JavaScript provides a handful of timer functions, including setTimeout, setInterval, clearTimeout, and clearInterval. These codes can be tested using Jest, too. Let's consider a simple function that displays the start of the game and ends the game after two seconds:

function takeXray(callback) {
 console.log("Ready, close your eye.");
 setTimeout(() => {
   console.log("Great you are done.");
   callback && callback();
 }, 2000);
}
module.exports = takeXray;

Now, let's test our function, called takeXray:

jest.useFakeTimers();
test("waits 2 second before taking the x-ray", () => {
 const takeXray = require("../time");
 takeXray();
 expect(setTimeout).toHaveBeenCalledTimes(1);
 expect(setTimeout).toHaveBeenLastCalledWith(expect.any(Function), 2000);
});