Jest testing multiple test file port 3000 already in use

Just runs test suites in parallel by default. However, the machine that the tests are running on only has a single port 3000 (or port 4000, etc.)

This means that if you tell your app to listen on a port, and then start multiple concurrent processes, there will be a port collision and Jest will exit with EADDRINUSE - port already in use.

Like some people already mentioned, you can solve the issue by running your tests in sequence with --runInBand. However, you will lose out on the performance benefits of parallelization.

There is another way... do not explicitly listen on a port when running tests.

if (process.env.NODE_ENV !== 'test') {
  app.listen(port, () => console.log(`Listening on port ${port}`)
}

Now, when you feed your app to supertest, it will run your app on port 0 since it's not already running on a port.

const app = require('../app')
const request = require('supertest')

it('gets todos', async () => {
  const response = await request(app).get('/todos')
  expect(response.body.length).toEqual(3)
}

What exactly is port 0 you ask? That is how you tell Unix machines...

Choose the first randomly available port that you find.

Now that each test suite is running on a randomly available port, there is no longer a risk of port collisions which means we've solved the EADDRINUSE - port already in use error and we can continue running tests in parallel.

I had this issue, and seem to have solved it by setting jest to only use one worker. I think the issue was caused by more than one instance of the server running with the same port, thereby causing the clash.

This is the line from my npm package.json file. The --maxWorkers=1 seems to have done the trick (https://jestjs.io/docs/en/cli#maxworkers-num-string).

"scripts": {
  "test": "jest --forceExit --detectOpenHandles  --watchAll --maxWorkers=1"
},