Is it possible to define the data type of object properties in TypeScript while also having the actual key types automatically inferred?

I am attempting to create a unique form of enumeration, where each key in the enum is associated with specific data whose type I want to define.

For example:

const Seasons = {
    winter: { temperature: 5, startMonth: "December" },
    spring: { temperature: 20, startMonth: "March" },
    summer: { temperature: 30, startMonth: "June" },
    fall: { temperature: 15, startMonth: "September" },
} as const

This setup allows me to utilize features like:

type Season = keyof typeof Seasons // "winter" | "spring" | "summer" | "fall"

and even implement a type guard such as

function isSeason(s: string): s is Season {
    return Object.keys(Seasons).includes(s)
}

However, I face an issue in ensuring that all season definitions adhere to a specified type. When I try:

type SeasonData = typeof Seasons[Season]

SeasonData becomes a union of all definition types, regardless of their shape.

So, I aim to find a concise and efficient way to define something similar to:

const Seasons: EnumWith<{temperature: number, startMonth: string}> = ... // as previously shown
               ^^^^^^^^ <- yet to be determined!

Specifically, I prefer to avoid duplicating the list of seasons in any other structure (like interface or array) and directly infer the type Season from the object definition (although open to alternatives!).

Any suggestions on how to achieve this?

typescriptenumstype-inferencesingleton-type

Answer №1

It seems like there might be a better way to model the relationship between your types based on the following approach:

type Weather =
  | "sunny"
  | "rainy"
  | "cloudy"

type Activity =
  | "hiking"
  | "swimming"
  | "skiing"

type WeatherConditions = {
  description: string
}

type Activities = { [K in Weather]: WeatherConditions }

const activities: Activities = {
    sunny: { description: "Perfect day for hiking" },
    rainy: { description: "Great weather for swimming" },
    cloudy: { description: "Ideal conditions for skiing" }
}

This structure provides a solid foundation to encompass all elements relevant to your scenario. Hopefully, this approach is beneficial to you.

Answer №2

Recently discovered a unique approach to extracting literal type information from keys while also validating values:

function EnumWith<P>() {
    return function <K extends keyof any, R extends Record<K, P>>(defs: R): R {
        return defs
    }
}

This method allows for the following syntax:

const Seasons = EnumWith<{
    temperature: number
    startMonth: Month // defined as in bugs's answer
}>()({
    winter: { temperature: 5, startMonth: "December" },
    spring: { temperature: 20, startMonth: "March" },
    summer: { temperature: 30, startMonth: "June" },
    fall: { temperature: 15, startMonth: "September" },
})
type Season = keyof typeof Seasons

The key insight was using K extends keyof any to capture key types in a generic signature, and splitting the two generic types into separate function calls to allow one to be specified while inferring the other (which is currently not possible in TypeScript with a single function call).

Admittedly, the line }>()({ may not be the most aesthetically pleasing...

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