Discussion on: Use an object instead of a switch

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Replies for: Nice article. I'll just add that unless MONTH_TO_TRANSLATION_KEY is a dynamic object that could be changed at some point, you could simplify this b...

It makes sense to add a const assertion:

const MONTH_TO_TRANSLATION_KEY = {
  January: 'JANUARY_TRANSLATION_KEY',
  February: 'FEBRUARY_TRANSLATION_KEY',
  March: 'MARCH_TRANSLATION_KEY',
  April: 'APRIL_TRANSLATION_KEY',
  May: 'MAY_TRANSLATION_KEY',
  June: 'JUNE_TRANSLATION_KEY',
  July: 'JULY_TRANSLATION_KEY',
  August: 'AUGUST_TRANSLATION_KEY',
  September: 'SEPTEMBER_TRANSLATION_KEY',
  October: 'OCTOBER_TRANSLATION_KEY',
  November: 'NOVEMBER_TRANSLATION_KEY',
  December: 'DECEMBER_TRANSLATION_KEY',
} as const;

type Month = keyof typeof MONTH_TO_TRANSLATION_KEY;
type TranslationKey = typeof MONTH_TO_TRANSLATION_KEY[Month];

const toTranslationKey: (m: Month) => TranslationKey = (month) =>
  MONTH_TO_TRANSLATION_KEY[month];

Given that object keys can only be strings or symbols (numbers are coerced to strings) Maps are useful as well:

const MONTH_KEY_ENTRIES = [
  ['January', 'JANUARY_TRANSLATION_KEY'],
  ['February', 'FEBRUARY_TRANSLATION_KEY'],
  ['March', 'MARCH_TRANSLATION_KEY'],
  ['April', 'APRIL_TRANSLATION_KEY'],
  ['May', 'MAY_TRANSLATION_KEY'],
  ['June', 'JUNE_TRANSLATION_KEY'],
  ['July', 'JULY_TRANSLATION_KEY'],
  ['August', 'AUGUST_TRANSLATION_KEY'],
  ['September', 'SEPTEMBER_TRANSLATION_KEY'],
  ['October', 'OCTOBER_TRANSLATION_KEY'],
  ['November', 'NOVEMBER_TRANSLATION_KEY'],
  ['December', 'DECEMBER_TRANSLATION_KEY'],
] as const;

type EntryKey<T> = T extends readonly any[]
  ? T[0] extends undefined
    ? never
    : T[0]
  : never;

type EntryValue<T> = T extends readonly any[]
  ? T[1] extends undefined
    ? never
    : T[1]
  : never;

type MonthKeyEntries = typeof MONTH_KEY_ENTRIES[number];
type Month = EntryKey<MonthKeyEntries>;
type TranslationKey = EntryValue<MonthKeyEntries>;

const TranslationKeyMap = new Map<Month, TranslationKey>(MONTH_KEY_ENTRIES);

function toTranslationKey(month: Month): TranslationKey {
  const XKey = TranslationKeyMap.get(month);
  if (typeof XKey === 'undefined')
    throw new Error(`Missing month ${month} in TranslationKeyMap`);

  return XKey;
}