format.js

export function stringify(x) {
  return x == null ? "" : x + "";
}

export function formatAuto(value) {
  return value == null ? ""
    : typeof value === "number" ? formatNumber(value)
    : value instanceof Date ? formatDate(value)
    : value + "";
}

export function formatNumber(value) {
  return value === 0 ? "0" : value.toLocaleString("en"); // handle negative zero
}

export function formatDate(date) {
  var hours = date.getUTCHours(),
      minutes = date.getUTCMinutes(),
      seconds = date.getUTCSeconds(),
      milliseconds = date.getUTCMilliseconds();
  return isNaN(date) ? "Invalid Date"
      : formatYear(date.getUTCFullYear(), 4) + "-" + pad(date.getUTCMonth() + 1, 2) + "-" + pad(date.getUTCDate(), 2)
      + (milliseconds ? "T" + pad(hours, 2) + ":" + pad(minutes, 2) + ":" + pad(seconds, 2) + "." + pad(milliseconds, 3) + "Z"
      : seconds ? "T" + pad(hours, 2) + ":" + pad(minutes, 2) + ":" + pad(seconds, 2) + "Z"
      : minutes || hours ? "T" + pad(hours, 2) + ":" + pad(minutes, 2) + "Z"
      : "");
}

function formatYear(year) {
  return year < 0 ? "-" + pad(-year, 6) : year > 9999 ? "+" + pad(year, 6) : pad(year, 4);
}

function pad(value, width) {
  return (value + "").padStart(width, "0");
}