Fix for month being reset when month prop is the same month (#695)

* Add isSameMonth to DateUtils

* Fix month props comparison

docs/src/pages/api/DateUtils.js

@@ -120,6 +120,14 @@ console.log(newRange.from) // 2015-05-24`}</CodeBlock>
         Return <code>true</code> if <code>day1</code> and
         <code>day2</code> are the same day.
       </p>
+      <h3>
+        <Anchor id="isSameMonth" />
+        isSameMonth <code>(day1: ?Date, day2: ?Date) ⇒ boolean</code>
+      </h3>
+      <p>
+        Return <code>true</code> if <code>day1</code> and
+        <code>day2</code> fall in the same month.
+      </p>
     </ApiDocs>
   </DocPage>
 );

src/DateUtils.js

@@ -41,6 +41,23 @@ export function isSameDay(d1, d2) {
   );
 }
 
+/**
+ * Return `true` if two dates fall in the same month.
+ *
+ * @export
+ * @param  {Date}  d1
+ * @param  {Date}  d2
+ * @return {Boolean}
+ */
+export function isSameMonth(d1, d2) {
+  if (!d1 || !d2) {
+    return false;
+  }
+  return (
+    d1.getMonth() === d2.getMonth() && d1.getFullYear() === d2.getFullYear()
+  );
+}
+
 /**
  * Returns `true` if the first day is before the second day.
  *
@@ -194,4 +211,5 @@ export default {
   isFutureDay,
   isPastDay,
   isSameDay,
+  isSameMonth,
 };

src/DayPicker.js

@@ -161,7 +161,7 @@ export default class DayPicker extends Component {
   }
 
   componentWillReceiveProps(nextProps) {
-    if (this.props.month !== nextProps.month) {
+    if (!DateUtils.isSameMonth(this.props.month, nextProps.month)) {
       this.setState(this.getStateFromProps(nextProps));
     }
   }

test/DateUtils.js

@@ -97,6 +97,26 @@ describe('DateUtils', () => {
     });
   });
 
+  describe('isSameMonth', () => {
+    it('returns true if two days differ only by time', () => {
+      const day1 = new Date(2015, 10, 11, 5, 25);
+      const day2 = new Date(2015, 10, 11, 7, 40);
+      const isSameMonth = DateUtils.isSameMonth(day1, day2);
+      expect(isSameMonth).toBe(true);
+    });
+    it('returns true for different days', () => {
+      const day1 = new Date(2015, 8, 12);
+      const day2 = new Date(2015, 8, 11);
+      const isSameMonth = DateUtils.isSameMonth(day1, day2);
+      expect(isSameMonth).toBe(true);
+    });
+    it('returns false if one of the days is not specified', () => {
+      const day1 = new Date(2015, 8, 12);
+      const isSameMonth = DateUtils.isSameMonth(day1, null);
+      expect(isSameMonth).toBe(false);
+    });
+  });
+
   describe('isPastDay', () => {
     it('detects a day is in the past', () => {
       const isPastDay = DateUtils.isPastDay(new Date(2015, 5, 11));