TS: Extension of Enumeration

TS: Extension of Enumeration

1. Enumeration calculation

• Enumeration members can be initialized using constant enumeration expressions :

  enum E {
        
        
      None ,
      OK = 1 << 1,
      Read = 1 << 2,
      Write = 2 << 1,
      ReadWrite = Read | Write,
      Pending = "123".length
  }
  

2. Enumeration member type

• After enumeration members are enumerated, they have a special semantics;

• Enumeration members become types:

  enum E {
        
         A , B }
  interface A {
        
        
      type : E.A;
      value : number
  }
  
  let c : A {
        
        
      type : E.B,	//error , A接口的type属性的类型为E.A
      value : 1
  }
  

• Enumerations are objects that really exist at runtime:

  enum E {
        
         X , Y}
  function foo(obj : {
        
        X : number}){
        
        
      return obj,X;
  }
  
  foo(E);		//ok,E是一个真实存在的对象，X他的一个属性，值为0。
  

4. Reverse mapping of enumeration

• In addition to the enumeration value obtained from the enumeration name, the enumeration can also obtain the enumeration name from the enumeration value.

  enum E {
        
        X , Y}
  console.log(E[0])	// "X"
  

5. Constant Enumeration

• In order to avoid the overhead of additional generated code and additional indirect access to enumeration members, constenumeration can be used .

  const enum E{
        
        
      A = 1,
      B = 2 * A
  }
  

• Constant enumeration can only use constant enumeration expressions;

• Constant enumeration will be deleted during the compilation phase and replaced by the value of the enumeration;

• External enumeration: External enumeration is used to describe the shape of an existing enumeration type.

  • There is an important difference between external enumeration and non-external enumeration: In normal enumeration, members without initialization methods are treated as constant members.
  • For non-constant external enumerations, when there is no initialization method, it is considered to be calculated.

  declare enum E{
        
        
      A = 1,
      B , 
      C = 3
  }