Jest testing multiple test file port 3000 already in use

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Jest 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.

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