Jest with async function calls before and after setTimeout

[lavish-jain]

🐛 Bug Report

The scenario:- Using jest with nodejs, the function to be tested calls one async function, then calls a sleep function (wrapper over setTimeout to wait for a specific period of time), and then calls another function (not necessarily async).

While testing this with jest.useFakeTimers() and jest.advanceTimersByTime()/jest.runAllTimers()/jest.runOnlyPendingTimers(), the first function and the sleep function gets called, but the code after the call to sleep function is not executed. The coverage report confirms that the lines after sleep function are not executed.
The test passes.

Everything works as expected when func1 is synchronous.

How Can I make this work?

To Reproduce

Here is a basic example:-

index.js

const func1 = async() => {
    setTimeout(()=>{console.log('func 1...')}, 1000);
}

const func2 = async() => {
    setTimeout(()=>{console.log('func 2...')}, 1000);

}

const sleep = ms => {
    console.log(`Sleeping for ${ms/1000} seconds`);
    return new Promise(resolve => {
        setTimeout(resolve, ms);
    })
}

const main = async() => {
    try {
    await func1();
    // Sleeping for a long long time
    console.log('Before Sleep');
    await sleep(2000000);
    console.log('After Sleep')
    await func2();
    return 'success';
    } catch(err) {
        console.log(err);
        return 'error'
    }
}

index.test.js

const index = require('./index');
jest.useFakeTimers();

describe('Testing index.js...', () => {
    test('Should return success', async() => {
        const promise = index();
        jest.advanceTimersByTime(2000000);
        promise.then(response => {
            expect(response).toBe('success');
        }) 
    });
})

Expected behavior

Test to pass with the following logging into console:-

func 1...
Before Sleep
Sleeping for 2000 seconds
After Sleep
func 2...

Actual behaviour

Test passes with the following logging into console:-

func 1...
Before Sleep
Sleeping for 2000 seconds

Link to repl or repo (highly encouraged)

GitHub Repo for the above code

envinfo

System:
    OS: Linux 5.4 Ubuntu 20.04 LTS (Focal Fossa)
    CPU: (4) x64 Intel(R) Core(TM) i5-6200U CPU @ 2.30GHz
  Binaries:
    Node: 13.14.0 - /bin/node
    npm: 6.14.4 - /bin/npm
  npmPackages:
    jest: ^25.5.4 => 25.5.4 

[lavish-jain]

The following workaround works when func2() is synchronous:-

In index.test.js, replace

jest.advanceTimersByTime(2000000);

with

Promise.resolve().then(()=>{jest.advanceTimersByTime(2000000)});

But the above change gives the following output when func2 is async:-

func 1...
Before Sleep
Sleeping for 2000 seconds
After Sleep

[sreetamdas]

this should help: https://stackoverflow.com/questions/52177631/jest-timer-and-promise-dont-work-well-settimeout-and-async-function

[lavish-jain]

Hi @sreetamdas I had already tried that before. The output hangs at the call of the sleep() method (with the console showing no output after Sleeping for 2000 seconds), but the test passes. The test passes even when we change the return value from success to some gibberish in the main() function.

Apparently, useFakeTimers breaks when used with native promises. It is a known issue in jest for almost 2 years now.

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