Use object literal as TypeScript enum values

Question

I have an enum:

export enum PizzaSize {
  SMALL =  0,
  MEDIUM = 1,
  LARGE = 2
}

But here I'd like to use some pair of values: e.g. SMALL I would like to say that it has a key of 0 and a value of 100. I endeavor to use:

export enum PizzaSize {
  SMALL =  { key: 0, value: 100 },
  // ...
}

But TypeScript doesn't accept this one. How can I do this?

Solution 1

Update: find @Javarome's answer below, which is more elegant. I suggest using his way.

If you need to use Type, try adding some code. usage: getPizzSizeSpec(PizzaSize.small).value

enum PizzaSize {
    small,
    medium,
    large
}
interface PizzaSizeSpec {
    key: number,
    value: number
}
function getPizzaSizeSpec(pizzaSize: PizzaSize): PizzaSizeSpec {
    switch (pizzaSize) {
        case PizzaSize.small:
            return {key:0, value: 25};
        case PizzaSize.medium:
            return {key:0, value: 35};
        case PizzaSize.large:
            return {key:0, value: 50};
    }
}

Solution 2

TypeScript supports numeric or string-based enums only, so you have to emulate object enums with a class (which will allow you to use it as a type in a function declaration):

export class PizzaSize {
  static readonly SMALL  = new PizzaSize('SMALL', 'A small pizza');
  static readonly MEDIUM = new PizzaSize('MEDIUM', 'A medium pizza');
  static readonly LARGE  = new PizzaSize('LARGE', 'A large pizza');

  // private to disallow creating other instances of this type
  private constructor(private readonly key: string, public readonly value: any) {
  }

  toString() {
    return this.key;
  }
}

then you can use the predefined instances to access their value:

const mediumVal = PizzaSize.MEDIUM.value;

or whatever other property/property type you may want to define in a PizzaSize.

and thanks to the toString() overriding, you will also be able to print the enum name/key implicitly from the object:

console.log(PizzaSize.MEDIUM);  // prints 'MEDIUM'

Solution 3

As of Typescript 3.4, you can use a combination of keyof typeof and const assertions to create objects that can have the same type safety as enums, and still hold complex values.

By creating a type with the same name as the const, you can have the same exhaustiveness checks that normal enums have.

The only wart is that you need some key in the complex object (I'm using value here) to hold the name of the enum member (if anyone can figure out a helper function that can build these objects in a typesafe way, I'd love to see it! I couldn't get one working).

export const PizzaSize = {
    small: { value: 'small', key: 0, size: 25 },
    medium: { value: 'medium', key: 1, size: 35 },
    large: { value: 'large', key: 2, size: 50 },
} as const

export type PizzaSize = keyof typeof PizzaSize

// if you remove any of these cases, the function won't compile
// because it can't guarantee that you've returned a string
export function order(p: PizzaSize): string {
    switch (p) {
        case PizzaSize.small.value: return 'just for show'
        case PizzaSize.medium.value: return 'just for show'
        case PizzaSize.large.value: return 'just for show'
    }
}

// you can also just hardcode the strings,
// they'll be type checked
export function order(p: PizzaSize): string {
    switch (p) {
        case 'small': return 'just for show'
        case 'medium': return 'just for show'
        case 'large': return 'just for show'
    }
}

In other files this can be used simply, just import PizzaSize.

import { PizzaSize } from './pizza'

console.log(PizzaSize.small.key)

type Order = { size: PizzaSize, person: string }

Also notice how even objects that are usually mutable can't be mutated with the as const syntax.

const Thing = {
    ONE: { one: [1, 2, 3] }
} as const

// this won't compile!! Yay!!
Thing.ONE.one.splice(1, 0, 0)

Solution 4

I think to get to what you want, something like this will work

interface PizzaInfo {
  name: string;
  cost_multiplier: number;
}

enum PizzaSize {
  SMALL,
  MEDIUM,
  LARGE,
}

const pizzas: Record<PizzaSize, PizzaInfo> = {
  [PizzaSize.SMALL]: { name: "Small", cost_multiplier: 0.7 },
  [PizzaSize.MEDIUM]: { name: "Medium", cost_multiplier: 1.0 },
  [PizzaSize.LARGE]: { name: "Large", cost_multiplier: 1.5 },
};

const order = PizzaSize.SMALL;
console.log(pizzas[order].name);  // "Small"

Solution 5

Object.freeze makes it read only and prevents more properties being added:

const pizzaSize = Object.freeze({
  small: { key: 0, value: 25 },
  medium: { key: 1, value: 35 },
  large: { key: 2, value: 50 }
})

View more solutions

Comments

[Frohlich]

Way more elegant than the accepted answer but, the constructor should be private constructor(private key: string, PUBLIC value: any) {} to be able to use PizzaSize.MEDIUM.value or define a getter to it

[Jérôme Beau]

True @Flohlich. Just added a getter to forbid change of such an enum constant.

[Alexander Mills]

this answer is unclear to me, can you show the enum declaration not just the PizzaSize class?

[Jérôme Beau]

@Alexander Mills There is no enum declaration, that’s the point : as there is no support for object enum you have to emulate it through a class declaration.

[qqilihq]

I'd suggest to declare the static items readonly as well. Else wise they could be reassigned externally (PizzaSize.SMALL = PizzaSize.LARGE; // get a big pizza, but pay small :-)).

[Joe Tse]

This is much more elegant. I would use your way too. Upvoted.

[j1s]

How can use this in angular html template?

[Jérôme Beau]

@j1s In the relevant component, declare a public property this has the name name as the enum and is equal to it: PizzaSize = PizzaSize. Then you'll be allowed to write PizzaSize.LARGE in your template.

[blaineh]

Check out my recent answer, it also has the benefit of exhausiveness checks

[Drenai]

+1 for the const ... as const, that's a new one for me! This approach is ok, but it's a bit more difficult to reason about than Javarome's

[JeffryHouser]

What is the purpose of the toString() method or the arguments into the constructor?

[JeffryHouser]

Nevermind, I figured it out. The internal read only variables are instances of the class. Got it!

[Drenai]

Be careful using this approach if you clone your objects with Ramda/lodash, e.g R.clone({pizza: PizzaSize.MEDIUM}) or _.cloneDeep. Triple equals and switch statements won't work like they do with TypeScript enum

[blaineh]

I wish this gave exhaustiveness checks :( That's often the main reason I want to use an enum typescriptlang.org/play/#code/…

[blaineh]

It does seem a little tricky. And it doesn't allow you to add functions to the "enum" values like the class does. But this give exhaustiveness checks where the class doesn't, so it's just a choice of tradeoffs I'm afraid.

[Basti]

FYI my tsc 3.9.7 is complaining about order(PizzaSize.large) with Argument of type '{ readonly value: "large"; readonly key: 2; readonly size: 50; }' is not assignable to parameter of type '"small" | "medium" | "large"'. So it doesn't behave like enums in that way.

[blaineh]

Yeah this solution isn't exactly like enums, it's just parallel, using the keys of a const object instead of an actual enum value. Pass order(PizzaSize.large.value) or order('large') and everything will compile. Doing that is just as typesafe as an enum.

[Onur Yıldırım]

Only first level. You should also freeze each deeper level.

[fIwJlxSzApHEZIl]

is there any way to make compilation fail if you add a value to PizzaSize if you forget to modify pizzas?

[Dennis]

In terms of building the values of the enumerated objects in a typesafe way, the best I could think to do was to cast each declaration as I defined it. It's a little more verbose than I'd like, but it seems to get the job done.

[bhantol]

This answer is no way close to object literal enum. stackoverflow.com/a/51398471/2103767 is by far the best by @Javarome