Typescript -- Section 2: variable declaration

Variable declarations

Variable declarations described in this section （var let const） Is based on JavaScript Of , The method used is exactly the same .

Simply familiarize yourself with the usage

1. var（ Not recommended ）
   · Used to declare the global scope and Local scope （ Or function scope ）, Block level scopes cannot be declared
   · There is variable promotion
   · have access to var Declare duplicate variable names , The front will be covered by the back .
2. let（ It is recommended to use ）
   · Used to declare a block level scope . for example {}, Is a scope
   · No variable promotion
   · Repeated declaration of variables in the same scope , Will report a mistake
   · Temporary dead zone （TDZ）. It means that the variable already exists in the scope , Must be in let / const Use... After the declaration ;TDZ It enables developers to form the habit of declaration before use , Make the program more stable
3. const（ It is recommended to use ）
   ·const Declare a read-only constant , Once declared , Constant values cannot be changed . therefore const Once declared, it must be initialized immediately .
   · Except for the first one , Others are similar to let
   · Little knowledge supplement ： Generally, variable names used to declare constants use upper case letters , Convention becomes custom , Just like the constructor's name is capitalized .

• Do simple things over and over again , Do it seriously ！！！ --wcc