Are tests inside one file run in parallel in Jest?

Question

Jest states in docs: \"Jest virtualizes JavaScript environments and runs tests in parallel across worker processes.\"

But what about multiple tests inside one file, do t

Answer 1

You can use test.concurrent('test run concurrently', () => { ... }) if you want to run them in parallel inside one file is too slow. It is a bit buggy and not well documented, but at least there's a way there.

One thing I notice is it does not wait for async stuff in the beforeAll() so you need some of your own trick(like setInterval to wait) to make it work as expected.

Answer 2

Yes, you can safely assume tests inside a single file will run in the order of appearance. You could prove this by putting a console.log in each it block.

It's probably worth mentioning that it's generally bad practice to rely on the order of execution / external state...and you never know, Jest (or the current underlying test runner, Jasmine) may decide to run them in a random order in a newer version.

Answer 3

Jest in 2020

To add a bit more information on this, async tests are run in series inside of a describe() statement. This is useful for IO/Database setup and cleanup functions. Just take a look at the following example:

some.spec.js

describe("my asynchronous tests", () => {
  beforeEach(async () => {
    console.log('> setup test')
    // SOMETHING ASYNCHRONOUS
  });
  afterEach(async () => {
    console.log('< teardown test')
    // SOMETHING ASYNCHRONOUS
  });

  test("test 1", async () => {
    console.log('-- starting test 1');
    // SOMETHING ASYNCHRONOUS
    console.log('-- finished test 1');
  }, 100000);

  test("test 2", async () => {
    console.log('-- starting test 2');
    // SOMETHING ASYNCHRONOUS
    console.log('-- finished test 2');
  }, 100000);
});

Outputs:

> setup test
-- starting test 1
-- finished test 1
< teardown test
> setup test
-- starting test 2
-- finished test 2
< teardown test

Multiple describe() statements will execute in parallel though, even if they're in the same file.