Inspect keys like "blah blah {placeholder}" and makes a type out of them

placeholder.ts

const en = {
    "key1": "my translation",
    "key2": "my translation {placeholder1} {placeholder2}",
    "key3": "{placeholder1} my translation",
    "key4": "x {placeholder1} x",
    "key5": "{placeholder1}{placeholder2}"
} as const;

type Translations = typeof en;
type TranslationKeys = keyof Translations;

// Consume {placeholders} by splitting the string in three parts: Left - Placeholder - Right
// If the string does not match Left - Placeholder - Right we return never
// Otherwise we recursively explore Left and Right and union it with Placeholder

type Placeholders<T extends string> =
    T extends `${infer Left}{${infer Placeholder}}${infer Right}`
    ? Placeholders<Left> | Placeholder | Placeholders<Right>
    : never;

type PlaceholdersOf<TKey extends TranslationKeys> =
    Placeholders<Translations[TKey]> extends never
    ? []
    : [Record<Placeholders<Translations[TKey]>, string>];

function translate<TKey extends TranslationKeys>(
    key: TKey,
    ...placeholders: PlaceholdersOf<typeof key>
) {
}

translate('key1');
translate('key2', { placeholder1: 'a', placeholder2: 'c' });
translate('key3', { placeholder1: 's' });
translate('key4', { placeholder1: 'x' });
translate('key5', { placeholder1: 'a', placeholder2: 'c' });