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

[chrisbobbe]

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.)

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⚠️	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.

[chrisbobbe]

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

[safaiyeh]

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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