Simple date format function

formatDate.js

// yyyy - full uyear, e.g. 1969
// mm - two digit month, e.g. 01, 12
// mmm - e.g. Jan, Feb, Mar
// dd - two digit date e.g. 01, 31
// ddd - 1st, 2nd 21st
// www  - e.g. Mon, Tue, Wed...
function formatDate(date, format = 'yyyy-mm-dd') { // mm/dd/yyyy, www MMM DDD YYYY
  const day = date.getDate();
  const weekdays = ["Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat"];
  const months = ["Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec"];

  const daySuffix = 
    day > 3 && day < 21 ? 'th' :
    day % 10 === 1 ? 'st': 
    day % 10 === 2 ? 'nd': 
    day % 10 === 3 ? 'rd':  'th';
  const yyyy = date.getFullYear()
  const mmm = months[date.getMonth()];
  const mm = ('0' + (date.getMonth() + 1)).slice(-2)
  const dd = ('0' + day).slice(-2);
  const ddd = day + daySuffix;
  const www = weekdays[date.getDay()];

  return format
    .replace('www', www).replace('www', www)
    .replace('yyyy', yyyy).replace('YYYY', yyyy)
    .replace('mmm', mmm).replace('MMM', mmm)
    .replace('mm', mm).replace('MM', mm)
    .replace('ddd', ddd).replace('DDD', ddd)
    .replace('dd', dd).replace('DD', dd)
}

formatDate(new Date(), 'yyyy-mm-dd');
formatDate(new Date(), 'mm/dd/yyyy');
formatDate(new Date(), 'www mmm ddd yyyy');