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Discussion on: Use an object instead of a switch

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charlesfries profile image
Charles Fries •

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 by using the keyof typeof keywords to pull the object keys directly from the object, so you don't need to explicitly define the keys:

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',
};

const getMonth = (month: keyof typeof MONTH_TO_TRANSLATION_KEY) => MONTH_TO_TRANSLATION_KEY[month];
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ubmit profile image
Guilherme de Andrade •

Oh thanks a lot for the tip!

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peerreynders profile image
peerreynders • • Edited

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];
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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;
}
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