Jest prints "Jest did not exit one second after the test run has completed" or "A worker process has failed to exit gracefully". What causes it and how do you fix it properly?

Something keeps Node's event loop alive after the tests finish. These open resources are called open handles. Common ones:

• an open database pool
• a Redis or message broker connection
• an HTTP server that was started and never closed
• a setInterval
• a scheduler that registered itself when a module was imported

To find them, run jest --detectOpenHandles. It shows where each handle was created, and it works best with --runInBand.

Then close everything in afterAll:

• await app.close() for a Nest app
• await dataSource.destroy()
• await redis.quit()
• server.close()
• clear any intervals

Also make background schedulers switch off in tests. Call unref() on timers that shouldn't keep the process alive.

--forceExit makes the message go away, but it only hides the leak. The leftover resources can still interfere with other tests in the same worker.