test formatting when microseconds or nanoseconds are the first numeric-styled unit

[ben-allen]

If microseconds are the first numeric-styled unit, they should be displayed as a fractional component of milliseconds. Likewise, if nanoseconds is the first numeric-styled unit, they should be displayed as a fractional component of microseconds.

[ptomato]

I didn't get all the way through this one, but I have a couple of questions:

1. If I'm understanding correctly, GetDurationUnitOptions step 4 basically forces {microseconds: 'numeric'} and {nanoseconds: 'numeric'} to {microseconds: 'fractional'} and {nanoseconds: 'fractional'} respectively. Do we need test coverage that these things produce the same output?
2. .format(d.microseconds.toString() + decimalSeparator + d.nanoseconds.toString())) seems OK for this particular input but it might be better to add .padStart(3, '0') to be robust against someone editing the values in d.

[ptomato]

Finished going through it, and I don't have any additional comments besides what I wrote the other day.

[ben-allen]

Added the padding for the fractional components -- good catch!

With regard to testing for "fractional" style: The "fractional" style is never directly used by users, who only have "numeric" available, and so we don't need to have tests for it. The internal-use-only "fractional" style exists for a couple of different reasons:

1. The "numeric" style means two dramatically different things, depending on the unit to which it's applied. If the unit is hours, minutes, or seconds, it means something like "format this as on a digital clock", when it's a subsecond unit it means "make this a fractional component of the previously displayed unit". Using the name "fractional" simplifies the logic for formatting numeric units, particularly once Editorial: Refactor handling of numeric-like styles, support differing hours/minutes vs minutes/seconds separators proposal-intl-duration-format#188 lands
2. A hypothetical DFv2 could allow users to style durations with fractional components of minutes or hours as well as seconds, milliseconds, and microseconds. In this case making "fractional" an actual user-specifiable style rather than just something for internal use would be necessary.

[ptomato]

Okay. fractional not being a user-facing option was what I was missing!