Add "isValidDate" function to date-utils

This commit moves the "isValidDate" function from within the
security security_dashboard folder to the shared date and time utils.

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

@@ -566,6 +566,14 @@ export const getDateInPast = (date, daysInPast) =>
 export const getDateInFuture = (date, daysInFuture) =>
   new Date(newDate(date).setDate(date.getDate() + daysInFuture));
 
+/**
+ * Checks if a given date-instance was created with a valid date
+ *
+ * @param  {Date} date
+ * @returns boolean
+ */
+export const isValidDate = date => date instanceof Date && !Number.isNaN(date.getTime());
+
 /*
  * Appending T00:00:00 makes JS assume local time and prevents it from shifting the date
  * to match the user's time zone. We want to display the date in server time for now, to

ee/app/assets/javascripts/security_dashboard/store/modules/unscanned_projects/utils.js

 /* eslint-disable import/prefer-default-export */
-import { getDayDifference } from '~/lib/utils/datetime_utility';
-
-/**
- * Checks if a given date instance has been created with a valid date
- *
- * @param date {Date}
- * @returns {boolean}
- */
-const isValidDate = date => !Number.isNaN(date.getTime());
+import { getDayDifference, isValidDate } from '~/lib/utils/datetime_utility';
 
 /**
  * Checks if a given number of days is within a date range

spec/frontend/lib/utils/datetime_utility_spec.js

@@ -474,6 +474,23 @@ describe('getDateInFuture', () => {
   });
 });
 
+describe('isValidDate', () => {
+  it.each`
+    valueToCheck                              | isValid
+    ${new Date()}                             | ${true}
+    ${new Date('December 17, 1995 03:24:00')} | ${true}
+    ${new Date('1995-12-17T03:24:00')}        | ${true}
+    ${new Date('foo')}                        | ${false}
+    ${5}                                      | ${false}
+    ${''}                                     | ${false}
+    ${false}                                  | ${false}
+    ${undefined}                              | ${false}
+    ${null}                                   | ${false}
+  `('returns $expectedReturnValue when called with $dateToCheck', ({ valueToCheck, isValid }) => {
+    expect(datetimeUtility.isValidDate(valueToCheck)).toBe(isValid);
+  });
+});
+
 describe('getDatesInRange', () => {
   it('returns an empty array if 1st or 2nd argument is not a Date object', () => {
     const d1 = new Date('2019-01-01');