jest unit tests hang when using jest.useFakeTimers

[rburgst]

Prerequisites

• I have written a descriptive issue title
• I have searched existing issues to ensure the bug has not already been reported

Fastify version

3.23.1

Plugin version

No response

Node.js version

14.18

Operating system

macOS

Operating system version (i.e. 20.04, 11.3, 10)

Monterey

Description

When using fastify in a jest unit test calling

    jest
      .useFakeTimers("modern")
      .setSystemTime(new Date("2020-02-08T20:10:21.520Z"));

causes the server.inject call to stall forever.

Steps to Reproduce

see repro repo: https://github.com/rburgst/fastify-jest-timers

Expected Behavior

it should be possible to mock the current system time using jest.useFakeTimers().setSystemTime(...)

[mcollina]

I don't know why this is happening. It's likely because of Jest own environment.

I personally won't have time to work on this, but a PR would be highly appreciated.

(Generically I do not recommend to use Jest for testing Node.js applications)

[mcollina]

I moved this issue to help.

[jsumners]

Please file an issue with Jest. They create a new environment. We target Node.js.

[rburgst]

just for the record, we have countless repos with both fastify and jest and so far this combination has worked pretty great. So I am quite surprised about the statement

(Generically I do not recommend to use Jest for testing Node.js applications)

@mcollina do you have further details on why this would be the case?

[mcollina]

jestjs/jest#2549 and a lot of other issues. To be honest it works fine 99% of the time but that remaining 1% keeps biting teams across the globe and they spend days trying to understand why things do not work as they expect to work as they do when running Node.js proper.

---

Regarding your problem, I think you are missing a call to run all the pending timers: https://jestjs.io/docs/timer-mocks.

[rburgst]

hmm, for the record, using
https://www.npmjs.com/package/jest-environment-node-single-context
seems to work as suggested in jestjs/jest#2549 (comment)

[kibertoad]

This seems to be related to plugin registration somehow.
The part that hangs is this: await fastify.ready(), and if I add fake timer increment somewhere in between, fastify.after block is invoked without plugins having been properly registered.

It also hangs if one does await fastify.register(plugin)

[onosendi]

It seems jest.useFakeTimers({ advanceTimers: true }) fixes the issue. Not really sure why, though.

From the docs:

  /**
   * If set to `true` all timers will be advanced automatically by 20 milliseconds
   * every 20 milliseconds. A custom time delta may be provided by passing a number.
   * The default is `false`.
   */
  advanceTimers?: boolean | number;