Add isWeekday code snippets

test/isWeekday.test.js

@@ -0,0 +1,16 @@
+const {isWeekday} = require('./_30s.js');
+
+test('isWeekDay is a Function', () => {
+  expect(isWeekday).toBeInstanceOf(Function);
+});
+test('Returns the correct type', () => {
+  expect(typeof isWeekday()).toBe('boolean');
+});
+const friday = new Date('2019-07-19');
+const saturday = new Date('2019-07-20');
+test('Returns true', () => {
+  expect(isWeekday(friday)).toBe(true);
+});
+test('Returns false', () => {
+  expect(isWeekday(saturday)).toBe(false);
+});