var date = new Date();
let hhmmss = date.toTimeString().replace(/.*(\d{2}:\d{2}:\d{2}).*/, "$1");

Convert seconds to HH:MM:SS format.

var date = new Date(1970,0,1);
let sec = 456475676868
date.setSeconds(sec);
hhmmss = date.toTimeString().replace(/.*(\d{2}:\d{2}:\d{2}).*/, "$1");
           //'07:07:48'