Make durations preserve keys across operations

src/duration.js

@@ -454,9 +454,8 @@ export default class Duration {
       result = {};
 
     for (const k of orderedUnits) {
-      const val = dur.get(k) + this.get(k);
-      if (val !== 0) {
-        result[k] = val;
+      if (dur.values.hasOwnProperty(k) || this.values.hasOwnProperty(k)) {
+        result[k] = dur.get(k) + this.get(k);
       }
     }
 

src/impl/diff.js

@@ -42,9 +42,7 @@ function highOrderDiffs(cursor, later, units) {
         cursor = highWater;
       }
 
-      if (delta > 0) {
-        results[unit] = delta;
-      }
+      results[unit] = delta;
     }
   }
 

test/datetime/diff.test.js

@@ -18,7 +18,7 @@ test('DateTime#diff defaults to milliseconds', () => {
 });
 
 test('DateTime#diff makes simple diffs', () => {
-  expect(diffObjs({ year: 2017 }, { year: 2017 }, 'years')).toEqual({});
+  expect(diffObjs({ year: 2017 }, { year: 2017 }, 'years')).toEqual({ years: 0 });
 
   expect(diffObjs({ year: 2017 }, { year: 2016 }, 'years')).toEqual({
     years: 1
@@ -102,14 +102,6 @@ test('DateTime#diff accepts multiple units', () => {
       'days'
     ])
   ).toEqual({ years: 5, days: 363 });
-
-  expect(
-    diffObjs({ year: 2015, month: 3, day: 14 }, { year: 2009, month: 3, day: 16 }, [
-      'years',
-      'days',
-      'hours'
-    ])
-  ).toEqual({ years: 5, days: 363 });
 });
 
 test('DateTime#diff handles unmatched units', () => {
@@ -119,23 +111,30 @@ test('DateTime#diff handles unmatched units', () => {
       { year: 2017, month: 6, day: 1, hour: 22 },
       ['weeks', 'days', 'hours']
     )
-  ).toEqual({ days: 5, hours: 23 });
+  ).toEqual({ weeks: 0, days: 5, hours: 23 });
 
   expect(
     diffObjs(
       { year: 2017, month: 6, day: 27, hour: 21 },
       { year: 2017, month: 6, day: 26, hour: 22 },
       ['days', 'hours']
     )
-  ).toEqual({ hours: 23 });
+  ).toEqual({ days: 0, hours: 23 });
 
   expect(
     diffObjs(
       { year: 2017, month: 6, day: 7, hour: 21 },
       { year: 2017, month: 6, day: 1, hour: 22 },
       ['weeks', 'hours']
     )
-  ).toEqual({ hours: 23 + 5 * 24 });
+  ).toEqual({ weeks: 0, hours: 23 + 5 * 24 });
+});
+
+test('DateTime#diff sets all its units to 0 if the duration is empty', () => {
+  const t = DateTime.fromObject({ year: 2018, month: 11, day: 5, hour: 0 });
+  expect(t.diff(t).toObject()).toEqual({ milliseconds: 0 });
+  expect(t.diff(t, 'hours').toObject()).toEqual({ hours: 0 });
+  expect(t.diff(t, 'days').toObject()).toEqual({ days: 0 });
 });
 
 test('DateTime#diff puts fractional parts in the lowest order unit', () => {
@@ -164,7 +163,7 @@ test('DateTime#diff is calendary for years, months, day', () => {
       'years',
       'days'
     ])
-  ).toEqual({ years: 6 });
+  ).toEqual({ years: 6, days: 0 });
 
   expect(
     diffObjs({ year: 2016, month: 3, day: 14 }, { year: 2010, month: 3, day: 16 }, [

test/duration/math.test.js

@@ -52,6 +52,14 @@ test('Duration#plus maintains invalidity', () => {
   expect(dur.invalidReason).toBe('because');
 });
 
+test('Duration#plus results in the superset of units', () => {
+  let dur = Duration.fromObject({ hours: 1, minutes: 0 }).plus({ seconds: 3, milliseconds: 0 });
+  expect(dur.toObject()).toEqual({ hours: 1, minutes: 0, seconds: 3, milliseconds: 0 });
+
+  dur = Duration.fromObject({ hours: 1, minutes: 0 }).plus({ });
+  expect(dur.toObject()).toEqual({ hours: 1, minutes: 0 });
+});
+
 //------
 // #minus()
 //------