TypeScript / JavaScript - immutable enums

In this short article, we would like to show how to create immutable enums in JavaScript / TypeScript.

Problem overview

During compilation, enums are converted to JavaScript objects that can be changed, which can lead to bugs.

Problem solution

It is necessary to freeze the enum object.

Practical example (JavaScript):

// ONLINE-RUNNER:browser;

const CurrencyEnum = {
    'USD': 1,
    'EUR': 2,
    'GBP': 3,
    'CHF': 4,
    'PLN': 5
};

Object.freeze(CurrencyEnum);  //  <---------- makes CurrencyEnum as read-only

// now CurrencyEnum is read-only

Practical example (TypeScript):

enum CurrencyEnum {
    USD = 1,
    EUR = 2,
    GBP = 3,
    CHF = 4,
    PLN = 5
};

Object.freeze(CurrencyEnum);  //  <---------- makes CurrencyEnum as read-only in pure JavaScript

// now CurrencyEnum is read-only from pure JavaScript

Note: Object.freeze() was introduced in ES5 (ECMAScript 2009).