Parts

Sometimes you need a duration's units separately, for example to show "1 day, 2 hours". msToParts splits milliseconds into a Parts object:

msToParts(93_784_005.006_007);
// => { weeks: 0, days: 1, hours: 2, minutes: 3, seconds: 4,
//      milliseconds: 5, microseconds: 6, nanoseconds: 7,
//      isNegative: false }

Each unit holds what's left after the larger units: hours is never more than 23, minutes never more than 59, and so on. Negative durations set isNegative and keep the units positive:

msToParts(-90_000);
// => { minutes: 1, seconds: 30, isNegative: true, …everything else 0 }

Joining parts

partsToMs does the reverse. Pass any of the units, and set isNegative for a negative duration:

partsToMs({ hours: 1, minutes: 30 });
// => 5_400_000

partsToMs({ minutes: 1, seconds: 30, isNegative: true });
// => -90_000

partsToMs(msToParts(ms)) returns the original ms.