setInterval not working when testing with jest

Question

I got a class on which I'm trying to run Jest tests. The class (not the test) is using an interval inside. I'm trying to test it with some method I created in the test like this:

const syncWait = ms => 
    {
        const end = Date.now() + ms;
        while (Date.now() < end) continue;
    };
    let myObj = new MyClass(); //the constructor starts the interval
    syncWait(5500);
    expect(...

However, it seems like the interval from the class itself is not being executed. I put some console.logs and saw it does arrive to the point in code in which the interval is set like this:

this.intervalId = setInterval(() => this.sendIntervalMessage(), this.timeUntilSendMessages); //time here is 3000

however it simply won't execute even after the time of the interval has passed. Why is that?

Answer 1

So for some reason checking time with jest won't work like that and you have to mock the passage of time.

You do it like this:

At the first line of the test I added this line:

jest.useFakeTimers();

The jest object doesn't need to be imported. It's automatically available in a jest test. This line means we are not using actual time, but rather gives us the option to simulate (mock) the passage of time.

While in the class's code I got a line calling for example setInterval(action, 3000), in the test we will call:

jest.advanceTimersByTime(3000);

Only then will the action of the interval be actually invoked.