Objects to unions converter

Let’s say we have a dictionary:

const FEATURES = {
  SORTING: 'Sorting',
  FILTERING: 'Filtering',
  REORDERING: 'Reordering',
};

It can be used like this:

const features = [];

// or it can be a method of a class like "addFeature"
features.push(FEATURES.FILTERING);

I want to say to my features array that it can accept values of the FEATURES object only. But in order to avoid creating a separate union type and support (possible) changes there and in FEATURES object, I’d like to collect values from the object and keep them as a union type. Thus if it is necessary to add/remove value, it can be done in only one place - in the FEATURES object.

To achieve this goal, in TypeScript it is necessary to define such monstrous type:

const FEATURES = {
  SORTING: 'Sorting',
  FILTERING: 'Filtering',
  REORDERING: 'Reordering',
// 1. const assertions
} as const;

// 2. if we have a lot of such dictionaries, we have to repeat this stuff every time
type Feature = (typeof FEATURES)[keyof typeof FEATURES];

// if we want to get union type of keys
// type FeatureKeys = keyof typeof FEATURES;

const features: Feature[] = [];

// OK
features.push(FEATURES.FILTERING);

/*
  Should throw a type error
  Argument of type '"foo"' is not assignable to parameter of type
  '"Sorting" | "Filtering" | "Reordering"'.
*/
features.push('foo')

Are there any thoughts of optimizing this type generation to avoid repeating such boilerplate in Hegel? Like a help function or something.

And I’d like to know if it is a common thing to work with dictionaries this way or not. Because I’m new to strong type checking and maybe I think the wrong way here…

Thanks in advance.

[thecotne]

@vasilii-kovalev i have reported Values type issue separately https://github.com/JSMonk/hegel/issues/269

[thecotne]

this works perfectly fine you don’t need to repeat anything …

const immutableId = <T>(v: T): $Immutable<T> => v

const FEATURES = immutableId({
  SORTING: 'Sorting',
  FILTERING: 'Filtering',
  REORDERING: 'Reordering'
})

type Feature = $Values<$TypeOf<FEATURES>>

const features: Array<Feature> = []

void features.push('Filtering') // ok
void features.push(FEATURES.FILTERING) // ok

void features.push('foo') // error as expected

try

you just need to use either Object.freeze or immutableId to achieve same result as as const in typescript.