configuration.js

let cfg_fnc = function(cfg = {
  duration: 10,
  delays: [3, 6]
}) {
  return "duration: " + cfg.duration + ", delays: " + cfg.delays;
}

// prints "duration: 5, delays: undefined"
console.log(cfg_fnc({duration:5}));

// prints "duration: 10, delays: 3,6"
console.log(cfg_fnc());