Declaring static constants in ES6 classes

Question

I want to implement constants in a class  because that s where it makes sense to locate them in the code   So far  I have been implementing the following workaround with static methods   class MyClass       static constant1     return 33        static constant2     return 2                   I know there is a possibility to fiddle with prototypes  but many recommend against this   Is there a better way to implement constants in ES6 classes

User · Answer

You can make the  constants  read-only  immutable  by freezing the class  e g   class Foo       static BAR    bat     public static read-only    Object freeze Foo        Uncaught TypeError  Cannot assign to read only property  BAR  of function  class Foo       static BAR    bat     public static read-only       Foo BAR    wut

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In this document it states      There is  intentionally  no direct declarative way to define either prototype data properties  other than methods  class properties  or instance property   This means that it is intentionally like this   Maybe you can define a variable in the constructor   constructor        this key   value

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Adding up to other answers you need to export the class to use in a different class  This is a typescript version of it   x000D   x000D    Constants tsx const DEBUG  boolean   true   export class Constants     static get DEBUG    boolean       return DEBUG           Anotherclass tsx import   Constants   from  Constants    if  Constants DEBUG      console log  debug mode     x000D   x000D   x000D

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Like https   stackoverflow com users 2784136 rodrigo-botti said  I think you re looking for Object freeze    Here s an example of a class with immutable statics   class User     constructor username  age        if  age  lt  User minimumAge          throw new Error  You are too young to be here               this username   username      this age   age      this state    active          User minimumAge   16  User validStates     active    inactive    archived     deepFreeze User    function deepFreeze value      if  typeof value      object   amp  amp  value     null        Object freeze value       Object getOwnPropertyNames value  forEach property   gt          deepFreeze value property                  return value

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Here is one more way you can do  x000D   x000D     one more way of declaring constants in a class  Note - the constants have to be declared after the class is defined    class Auto       other methods   Auto CONSTANT1    const1   Auto CONSTANT2    const2    console log Auto CONSTANT1  console log Auto CONSTANT2   x000D   x000D   x000D   Note - the Order is important  you cannot have the constants above Usage console log Auto CONSTANT1

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You can create a way to define static constants on a class using an odd feature of ES6 classes   Since statics are inherited by their subclasses  you can do the following   const withConsts    map  BaseClass   Object    gt      class ConstClass extends BaseClass       Object keys map  forEach key   gt        Object defineProperty ConstClass  key          value  map key         writable   false        enumerable   true        configurable   false                 return ConstClass      class MyClass extends withConsts   MY CONST   this is defined         foo         console log MyClass MY CONST

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Maybe just put all your constants in a frozen object   class MyClass        constructor             this constants   Object freeze               constant1  33              constant2  2                         static get constant1             return this constants constant1             doThisAndThat                           let value   this constants constant2

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I m using babel and the following syntax is working for me   class MyClass       static constant1   33      static constant2            case1  1         case2  2                       MyClass constant1     33 MyClass constant2 case1     1   Please consider that you need the preset  stage-0   To install it     npm install --save-dev babel-preset-stage-0     in  babelrc        presets     stage-0       Update   currently use stage-3

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You can define it like this   class Foo     static MyConst   200     myFunc         const doubleConst   Foo MyConst   2

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The cleanest way I ve found of doing this is with TypeScript - see How to implement class constants  class MyClass       static readonly CONST1  string    quot one quot       static readonly CONST2  string    quot two quot       static readonly CONST3  string    quot three quot

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class Whatever       static get MyConst     return 10       let a   Whatever MyConst    Seems to work for me

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Here s a few things you could do   Export a const from the module  Depending on your use case  you could just   export const constant1   33    And import that from the module where necessary  Or  building on your static method idea  you could declare a static get accessor   const constant1   33        constant2   2  class Example      static get constant1         return constant1         static get constant2         return constant2          That way  you won t need parenthesis   const one   Example constant1    Babel REPL Example  Then  as you say  since a class is just syntactic sugar for a function you can just add a non-writable property like so   class Example     Object defineProperty Example   constant1         value  33      writable   false      enumerable   true      configurable   false     Example constant1     33 Example constant1   15     TypeError   It may be nice if we could just do something like   class Example       static const constant1   33      But unfortunately this class property syntax is only in an ES7 proposal  and even then it won t allow for adding const to the property

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You could use import   as syntax  Although not a class  they are real const variables   Constants js  export const factor   3  export const pi   3 141592    index js  import   as Constants from  Constants js  console log  Constants factor

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It is also possible to use Object freeze on you class es6  constructor function es5  object to make it immutable   class MyConstants    MyConstants staticValue   3  MyConstants staticMethod   function       return 4    Object freeze MyConstants      after the freeze  any attempts of altering the MyConstants class will have no result     either trying to alter  add or delete a property  MyConstants staticValue     3     true MyConstants staticValue   55     will have no effect MyConstants staticValue     3     true  MyConstants otherStaticValue    other     will have no effect MyConstants otherStaticValue     undefined    true  delete MyConstants staticMethod    false typeof MyConstants staticMethod       function     true   Trying to alter the class will give you a soft-fail  won t throw any errors  it will simply have no effect

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I did this     class Circle       constuctor radius                this radius   radius            static get PI                 return 3 14159            The value of PI is protected from being changed since it is a value being returned from a function   You can access it via Circle PI    Any attempt to assign to it is simply dropped on the floor in a manner similar to an attempt to assign to a string character via

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If you are comfortable mixing and matching between function and class syntax you can declare constants after the class  the constants are  lifted     Note that Visual Studio Code will struggle to auto-format the mixed syntax   though it works     x000D   x000D  class MyClass   x000D             x000D   x000D    x000D  MyClass prototype consts      x000D      constant1   33  x000D      constant2  32 x000D     x000D  mc   new MyClass    x000D  console log mc consts constant2       x000D   x000D   x000D

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