This code seems to be a good replacement for the "formatTime" function:
var toHHMMSS = (secs) => {
var sec_num = parseInt(secs, 10)
var hours = Math.floor(sec_num / 3600)
var minutes = Math.floor(sec_num / 60) % 60
var seconds = sec_num % 60
return [hours,minutes,seconds]
.map(v => v < 10 ? "0" + v : v)
.filter((v,i) => v !== "00" || i > 0)
.join(":")
}