.define-i18n.ts

type LanguageKey = keyof Definitions<any>

let currentLanguage: LanguageKey = "en"

/**
 * @param namespace most underlying i18n solutions like to have a namespace
 */
function define<T>(namespace: string, definitions: Definitions<T>): Translate<T> {
  // ... set up with your existing translation library which supports fallbacks and such...
  return key => `Key (${key}) has no definition`
}


type Translate<T> = (key: keyof T, params?: any) => string

/** development type */
type Definitions<T> = {
  en: T,
  fr: Partial<T>,
}

// greeting.i18n.ts
// separate your translation files based on part of the product being worked on
const greetingI18n = define('greeting', {
    en: {
        // depending on the way your translation lib works, you could use functions and template strings here, too.
        hello$name: "Hello, $name!",

        // many libraries use key suffixes to mean special things
        cartItems$count: "one item",
        cartItems$count_2: "two items",
        cartItems$count_plural: "$count items",
    },
    fr: {
        hello$name: "Bonjour, $name!",

        cartItems$count: "un article",
        cartItems$count_2: "deux articles",
        cartItems$count_plural: "$count articles",
    }
})

// see how the first argument can be autocompleted
// then, the key itself ("cartItems$count") describes the parameters
// we're expecting. Although not "type-checked", it is simpler, very compatible,
// and gets us 90% of the way there (as far as defining expectations)
console.log(greetingI18n("cartItems$count", { count: 2 }))

typescript-playground-demo.url

https://www.typescriptlang.org/play/#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-JD75pGsU5pEkECXA6SA9DuUNIMucT3Y9PXPQeAHqSo1lQLyCtAA