[Feature]: Separate jsdom instances for parallel running test suits to avoid flakiness

🚀 Feature Proposal

Add an option to use a jsdom instance per test suite to avoid test flakiness when test suites interfere with each other (e.g. both use localStorage).

Motivation

Currently when two test suites read/write localStorage in their executed application code, it might cause flaky tests because the tests use a shared jsdom instance and thus shared localStorage. Clearing localStorage in beforeEach does not solve the issue. —runInBand solves the issue but slows down test execution massively (e.g. 14min instead of 7min in our case). A clean solution would be to use separate jsdom instances for each test suite.

Example

No response

Pitch

Jest initializes the jsdom environment, so initializing environments per test suite needs to be done by Jest itself.

[SimenB]

We already instantiate per test suite (test file): https://github.com/facebook/jest/blob/491e7cb0f2daa8263caccc72d48bdce7ba759b11/packages/jest-environment-jsdom/src/index.ts#L34-L53

Sounds like JSDOM has some global state which interfere? Can you put together a reproduction which shows the issue?

[fabb]

Hm, you are right, writing/reading localStorage in separate test suites works fine in a test project: https://github.com/fabb/jest-jsdom-flaky

So the flakiness in my project needs to be caused by something else, I need to investigate further. Sorry to bother you.