Here's my version of this, you can use formatToParts() function to get them by part assuming you will be using the same format every time, and then get them by object array so that you can create your own format allowing you to add custom text. Although it would be much more easier and efficient if you will use some other Date formatter.

let checkoutDate = new Date();
var th = 'th';
var formatter = new Intl.DateTimeFormat('en-us', {
  weekday: 'long',
  year: 'numeric',
  month: 'long',
  day: 'numeric',
  fractionalSecondDigits: 3,
  timeZone: 'UTC'
});
var formatted = formatter.formatToParts(checkoutDate);

var day = formatted[4].value;
var wr = checker(day);

console.log(wr);
console.log(formatted[0].value+ ',', day +wr, formatted[2].value, formatted[6].value);

function checker(x){
    if (x > 3 && x < 21) return 'th';
    switch (x % 10) {
        case 1:  return "st";
        case 2:  return "nd";
        case 3:  return "rd";
        default: return "th";
    }
}
//output Friday, 13th March 2020