Start using Jest's "modern" fake-timer implementation.

Recent improvements have been made to Jest's implementation of fake timers, on an opt-in basis, with a plan to make them the default in the future: jestjs/jest#7776.

Some prep work has been done toward this (#29011), but I don't find an existing issue for the actual change.

Simply flipping the switch to the "modern" implementation gives several test failures, likely due to some combination of

dependency on wrong semantics of the legacy implementation, and
"Modern" fake-timer implementation doesn't work with PromiseJS. jestjs/jest#10221

I'm excited for facebook/react-native's own tests to be made more correct, but I particularly hope to draw attention to the fact that React Native apps can't currently use the new implementation (jestjs/jest#10221, #29303, #29754). This is a problem for apps that can't use the legacy implementation either, because of its issues (not having Date.now() faked is a big one).

React Native version:

v0.61.5. (See #29767 (comment), below, for more environment info.)

⚠️	Missing Environment Information
ℹ️	Your issue may be missing information about your development environment. You can obtain the missing information by running `react-native info` in a console.

Ah, here's the missing environment info for the bot 🙂:

System:
    OS: macOS 10.15.6
    CPU: (12) x64 Intel(R) Core(TM) i7-9750H CPU @ 2.60GHz
    Memory: 2.87 GB / 16.00 GB
    Shell: 3.2.57 - /bin/bash
  Binaries:
    Node: 12.13.1 - ~/.nvm/versions/node/v12.13.1/bin/node
    Yarn: 1.22.4 - /usr/local/bin/yarn
    npm: 6.12.1 - ~/.nvm/versions/node/v12.13.1/bin/npm
    Watchman: 4.9.0 - /usr/local/bin/watchman
  SDKs:
    iOS SDK:
      Platforms: iOS 13.7, DriverKit 19.0, macOS 10.15, tvOS 13.4, watchOS 6.2
    Android SDK:
      API Levels: 23, 25, 26, 27, 28, 29
      Build Tools: 27.0.3, 28.0.3, 29.0.2
      System Images: android-28 | Google Play Intel x86 Atom
  IDEs:
    Android Studio: 3.6 AI-192.7142.36.36.6241897
    Xcode: 11.7/11E801a - /usr/bin/xcodebuild
  npmPackages:
    react: 16.9.0 => 16.9.0 
    react-native: 0.61.5 => 0.61.5

@chrisbobbe

Hi @chrisbobbe thanks for the request & all the details.

You opened a PR here: https://github.com/facebook/react-native/pull/29754/files

Would this resolve the Jest issue, if merged in?

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Start using Jest's "modern" fake-timer implementation. #29767

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Start using Jest's "modern" fake-timer implementation. #29767

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React Native version:

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github-actions commented on Aug 25, 2020

chrisbobbe commented on Sep 11, 2020

safaiyeh commented on Oct 28, 2020

github-actions commented on Jun 30, 2023

github-actions commented on Jul 7, 2023

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