reordenamiento de un array segun los dias de la semana y las horas dada.

userSchedule.js

const days = ["Lun","Mar","Mie","Juv","Vie","Sab","Dom"]
const userSchedule = [
	[],
	['17:00', '18:00', '19:00'],
	['17:00', '18:00', '19:00'],
	['17:00'],
	['17:00', '18:00', '19:00'],
	['17:00', '18:00', '19:00'],
	[]
];

const result = userSchedule.reduce((acc={}, item, i) => {
	item.forEach(x =>acc[x] = acc[x] ? acc[x].concat(days[i-1]) : [days[0]]);
	return acc
},{});

console.log(result);

/*
{
  '17:00': [ 'Lun', 'Mar', 'Mie', 'Juv', 'Vie' ],
  '18:00': [ 'Lun', 'Mar', 'Juv', 'Vie' ],
  '19:00': [ 'Lun', 'Mar', 'Juv', 'Vie' ]
}
*/