Capitalize First Letter

const capitalizeFirstLetter = ([firstLetter, ...restOfWord]) =>
  firstLetter.toUpperCase() + restOfWord.join('');

const capitalizeFirstLetter = <T extends string>([
  firstLetter,
  ...restOfWord
]: T) => {
  return (firstLetter.toUpperCase() + restOfWord.join('')) as Capitalize<T>;
};