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 they run in parallel or this statement applies just to the test files? Can I assume that tests in one file run in order of appearance and in serial?

Answer 1

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 2

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.