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?

Answer 1

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.

Answer 2

class Whatever {
    static get MyConst() { return 10; }
}

let a = Whatever.MyConst;

Seems to work for me.

Answer 3

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 for stage:

it was moved on stage-3.

Update Babel 7:

As per Babel 7 stage presets are deprecated.

The Babel plugin to use is @babel/plugin-proposal-class-properties.

npm i --save-dev @babel/plugin-proposal-class-properties

{
    "plugins": ["@babel/plugin-proposal-class-properties"]
}

Note: This plugin is included in @babel/preset-env

Answer 4

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
}

Answer 5

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).

Answer 6

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;
        //...
    }
}

Answer 7

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) => {
  class ConstClass extends BaseClass { }
  Object.keys(map).forEach(key => {
    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);
  }
}

Answer 8

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 < 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' && value !== null) {
    Object.freeze(value);
    Object.getOwnPropertyNames(value).forEach(property => {
      deepFreeze(value[property]);
    });
  }
  return value;
}

Answer 9

Here is one more way you can do

/*
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);

Note - the Order is important, you cannot have the constants above

Usage

console.log(Auto.CONSTANT1);

Answer 10

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 [].

Answer 11

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 );

Answer 12

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";

Answer 13

The cleanest way I've found of doing this is with TypeScript - see How to implement class constants?

class MyClass {
    static readonly CONST1: string = "one";
    static readonly CONST2: string = "two";
    static readonly CONST3: string = "three";
}

Answer 14

Just declare your variables as private and use a get method to retrieve them.

class MyClass {

   #myConst = 'Something';

   static #anotherConst = 'Something Else';

   get myConst() {
      return this.#myConst; // instance method
   }

   static get anotherConst() {
      return MyClass.#anotherConst; // static method
   }
}

let myClass = new MyClass();

console.log( myClass.myConst + ' is not ' + MyClass.anotherConst );

Users cannot change the original variable, and you can write the class to use the get methods rather than the private variables themselves.

Answer 15

One pattern that I use to expose error codes, i.e.,

• I have many constants inside the module

• I may not want to expose all constants to callers

• I do not want to provide 1 static constant for one exposed constant

   // inside the module 
   const Errors = {
  
       INTERNAL: 100,
       EMPTY_QUEUE: 101,
       UNKNOWN_COMMAND: 102,
       OK: 200, 
       MOVE: 201,
       CREATE_DOT: 202,
       PIXEL_MAPPING: 203
  
   }
  
   Object.freeze(Errors);
  
   class  PlotterError extends Error {
     // use constant inside the module 
     code = Errors.INTERNAL;
  
     constructor(message, code) {
       super(message);
       this.name = 'PlotterError';
       this.code = code 
     }
  
   }
  
    // expose via static constant 
    Class Plotter {
     .....
     static get ERRORS() {
       return Errors;
      }
     ....
     export Plotter;
     // module ends 
  
  
     // in the caller 
     import {Plotter} from ... 
      try {
  
       this.plotter.execute();
  
      } catch(error) {
  
       if(error.code == Plotter.ERRORS.EMPTY_QUEUE) {
       // 
      }
    }
  

We can also decide to expose only the constants we want by breaking the constants acr two objects.

Answer 16

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).

class MyClass {
    // ...

}
MyClass.prototype.consts = { 
    constant1:  33,
    constant2: 32
};
mc = new MyClass();
console.log(mc.consts.constant2);    

Answer 17

Adding up to other answers you need to export the class to use in a different class. This is a typescript version of it.

//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")
}

Answer 18

If trying to make a const/variable static to a class; try using the hash (#) to define a place holder, than a function to access it.

class Region {
    // initially empty, not accessible from outside
    static #empty_region = null; 

    /* 
        Make it visible to the outside and unchangeable 
        [note] created on first call to getter.
    */

    static EMPTY() {
        if (!this.#empty_region)
            this.#empty_region = new Region(0, 0, 0, 0);
        return this.#empty_region;
    }

    #reg = {x0:0, y0:0, x1:0, y1:0};

    constructor(x0, y0, x1, y1) { 
        this.setRegion(x0, y0, x1, y1);
    }

    // setters/getters
}

Implementation:

let someRegion = Region.EMPTY();

let anotherRegion = Region.EMPTY();

Answer 19

In order to keep consistency with native class, consider ECMAScript proposal Decorators to declare static constants, but before that day comes, use a helper function instead:

function toConstantDescriptors(entries){
  let arr = Array.isArray(entries) ? entries:
    Object.entries(entries);
  let descriptors = {};
  for(let [name, value] of arr) {
    descriptors[name] = {value, writable: false, enumerable: true, configurable: false};
  }
  return descriptors;
}

example usage 1

// define an "enum" of constants
const ReadyState = {
  CONNECTING: 0,
  OPEN: 1,
  CLOSED: 2,
};
export class ServerSentEvents extends EventTarget {
  // ...
  #readyState;
  #connect(){
    this.#readyState = ReadyState.CONNECTING; // internal use of constants
    // ...
  }
  get readyState(){
    return this.#readyState;
  }
}
// attach constants to a class
let descriptors = toConstantDescriptors(ReadyState);
Object.defineProperties(ServerSentEvents, descriptors);
Object.defineProperties(ServerSentEvents.prototype, descriptors);

example usage 2

// define constants
const CONNECTING = 0;
const OPEN = 1;
const CLOSED = 2;

export class ServerSentEvents extends EventTarget {
  // ...
  #readyState;
  #connect(){
    this.#readyState = CONNECTING; // internal use of constants
    // ...
  }
  get readyState(){
    return this.#readyState;
  }
}
// attach constants to a class
let descriptors = toConstantDescriptors({CONNECTING, OPEN, CLOSED});
Object.defineProperties(ServerSentEvents, descriptors);
Object.defineProperties(ServerSentEvents.prototype, descriptors);

Answer 20

Here You Go!

const Status = Object.freeze(class Status {
  static Disabled = 0
  static Live = 1
})