How to create a const object that conforms to a specific type in Typescript

Question

I am searching for a better solution of the following... I need to do this:

const FRUIT_PROPERTIES = {
   "apple": { color: "red" },
   "banana": { color: "yellow" }
} as const

const myFavoriteFruitColor = FRUIT_PROPERTIES.apple.color

That is fine and dandy, but I also want the FRUIT_PROPERTIES object to conform to a type like this:

type FruitEnum = Record<string, { color: string }>

const FRUIT_PROPERTIES: FruitEnum = {
   "apple": { color: "red" },
   "banana": { color: "yellow" }
} as const

But this way I am loosing the original inferred type and autocompletion of the "apple" and "banana" keys.

So I have solved this by doing the following

type FruitEnum = Record<string, { color: string }>;

const validateFruitEnum = <TParam extends FruitEnum>(param: TParam) => {
   return param;
};

const FRUIT_PROPERTIES = validateFruitEnum({
   apple: { color: 'red' },
   banana: { color: 'yellow' },
} as const)

This way I retain the original type...

But I was wondering if there is some better way of doing this? Does anybody have any better solution?

Answer 1

It is also possible to infer literal type without using as const assertion:

const validateFruitEnum = <
  Color extends string,
  Value extends { color: Color },
  Prop extends string,
  Obj extends Record<Prop, Value>
>(param: Obj) => param

const FRUIT_PROPERTIES = validateFruitEnum({
  apple: { color: 'red' },
  banana: { color: 'yellow' },
})

FRUIT_PROPERTIES.apple.color  // red

Playground

Using explicit type disables as const assertion. For instance string[] as const can't give you literal type as well as const a:string[]=['a','b'] as const

Answer 2

You need a source of truth for the fruit types. So here is what I would do instead of using the validateFruitEnum function, although not exactly what you were looking for:

Define a fruits enum before and use that:

const FruitEnum = {
  apple: "apple",
  banana: "banana",
} as const;

type FruitToColorMap = Record<keyof typeof FruitEnum, { color: string }>

const FRUIT_PROPERTIES: FruitToColorMap = {
   "apple": { color: "red" },
   "banana": { color: "yellow" }
};

link: playground