[DatePicker] fix onChange on month change after initial day selection (…

…#3343)

* fix onChange doesn't fire on month change after initial day selection
Fixes #3340

* change !== to != operator

* add test onChange fired when month is changed for uncontrolled datepicker

packages/datetime/src/datePicker.tsx

@@ -246,7 +246,7 @@ export class DatePicker extends AbstractPureComponent<IDatePickerProps, IDatePic
                 selectedDay: day.getDate(),
             });
         }
-        if (this.state.value == null || this.state.value.getMonth() !== day.getMonth()) {
+        if (this.state.value != null && this.state.value.getMonth() !== day.getMonth()) {
             this.ignoreNextMonthChange = true;
         }
 

packages/datetime/test/datePickerTests.tsx

@@ -413,6 +413,16 @@ describe("<DatePicker>", () => {
             assert.isTrue(onChange.calledOnce);
         });
 
+        it("onChange fired when month is changed", () => {
+            const onChange = sinon.spy();
+            const { getDay, clickNextMonth } = wrap(<DatePicker onChange={onChange} />);
+            assert.isTrue(onChange.notCalled);
+            getDay().simulate("click");
+            assert.isTrue(onChange.calledOnce, "expected onChange called");
+            clickNextMonth();
+            assert.isTrue(onChange.calledTwice, "expected onChange called again");
+        });
+
         it("selected day updates are automatic", () => {
             const { assertSelectedDays, getDay } = wrap(<DatePicker />);
             assertSelectedDays();