Add `newDate` helper, update `getTimeframeWindowFrom`

Adds `newDate` helper to copy date object without keeping reference.
Adds negative length support in `getTimeframeWindowFrom` to go
back in time.

app/assets/javascripts/lib/utils/datetime_utility.js

@@ -7,6 +7,14 @@ import { languageCode, s__ } from '../../locale';
 
 window.timeago = timeago;
 
+/**
+ * This method allows you to create new Date instance from existing
+ * date instance without keeping the reference.
+ *
+ * @param {Date} date
+ */
+export const newDate = date => (date instanceof Date ? new Date(date.getTime()) : new Date());
+
 /**
  * Returns i18n month names array.
  * If `abbreviated` is provided, returns abbreviated
@@ -321,23 +329,35 @@ export const getSundays = date => {
 
 /**
  * Returns list of Dates representing a timeframe of months from startDate and length
+ * This method also supports going back in time when `length` is negative number
  *
- * @param {Date} startDate
+ * @param {Date} initialStartDate
  * @param {Number} length
  */
-export const getTimeframeWindowFrom = (startDate, length) => {
-  if (!(startDate instanceof Date) || !length) {
+export const getTimeframeWindowFrom = (initialStartDate, length) => {
+  if (!(initialStartDate instanceof Date) || !length) {
     return [];
   }
 
+  const startDate = newDate(initialStartDate);
+  const moveMonthBy = length > 0 ? 1 : -1;
+
+  startDate.setDate(1);
+  startDate.setHours(0, 0, 0, 0);
+
   // Iterate and set date for the size of length
   // and push date reference to timeframe list
-  const timeframe = new Array(length)
-    .fill()
-    .map((val, i) => new Date(startDate.getFullYear(), startDate.getMonth() + i, 1));
+  const timeframe = new Array(Math.abs(length)).fill().map(() => {
+    const currentMonth = startDate.getTime();
+    startDate.setMonth(startDate.getMonth() + moveMonthBy);
+    return new Date(currentMonth);
+  });
 
   // Change date of last timeframe item to last date of the month
-  timeframe[length - 1].setDate(totalDaysInMonth(timeframe[length - 1]));
+  // when length is positive
+  if (length > 0) {
+    timeframe[timeframe.length - 1].setDate(totalDaysInMonth(timeframe[timeframe.length - 1]));
+  }
 
   return timeframe;
 };

spec/javascripts/lib/utils/datetime_utility_spec.js

@@ -156,7 +156,7 @@ describe('getSundays', () => {
 });
 
 describe('getTimeframeWindowFrom', () => {
-  it('returns array of date objects upto provided length start with provided startDate', () => {
+  it('returns array of date objects upto provided length (positive number) into the future starting from provided startDate', () => {
     const startDate = new Date(2018, 0, 1);
     const mockTimeframe = [
       new Date(2018, 0, 1),
@@ -174,6 +174,25 @@ describe('getTimeframeWindowFrom', () => {
       expect(timeframeItem.getDate()).toBe(mockTimeframe[index].getDate());
     });
   });
+
+  it('returns array of date objects upto provided length (negative number) into the past starting from provided startDate', () => {
+    const startDate = new Date(2018, 0, 1);
+    const mockTimeframe = [
+      new Date(2018, 0, 1),
+      new Date(2017, 11, 1),
+      new Date(2017, 10, 1),
+      new Date(2017, 9, 1),
+      new Date(2017, 8, 1),
+    ];
+    const timeframe = datetimeUtility.getTimeframeWindowFrom(startDate, -5);
+
+    expect(timeframe.length).toBe(5);
+    timeframe.forEach((timeframeItem, index) => {
+      expect(timeframeItem.getFullYear()).toBe(mockTimeframe[index].getFullYear());
+      expect(timeframeItem.getMonth()).toBe(mockTimeframe[index].getMonth());
+      expect(timeframeItem.getDate()).toBe(mockTimeframe[index].getDate());
+    });
+  });
 });
 
 describe('formatTime', () => {
@@ -376,3 +395,22 @@ describe('calculateRemainingMilliseconds', () => {
     expect(milliseconds).toBe(0);
   });
 });
+
+describe('newDate', () => {
+  it('returns new date instance from existing date instance', () => {
+    const initialDate = new Date(2019, 0, 1);
+    const copiedDate = datetimeUtility.newDate(initialDate);
+
+    expect(copiedDate.getTime()).toBe(initialDate.getTime());
+
+    initialDate.setMonth(initialDate.getMonth() + 1);
+
+    expect(copiedDate.getTime()).not.toBe(initialDate.getTime());
+  });
+
+  it('returns date instance when provided date param is not of type date or is undefined', () => {
+    const initialDate = datetimeUtility.newDate();
+
+    expect(initialDate instanceof Date).toBe(true);
+  });
+});