milliseconds to time in javascript

Here is my favourite one-liner solution:

new Date(12345 * 1000).toISOString().slice(11, -1);  // "03:25:45.000"

Method Date.prototype.toISOString() returns a string in the simplified extended ISO format (ISO 8601), which is always 24 characters long: YYYY-MM-DDTHH:mm:ss.sssZ. This method is supported in all modern browsers (IE9+) and Node.

This one-liner is limited to a range of one day, which is fine if you use it to format milliseconds up to 24 hours (i.e. ms < 86400000). The following code is able to format correctly any number of milliseconds (shaped in a handy prototype method):

/**
 * Convert (milli)seconds to time string (hh:mm:ss[:mss]).
 *
 * @param Boolean seconds
 *
 * @return String
 */
Number.prototype.toTimeString = function(seconds) {
    var _24HOURS = 8.64e7;  // 24*60*60*1000

    var ms = seconds ? this * 1000 : this,
        endPos = ~(4 * !!seconds),  // to trim "Z" or ".sssZ"
        timeString = new Date(ms).toISOString().slice(11, endPos);

    if (ms >= _24HOURS) {  // to extract ["hh", "mm:ss[.mss]"]
        var parts = timeString.split(/:(?=\d{2}:)/);
        parts[0] -= -24 * Math.floor(ms / _24HOURS);
        timeString = parts.join(":");
    }

    return timeString;
};

console.log( (12345 * 1000).toTimeString()     );  // "03:25:45.000"
console.log( (123456 * 789).toTimeString()     );  // "27:03:26.784"
console.log(  12345.       .toTimeString(true) );  // "03:25:45"
console.log(  123456789.   .toTimeString(true) );  // "34293:33:09"

Lots of unnecessary flooring in other answers. If the string is in milliseconds, convert to h:m:s as follows:

function msToTime(s) {
  var ms = s % 1000;
  s = (s - ms) / 1000;
  var secs = s % 60;
  s = (s - secs) / 60;
  var mins = s % 60;
  var hrs = (s - mins) / 60;

  return hrs + ':' + mins + ':' + secs + '.' + ms;
}

If you want it formatted as hh:mm:ss.sss then use:

function msToTime(s) {

  // Pad to 2 or 3 digits, default is 2
  function pad(n, z) {
    z = z || 2;
    return ('00' + n).slice(-z);
  }

  var ms = s % 1000;
  s = (s - ms) / 1000;
  var secs = s % 60;
  s = (s - secs) / 60;
  var mins = s % 60;
  var hrs = (s - mins) / 60;

  return pad(hrs) + ':' + pad(mins) + ':' + pad(secs) + '.' + pad(ms, 3);
}

console.log(msToTime(55018))

Using some recently added language features, the pad function can be more concise:

function msToTime(s) {
    // Pad to 2 or 3 digits, default is 2
  var pad = (n, z = 2) => ('00' + n).slice(-z);
  return pad(s/3.6e6|0) + ':' + pad((s%3.6e6)/6e4 | 0) + ':' + pad((s%6e4)/1000|0) + '.' + pad(s%1000, 3);
}

// Current hh:mm:ss.sss UTC
console.log(msToTime(new Date() % 8.64e7))

function millisecondsToTime(milli)
{
      var milliseconds = milli % 1000;
      var seconds = Math.floor((milli / 1000) % 60);
      var minutes = Math.floor((milli / (60 * 1000)) % 60);

      return minutes + ":" + seconds + "." + milliseconds;
}