list format

list.ts

function namesList(names: string | readonly string[]): string {
  if (typeof names === 'string') return names;
  if (names instanceof Array) return names.join(', ');
  throw new TypeError(
    `Invalid parameter, expected a string or array of strings, received: ${names}`,
  );
}

const nameList2 = (names: string | Iterable<string>) =>
  typeof names === 'string'
    ? names
    : new Intl.ListFormat('en-GB', { type: 'unit' }).format(names);