monetizeMask.js

const DEFAULT_PREFIX = 'R$ ';
const DEFAULT_SUFFIX = '';

function prepend(value = 0) {
  if (value < 10) {
    return '00';
  }

  if (value < 100) {
    return '0';
  }

  return '';
}

function demonetize(value = '') {
  return parseInt(String(value).replace(/\D/g, ''), 10) || 0;
}

function monetize(value = '', options = {}) {
  const prefix = options.prefix !== undefined ? options.prefix : DEFAULT_PREFIX;
  const suffix = options.suffix !== undefined ? options.suffix : DEFAULT_SUFFIX;

  let cents = demonetize(value);

  if (!cents) {
    return `${prefix}0,00${suffix}`;
  }

  cents = `${prepend(cents)}${cents}`
    .replace(/(\d{2})$/, ',$1')
    .replace(/(\d+)(\d{3},\d{2})$/g, '$1.$2');

  for (let i = 0, l = (cents.length - 3) / 3; l > i; i += 1) {
    cents = cents.replace(/(\d+)(\d{3}.*)/, '$1.$2');
  }

  return `${prefix}${cents}${suffix}`;
}