duration.valueOf() shall return number of milliseconds

[chengB12]

Describe the bug
javascript convention is valueOf() shall return comparable value

const d1 = dayjs.duration(1, 'second')
const d2 = dayjs.duration(2, 'second')
console.log(d2 > d1)

Expected behavior
above script shall print true, however, false would be printed

Information

• Day.js Version [e.g. v1.9.7]
• OS: [windows 10]
• Browser [node.js 14.15.0]
• Time zone: [e.g. GMT-07:00 DST (Pacific Daylight Time)]

[franky47]

This would be problematic for some cases, where additional context is needed:

// Months have 28 to 31 days
dayjs.duration(1, 'month').valueOf()

// Minutes can include leap seconds
dayjs.duration(1, 'minute').valueOf()

// Years can include leap days
dayjs.duration(1, 'year').valueOf()

[chengB12]

if dayjs.duration(1, 'month').valueOf() has problem, dayjs.duration(1, 'month').asMilliseconds() would have same problem. So I didn't see where is problem besides apparently breaking change

[addisonElliott]

I agree with adding a valueOf() function to duration to facilitate comparisons between durations. I think it makes the most sense to return the number of milliseconds since that's how the durations are primarily stored/handled under-the-hood.

Moment's duration has a valueOf() that returns the number of milliseconds.

@franky47 Regarding your comment, I don't think the valueOf() function is problematic. Rather, the constructor is problematic. The Day.js duration stores a milliseconds value and then calculates the various units from that (e.g. years, months, days, minutes). If you call duration.asMilliseconds(), the value is constant. For example, I believe months are assumed to be 30 days and the milliseconds is calculated from that.

Even Moment.js has this issue with it's duration. See here for more information: https://momentjs.com/docs/#/durations/. They make a reasonable assumption as to the length of a month & year and go from there.

If anyone is interested, here's code for extending the Day.js duration to add a valueOf() and equals() function. Written in Typescript.

// What a wonderful hack because Day.js decides NOT to export the Duration class
const Duration = dayjs.duration(0, 'ms').constructor;

Duration.prototype.equals = function(other: dayjs.Duration) {
    return this.$ms === (other as any).$ms;
};

// valueOf is used for comparison between durations
Duration.prototype.valueOf = function() {
    // Return total number of ms
    return this.$ms;
};