Add timer example for jest testing

Author: jameshosken

clock.js

@@ -0,0 +1,11 @@
+function countdown(t, callback){
+  console.log("Countdown started")
+  setTimeout(function(){
+    console.log('Countdown done!');
+    callback();
+  }, t);
+}
+
+module.exports = {
+  countdown: countdown
+}

clock.test.js

@@ -0,0 +1,44 @@
+/* Based on Jest documentation: https://jestjs.io/docs/en/timer-mocks */
+'use strict';
+
+const { countdown } = require('./clock');
+const fs = require("fs");
+const path = require("path");
+const fetch = require("node-fetch");
+
+// Jest can fake the passage of time for javascript timers
+jest.useFakeTimers();
+
+/**
+ * Countdown to be defined
+ */
+
+test('Countdown function exists', countdownExistsTest);
+
+function countdownExistsTest(){
+  expect(countdown).toBeDefined();
+}
+
+/**
+ * Countdown to delay execution of callback by 2000 millis
+ */
+
+test('tests delay of countdown calls by 2 seconds', countdownDelayTest);
+
+function countdownDelayTest() {
+  const t = 2000;
+  const callback = jest.fn();
+
+  countdown(t, callback);
+
+  // At this point in time, the callback should not have been called yet
+  expect(callback).not.toBeCalled();
+
+  // Fast-forward until all timers have been executed
+   jest.advanceTimersByTime(t);
+
+  // Now our callback should have been called!
+  expect(callback).toBeCalled();
+  expect(callback).toHaveBeenCalledTimes(1);
+
+}