public static const in TypeScript

Question

Is there such a thing as public static constants in TypeScript   I have a class that looks like   export class Library     public static BOOK SHELF NONE  string    None     public static BOOK SHELF FULL  string    Full       In that class  I can do Library BOOK SHELF NONE and the tsc doesn t complain   But if I try to use the class Library elsewhere  and try to do the same thing  it doesn t recognize it

User · Answer

You can do it using namespaces  like this   export namespace Library       export const BOOK SHELF NONE  string    NONE       Then you can import it from anywhere else   import  Library  from    Library   console log Library BOOK SHELF NONE     If you need a class there as well include it inside the namespace  export class Book

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Meanwhile this can be solved through a decorator in combination with Object freeze or Object defineProperty  I m using this  it s a little bit prettier than using tons of getters  You can copy paste this directly TS Playground to see it in action  - There are two options    Make individual fields  final   The following decorator converts both  annotated static and non-static fields to  getter-only-properties    Note  If an instance-variable with no initial value is annotated  final  then the first assigned value  no matter when  will be the final one      example class MyClass        final     public finalProp  string    You shall not change me          final     public static FINAL FIELD  number   75       public static NON FINAL  string    I am not final      var myInstance  MyClass   new MyClass    myInstance finalProp    Was I changed    MyClass FINAL FIELD   123  MyClass NON FINAL    I was changed     console log myInstance finalProp         gt  You shall not change me  console log MyClass FINAL FIELD          gt  75 console log MyClass NON FINAL            gt  I was changed    The Decorator  Make sure you include this in your code          Turns static and non-static fields into getter-only  and therefor renders them  final     To use simply annotate the static or non-static field with   final    function final target  any  propertyKey  string        const value  any   target propertyKey          if it currently has no value  then wait for the first setter-call        usually the case with non-static fields     if   value            Object defineProperty target  propertyKey                set  function  value  any                    Object defineProperty this  propertyKey                        get  function                              return value                                             enumerable  true                      configurable  false                                                enumerable  true              configurable  true                   else      else  set it immediatly         Object defineProperty target  propertyKey                get  function                      return value                             enumerable  true                         As an alternative to the decorator above  there would also be a strict version of this  which would even throw an Error when someone tried to assign some value to the field with  use strict   being set   This is only the static part though          Turns static fields into getter-only  and therefor renders them  final      Also throws an error in strict mode if the value is tried to be touched     To use simply annotate the static field with   strictFinal     function strictFinal target  any  propertyKey  string        Object defineProperty target  propertyKey            value  target propertyKey           writable  false          enumerable  true               Make every static field  final   Possible Downside  This will only work for ALL statics of that class or for none  but cannot be applied to specific statics         Freezes the annotated class  making every static  final     Usage     StaticsFinal   class MyClass          public static SOME STATIC  string    SOME STATIC                       function StaticsFinal target  any        Object freeze target          Usage here  StaticsFinal class FreezeMe       public static FROZEN STATIC  string    I am frozen      class EditMyStuff       public static NON FROZEN STATIC  string    I am frozen         Test here FreezeMe FROZEN STATIC    I am not frozen    EditMyStuff NON FROZEN STATIC    I am not frozen     console log FreezeMe FROZEN STATIC        gt   I am frozen   console log EditMyStuff NON FROZEN STATIC        gt   I am not frozen

User · Answer

The following solution also works as of TS 1 7 5      Constancts ts     export const kNotFoundInArray   -1  export const AppConnectionError   new Error  The application was unable to connect     export const ReallySafeExtensions     exe    virus    1337h4x      To use      Main ts     import  ReallySafeExtensions  kNotFoundInArray  from    Constants    if  ReallySafeExtensions indexOf  png       kNotFoundInArray        console log  PNG s are really unsafe

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Here s what s this TS snippet compiled into  via TS Playground    define   require    exports    function require  exports        var Library    function              function Library                       Library BOOK SHELF NONE    None           Library BOOK SHELF FULL    Full           return Library                exports Library   Library        As you see  both properties defined as public static are simply attached to the exported function  as its properties   therefore they should be accessible as long as you properly access the function itself

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You can use a getter  so that your property is going to be reading only  Example    export class MyClass       private  LEVELS             level1   level1           level2   level2           level2   level2              public get STATUSES             return this  LEVELS            Used in another class    import   MyClass   from  myclasspath   class AnotherClass       private myClass   new MyClass         tryLevel            console log this myClass STATUSES level1

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If you did want something that behaved more like a static constant value in modern browsers  in that it can t be changed by other code   you could add a get only accessor to the Library class  this will only work for ES5  browsers and NodeJS    export class Library       public static get BOOK SHELF NONE   string   return  None         public static get BOOK SHELF FULL   string   return  Full           var x   Library BOOK SHELF NONE  console log x   Library BOOK SHELF NONE    Not Full   x   Library BOOK SHELF NONE  console log x     If you run it  you ll see how the attempt to set the BOOK SHELF NONE property to a new value doesn t work    2 0  In TypeScript 2 0  you can use readonly to achieve very similar results   export class Library       public static readonly BOOK SHELF NONE    None       public static readonly BOOK SHELF FULL    Full       The syntax is a bit simpler and more obvious  However  the compiler prevents changes rather than the run time  unlike in the first example  where the change would not be allowed at all as demonstrated

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Just simply  export  variable and  import  in your class  export var GOOGLE API URL    https   www googleapis com admin directory v1       default err string message export var errStringMsg    Something went wrong     Now use it as   import appConstants   require     core AppSettings    console log appConstants errStringMsg   console log appConstants GOOGLE API URL

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Thank you WiredPrairie    Just to expand on your answer a bit  here is a complete example of defining a constants class      CYConstants ts  class CYConstants       public static get NOT FOUND    number      return -1        public static get EMPTY STRING    string   return          export   CYConstants    To use     main ts  import CYConstants   require    CYConstants     console log CYConstants NOT FOUND         Prints -1 console log CYConstants EMPTY STRING      Prints     Nothing

[typescript] public static const in TypeScript

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