Allow to share global state between tests from globalSetup

🚀 Feature Proposal

Add a global property to the this of globalSetup and globalTeardown async functions that can be used to set global variables that can be accessed in all tests via global. The same global is shared across all tests.

Motivation

While jest was in the beginning used only for frontend testing, it has moved in the direction of becoming a general test framework. Especially for backend integration tests there is a tradeoff between test speed and departmentalization: Starting up a new backend instance for each individual test usually isn’t feasible. Therefore most other test frameworks like mocha or jasmine provide possibilities to share state between tests, e. g. the backend instance. Usage examples include mocking http requests via nock.

Example

Let’s assume an integration test that tests backend and database integration. The setup could look like this:

const backend = require('backend')

async function setupApp () {
  await new Promise((resolve, reject) => {
    backend.start().then((instance) => {
      this.global.backend = instance
    })
  })
}

module.exports = setupApp

And using the global could be done like this:

const request = require('supertest')
test('should call out to that fancy other api', () => {
  request(jest.globals.backend.url)
    .post('/some-endpoint')
    expect(200)
})

Pitch

As far as I know this change currently cannot be implemented outside of the main jest framework. Closest is an environment, but environments are sandboxed and do not share global state.

Open questions

How to best implement it?

I don’t know the jest code well enough to have an idea how to best implement this. It might e. g. be easier to make the global available via global, or even jest.getGlobals().

Can we prevent misuse?

Sharing state between tests can lead to sideffects and random test breakage. One possible solution would be to make the jest.globals read-only, but I am not sure whether this is feasible without massively reducing which kind of objects can be stored.

[DanLambe]

I agree with @dbartholomae on this issue, I find it hard to recommend jest for all types of testing without the ability to share state between tests. I have a real usecase currently where the company I work for wanted to standardize our testing frameworks so I do to start using Jest over Mocha for my functional API testing for our react app. that was a mistake given that I have to fetch a new bearer token for every test file with no way of retaining that token to a variable “globally”.

[adrianmcli]

I think my desired use-case is probably not possible, but I figured I’d chime in here as well.

I need to run integration tests with a server that is instantiated in-memory. That means I’m doing something like this:

const server = await startServer()

And then I need to use that server instance over a bunch of my tests files. To make matters worse, that particular startServer function takes about 5 to 10 seconds to startup (this is non-negotiable).

If instances cannot be passed around, then it sounds like I’m out of luck. But I would really love it if someone could tell me that I am wrong and that a solution is just around the corner.