Optimize isDayVisible

Cloning is a little expensive, so we want to do it as little as possible.
Here we clone the month once and keep mutating that moment object.

In my profiling this change by itself appears to reduce the amount of
time spent in this function when updating DayPickeRangeController from
~16ms to ~11ms, with almost all of the time being spent in Moment clone.

src/utils/isDayVisible.js

@@ -5,11 +5,23 @@ import isAfterDay from './isAfterDay';
 
 export default function isDayVisible(day, month, numberOfMonths, enableOutsideDays) {
   if (!moment.isMoment(day)) return false;
-  let firstDayOfFirstMonth = month.clone().startOf('month');
-  if (enableOutsideDays) firstDayOfFirstMonth = firstDayOfFirstMonth.startOf('week');
+
+  // Cloning is a little expensive, so we want to do it as little as possible.
+  // Here we clone the month once and keep mutating that moment object.
+  const mutableMonth = month.clone();
+
+  const firstDayOfFirstMonth = enableOutsideDays
+    ? mutableMonth.startOf('month').startOf('week')
+    : mutableMonth.startOf('month');
+
   if (isBeforeDay(day, firstDayOfFirstMonth)) return false;
 
-  let lastDayOfLastMonth = month.clone().add(numberOfMonths - 1, 'months').endOf('month');
-  if (enableOutsideDays) lastDayOfLastMonth = lastDayOfLastMonth.endOf('week');
+  const lastDayOfLastMonth = enableOutsideDays
+    // We need to call endOf('week') when enableOutsideDays is true, because we
+    // are reusing the moment object, and our earlier mutation may have moved it
+    // to a previous month. This should snap us back to a good starting place.
+    ? mutableMonth.endOf('week').add(numberOfMonths - 1, 'months').endOf('month').endOf('week')
+    : mutableMonth.add(numberOfMonths - 1, 'months').endOf('month');
+
   return !isAfterDay(day, lastDayOfLastMonth);
 }

test/utils/isDayVisible_spec.js

@@ -21,4 +21,32 @@ describe('#isDayVisible', () => {
     const currentMonth = moment().add(2, 'months');
     expect(isDayVisible(test, currentMonth, 2)).to.equal(false);
   });
+
+  describe('enableOutsideDays', () => {
+    it('returns true if arg is in partial week before visible months', () => {
+      const test = moment('2019-04-30');
+      const currentMonth = moment('2019-05-01');
+      expect(isDayVisible(test, currentMonth, 1, false)).to.equal(false);
+      expect(isDayVisible(test, currentMonth, 1, true)).to.equal(true);
+    });
+
+    it('returns true if arg is in partial week after visible months', () => {
+      const test = moment('2019-06-01');
+      const currentMonth = moment('2019-05-01');
+      expect(isDayVisible(test, currentMonth, 1, false)).to.equal(false);
+      expect(isDayVisible(test, currentMonth, 1, true)).to.equal(true);
+    });
+
+    it('returns false if arg is before partial week before visible months', () => {
+      const test = moment('2019-04-27');
+      const currentMonth = moment('2019-05-01');
+      expect(isDayVisible(test, currentMonth, 1, true)).to.equal(false);
+    });
+
+    it('returns false if arg is after partial week after visible months', () => {
+      const test = moment('2019-06-03');
+      const currentMonth = moment('2019-05-01');
+      expect(isDayVisible(test, currentMonth, 1, true)).to.equal(false);
+    });
+  });
 });