Jest: Odd behavior in watch mode between "Enter" and "save" with React

[kentcdodds]

🐛 Bug Report

To Reproduce

Steps to reproduce the behavior:

git clone https://github.com/kentcdodds/jest-save-enter
cd jest-save-enter
npm install
npm run test

Then open the example.test.js and save it a few times until you notice the issue. The test passes, but every now and then, we'll get a console.error from React.

Here's the code in the example.test.js file:

const React = require('react')
const ReactDOM = require('react-dom')
const {act} = require('react-dom/test-utils')

function Comp() {
  const [state, setState] = React.useState()
  React.useEffect(() => {
    setTimeout(() => {
      console.log('updating state')
      setState('blah')
    })
  }, [state])
  return null
}

test('example', async () => {
  const container = document.createElement('div')

  // using `act` is required to get effect callbacks run when rendering components
  // (React Testing Library does this automatically)
  act(() => {
    ReactDOM.render(React.createElement(Comp), container)
  })

  // the `cleanup` function from React Testing Library (which also happens automatically)
  // flushes all the in-flight effects by waiting for the next tick of the event loop
  // this is basically a simplified version of: https://github.com/facebook/react/blob/master/packages/shared/enqueueTask.js
  await new Promise((resolve) => setImmediate(resolve))

  // this is the last thing that happens in cleanup (to avoid memory leaks)
  ReactDOM.unmountComponentAtNode(container)
}

And here's some output:

When it works

Without changing anything, sometimes this happens

Expected behavior

I expect the output to be consistent.

Link to repl or repo (highly encouraged)

https://github.com/kentcdodds/jest-save-enter

This issue is best explained in video form (5 minutes): https://youtu.be/hRxf6QUwkow

envinfo

  System:
    OS: macOS 10.15.4
    CPU: (16) x64 Intel(R) Core(TM) i9-9980HK CPU @ 2.40GHz
  Binaries:
    Node: 12.13.1 - ~/n/bin/node
    Yarn: 1.21.1 - ~/.yarn/bin/yarn
    npm: 6.14.5 - /usr/local/bin/npm
  npmPackages:
    jest: ^26.0.1 => 26.0.1 

I honestly have no idea what's going on here, but I definitely observe this happening sometimes in less contrived examples.

Related issues:

• https://github.com/tannerlinsley/react-query/issues/432
• https://github.com/testing-library/react-testing-library/pull/632

[keyboard-clacker]

Looks like it may have to do with the setTimeout, I was able to modify the test to make sure it's using fake timers when it's rendering to seemingly remove the randomness from it-- no idea if this is the best solution, but it's worked for me so far! haha https://github.com/kentcdodds/jest-save-enter/pull/1

[kentcdodds]

Thanks, but changing the test is irrelevant. The fact jest is inconsistent with this is the bug in jest.