediately after the declaration

3,constCan I change a defined constant?

  1. constDefining base types cannot be modified;
  2. constDefining a reference type allows you to modify the value inside the reference type.

If I want toconstDefining a reference type does not change its value.

  1. Object.freeze;
  2. Agent (proxy/Object.defineProperty);
  3. Modify objectconfigurable,writableProperties.

5, how inES5In this caselet 和 const?

1. The implementationlet

This can be done by self-executing functions.

2. Implementconst

This can be done with Object.defineProperty() to set writable.