Back to home
Sergio Carracedo

Sergio Carracedo

Typescript enums, const enums and readonly maps

Typescript enums, const enums and readonly maps

Typescript enums, const enums and readonly maps

Enum basics

Enums is one of the nice things Typescript bring to the Javascript development’s environment. Enums allows you to define a set of named values (constants), generally with a semantic meaning.

One of the advantages over the regular constants is the grouping, making easy to know the different values you can use in certain place (and limiting the possible values to use)

In Typescript an enum has this shape:

enum HttpResponseStatus {

In the example, the enum represent a list of possible (a simplified list) HTTP response status, and using the names it’s easy to remember which status we want to use in each case.

The way to use a enum, it’s very straightforward, just: Enum name + dot + Enum value name: HttpResponseStatus.NotFound, Typescript will replace it by a value

By default, and if you don’t specify more, Typescript converts each the enum value to a number, starting on 0. In or example NotFound value is 0, Forbidden is 1, etc.

This doesn’t fit the expected use case, we expect, for example NotFound’s value be 404, Ok be 200, etc.

You can even define the starting number

enum Chapters {
  Four = 4,

But, to meet the expected values we only need to assign the desired values for each enum’s value name like:

enum HttpResponseStatus {
  NotFound = 404,
  Forbidden = 403,
  Ok = 200,
  InternalServerError = 500

You can use expressions to define the values, for example calculations, bit operations, etc. Event random numbers (the random values is evaluated just one time, so it will be constant in the runtime), or values returned by a function

enum MyEnum {
  A = 404,
  B = 1 << 2,
  C = 1 * 3,
  D = Math.random(),
  E = someFunction(123)

After defining an enum you can use it as a type, with limitations*, for example:

function handleResponse(responseCode: HttpResponseStatus)

(*) The main limitation is you can assign any number to a numeric-enum type and that is intended (

For example handleResponse(123) is valid, even if the value 123 is not a value in the enum HttpResponseStatus. This doesn’t happen with string-enums

Now you know the enum basics, let go deeper

Enums in runtime

Typescript’s enums have a representation in runtime, but maybe is not as you can expect, let’s see how the HttpResponseStatus enum is compiled to vanilla JS:

var HttpResponseStatus;
(function (HttpResponseStatus) {
    HttpResponseStatus[HttpResponseStatus["NotFound"] = 404] = "NotFound";
    HttpResponseStatus[HttpResponseStatus["Forbidden"] = 403] = "Forbidden";
    HttpResponseStatus[HttpResponseStatus["Ok"] = 200] = "Ok";
    HttpResponseStatus[HttpResponseStatus["InternalServerError"] = 500] = "InternalServerError";
})(HttpResponseStatus || (HttpResponseStatus = {}));

and if you do console.log(HttpResponseStatus) this is the result:

  "200": "Ok",
  "403": "Forbidden",
  "404": "NotFound",
  "500": "InternalServerError",
  "NotFound": 404,
  "Forbidden": 403,
  "Ok": 200,
  "InternalServerError": 500,
  "Test": 0.48543608526338566,
  "0.48543608526338566": "Test"

Even the RxJS core team lead wrote a tweet about that:

The reason of this behavior is mainly because:

  • Reverse Mapping this allows to use the enum in both directions, get the value from the value’s name or get the name from the value’s name from the value.
  • Computed values: The object that represents the enum is computed in a function to allow computed values, like the random we used before in the example.

Reverse mapping can be useful in some cases, for example if you want to show the value’s name instead of the value, even in a dropdown, to list all values’ names and get the selected value, but in this case you need to do an extra conversion to avoid repeated values:

Const enums

In most use cases you don’t need Reverse mapping neither computed values, then you can use const enums, just adding the keyword const before the enum definition.

const enum HttpResponseStatus {
  NotFound = 404,
  Forbidden = 403,
  Ok = 200,
  InternalServerError = 500

In this case Typescript’s compiler just will replace the uses of the enum items by the value, for example:

// Compiler output
someFunction(404 /* NotFound */)

Union types

Other simple way to “emulate” enum’s behaviour keeping type safety and without overload the bundle with extra code is just using union types

type HttpResponseStatus = 404 | 403 | 200 | 500

This solution lost the spirit of an enum, but the IDE can to the “magic” suggestion the available values when you try to fill a function argument typed as HttpResponseStatus

Both solutions are nice in terms of the bundle size, but removes the possibility of knowing the value’s name.

Const object

In the case we want to have the value’s name in runtime we can use a plain object to emulate the enum behavior, then our enum becomes:

const HttpResponseStatus = {
  NotFound: 404,
  Forbidden: 403,
  Ok: 200,
  InternalServerError: 500

We can use it as an enum, referencing a value in the same way HttpResponseStatus.NotFound, but what about the typing?

If we check the type of the object we get this:

const HttpResponseStatus: {
    NotFound: number;
    Forbidden: number;
    Ok: number;
    InternalServerError: number;

We lost the possibility of use the type, for example in a function’s argument: function someFunction(status: HttpResponseStatus) will not work, and we should use number as status’ type.

Const assertion

The Typescript’s const assertion solves this as forces the object’s properties to be readonly, and for that Typescript’s compiler is able to infer the property types as and convert it as type:

const HttpResponseStatus = {
  NotFound: 404,
  Forbidden: 403,
  Ok: 200,
  InternalServerError: 500
} as const

// The type infered by typescript
const HttpResponseStatus: {
  readonly NotFound: 404;
  readonly Forbidden: 403;
  readonly Ok: 200;
  readonly InternalServerError: 500;

This is an object and we still having the keys in runtime as in a regular enum

Now we can create a type that will content the union of all properties values as type

type HttpResponseStatusEnum = typeof HttpResponseStatus[keyof typeof HttpResponseStatus]

Explaining it a bit in detail:

and then we can use this “Enum type” as type:

function someFunction(status: HttpResponseStatusEnum)


As you read, there several ways to achieve the same or similar behavior, and now you have information enough to decide which solution use depending on the use case, if you are worried about the bundle size the code output maybe it’s the moment to start to use another solution, Typescript documentation recommends the object with as const solution, but if you don’t need the enum values’ names you can use the const enum or just the union type.

In modern TypeScript, you may not need an enum when an object with as const could suffice (from