1-formatDate.js

export const formatDate = (dateOrString, opts) => {
  const date = new Date(dateOrString);

  return new Intl.DateTimeFormat('pt-BR', opts).format(date);
}

2-formatDate.js

const tokens = {
  DD: (date) => date.getDate().toString().padStart(2, '0'),
  MM: (date) => (date.getMonth() + 1).toString().padStart(2, '0'),
  YYYY: (date) => date.getFullYear().toString().padStart(4, '0'),
};

export const formatDate = (dateOrString, format = 'DD/MM/YYYY') => {
  const date = new Date(dateOrString);

  return Object.entries(tokens).reduce(
    (result, [pattern, fn]) => result.replace(new RegExp(pattern, 'g'), fn(date)),
    format
  );
};

3-formatDate.js

const monthNames = ['Janeiro', 'Fevereiro', 'Março', 'Abril', 'Maio', 'Junho', 'Julho', 'Agosto', 'Setembro', 'Outubro', 'Novembro', 'Dezembro'];
const weekDayNames = ['Domingo', 'Segunda', 'Terça', 'Quarta', 'Quinta', 'Sexta', 'Sábado'];

// Add literally any custom token... have fun
const tokens = {
  DD: (date) => date.getDate().toString().padStart(2, '0'),
  MM: (date) => (date.getMonth() + 1).toString().padStart(2, '0'),
  YYYY: (date) => date.getFullYear().toString().padStart(4, '0'),

  HH: (date) => date.getHours().toString().padStart(2, '0'),
  mm: (date) => date.getMinutes().toString().padStart(2, '0'),

  Mmmm: (date) => monthNames[date.getMonth()],
  Mmm: (date) => monthNames[date.getMonth()].slice(0, 3),
  Wwww: (date) => weekDayNames[date.getDay()],
  Www: (date) => weekDayNames[date.getDay()].slice(0, 3),
};

export const formatDate = (dateOrString, format = 'DD/MM/YYYY') => {
  const date = new Date(dateOrString);

  return Object.entries(tokens).reduce(
    (result, [pattern, fn]) => result.replace(new RegExp(pattern, 'g'), fn(date)),
    format
  );
};