Date utils improvements and specs

Date utility functions `prevMonth` and `nextMonth` change the source
date when using `src.set[Date|Month|FullYear]()`. This change creates a
copy of the date at entrance of function so we have a pure function.

Made some code refactor and added specs for these functions.

src/components/date-picker/util.js

@@ -17,19 +17,7 @@ export const parseDate = function(string, format) {
 };
 
 export const getDayCountOfMonth = function(year, month) {
-    if (month === 3 || month === 5 || month === 8 || month === 10) {
-        return 30;
-    }
-
-    if (month === 1) {
-        if (year % 4 === 0 && year % 100 !== 0 || year % 400 === 0) {
-            return 29;
-        } else {
-            return 28;
-        }
-    }
-
-    return 31;
+    return new Date(year, month + 1, 0).getDate();
 };
 
 export const getFirstDayOfMonth = function(date) {
@@ -38,48 +26,30 @@ export const getFirstDayOfMonth = function(date) {
     return temp.getDay();
 };
 
-export const prevMonth = function(src) {
-    const year = src.getFullYear();
-    const month = src.getMonth();
-    const date = src.getDate();
-
-    const newYear = month === 0 ? year - 1 : year;
-    const newMonth = month === 0 ? 11 : month - 1;
-
-    const newMonthDayCount = getDayCountOfMonth(newYear, newMonth);
-    if (newMonthDayCount < date) {
-        src.setDate(newMonthDayCount);
+export const siblingMonth = function(src, diff) {
+    const temp = new Date(src); // lets copy it so we don't change the original
+    const newMonth = temp.getMonth() + diff;
+    const newMonthDayCount = getDayCountOfMonth(temp.getFullYear(), newMonth);
+    if (newMonthDayCount < temp.getDate()) {
+        temp.setDate(newMonthDayCount);
     }
+    temp.setMonth(newMonth);
 
-    src.setMonth(newMonth);
-    src.setFullYear(newYear);
+    return temp;
+};
 
-    return new Date(src.getTime());
+export const prevMonth = function(src) {
+    return siblingMonth(src, -1);
 };
 
 export const nextMonth = function(src) {
-    const year = src.getFullYear();
-    const month = src.getMonth();
-    const date = src.getDate();
-
-    const newYear = month === 11 ? year + 1 : year;
-    const newMonth = month === 11 ? 0 : month + 1;
-
-    const newMonthDayCount = getDayCountOfMonth(newYear, newMonth);
-    if (newMonthDayCount < date) {
-        src.setDate(newMonthDayCount);
-    }
-
-    src.setMonth(newMonth);
-    src.setFullYear(newYear);
-
-    return new Date(src.getTime());
+    return siblingMonth(src, 1);
 };
 
-export const initTimeDate = function () {
+export const initTimeDate = function() {
     const date = new Date();
     date.setHours(0);
     date.setMinutes(0);
     date.setSeconds(0);
     return date;
-};
+};

test/unit/specs/date-picker-utils.spec.js

@@ -0,0 +1,67 @@
+const {prevMonth, nextMonth, getDayCountOfMonth} = require('../../../src/components/date-picker/util.js');
+
+// yyyy-mm-dd -> Date
+function dateFromString(str) {
+  str = str.split('-').map(Number);
+  str[1] = str[1] - 1;
+  return new Date(...str);
+}
+
+// Date -> yyyy-mm-dd
+function dateToString(date) {
+  return [date.getFullYear(), date.getMonth() + 1, date.getDate()].join('-');
+}
+
+describe('DatePicker utility functions', () => {
+  const assets = [
+		{date: '2030-3-31', prevMonth: '2030-2-28', nextMonth: '2030-4-30', count: 31},
+		{date: '2030-3-28', prevMonth: '2030-2-28', nextMonth: '2030-4-28', count: 31},
+		{date: '2030-3-1', prevMonth: '2030-2-1', nextMonth: '2030-4-1', count: 31},
+		{date: '2030-2-1', prevMonth: '2030-1-1', nextMonth: '2030-3-1', count: 28},
+		{date: '2030-1-1', prevMonth: '2029-12-1', nextMonth: '2030-2-1', count: 31},
+		{date: '2030-12-31', prevMonth: '2030-11-30', nextMonth: '2031-1-31', count: 31},
+		{date: '2030-6-30', prevMonth: '2030-5-30', nextMonth: '2030-7-30', count: 30},
+		{date: '2030-5-31', prevMonth: '2030-4-30', nextMonth: '2030-6-30', count: 31},
+		{date: '2032-3-31', prevMonth: '2032-2-29', nextMonth: '2032-4-30', count: 31},
+		{date: '2032-2-1', prevMonth: '2032-1-1', nextMonth: '2032-3-1', count: 29}
+  ];
+
+  it('Should behave as pure functions and not change source date', () => {
+    const date = new Date(2030, 4, 10);
+    const original = date.getMonth();
+    const foo = prevMonth(date);
+
+    expect(original).to.equal(date.getMonth());
+
+    const bar = nextMonth(date);
+    expect(original).to.equal(date.getMonth());
+    expect(bar.getMonth() - foo.getMonth()).to.equal(2);
+  });
+
+  it('Should calculate the previous month', () => {
+    for (const asset of assets) {
+      const date = dateFromString(asset.date);
+      const previous = prevMonth(date);
+
+      expect(dateToString(previous)).to.equal(asset.prevMonth);
+    }
+  });
+
+  it('Should calculate the next month', () => {
+    for (const asset of assets) {
+      const date = dateFromString(asset.date);
+      const next = nextMonth(date);
+
+      expect(dateToString(next)).to.equal(asset.nextMonth);
+    }
+  });
+
+  it('Should calculate the month length', () => {
+    for (const asset of assets) {
+      const date = dateFromString(asset.date);
+      const monthLength = getDayCountOfMonth(date.getFullYear(), date.getMonth());
+
+      expect(monthLength).to.equal(asset.count);
+    }
+  });
+});