strftime.ts

interface Date {
  strftime (format: string): string;
}

Date.prototype.strftime = function (format: string) {

  const date = this;

    return (format + "").replace(/%([a-zA-Z])/g, function(m, f) {
      var formatter = formats[f];
      
      if (typeof formatter == "function") {
        return formatter.call(formats, date);
      } else if (typeof formatter == "string") {
        return date.strftime(formatter);
      }
      return f;
    });
  };

  // Internal helper
   const zeroPad = (num) => {
    return (+num < 10 ? "0" : "") + num;
  }

   const formats = {
      // formatting methods
      d: function (date) {
          return zeroPad(date.getDate());
      },

      m: function (date) {
          return zeroPad(date.getMonth() +1);
      },

      y: function (date) {
          return zeroPad(date.getYear() % 100);
      },

      Y: function (date) {
          return date.getFullYear();
      },

      // Format shorthands
      F: "%Y-%m-%d",
      D: "%m/%d/%y"
  };