Babel parse error on flow syntax

[benadamstyles]

Summary

I am getting a BABEL_PARSE_ERROR on flow type syntax, and I think it's because stryker is adding instrumentation to the top of my files, which means that the // @flow pragma is no longer at the top of those files, so the babel flow plugin no longer runs against those files.

Stryker config

{
  packageManager: 'yarn',
  reporters: ['html', 'clear-text', 'progress', 'dashboard'],
  testRunner: 'jest',
  coverageAnalysis: 'perTest',
  disableTypeChecks: false,

  sandbox: {
    fileHeaders: {
      // I think this is ignored?
      '**/*.js': '// @flow\n\n',
    },
  }
}

Stryker environment

├─ @stryker-mutator/api@4.3.1
├─ @stryker-mutator/core@4.3.1
│  ├─ @stryker-mutator/api@4.3.1
│  ├─ @stryker-mutator/instrumenter@4.3.1
│  ├─ @stryker-mutator/util@4.3.1
├─ @stryker-mutator/instrumenter@4.3.1
│  ├─ @stryker-mutator/api@4.3.1
│  ├─ @stryker-mutator/util@4.3.1
├─ @stryker-mutator/jest-runner@4.3.1
│  ├─ @stryker-mutator/api@4.3.1
│  ├─ @stryker-mutator/util@4.3.1
├─ @stryker-mutator/util@4.3.1

├─ @jest/core@25.4.0

Your Environment

software	version(s)
node	14
npm	6
Operating System	macos

Add stryker.log

Will add if it's absolutely necessary

[benadamstyles]

I have just confirmed that my suspicion was correct – that this is due to the // @flow pragma no longer being at the top of the file – by adding a function above the pragma in one of my source files and running my jest tests as normal: the same error occurs.

[nicojs]

Thanks for bug report and triage! This should be a simple fix.

Could you maybe create a small hello-world-like example that reproduces this issue? We would be happy to add it to our e2e test suite so it won't ever break again.

[nicojs]

flow-project.zip

I tried my hand at reproducing this myself, but couldn't. See zip.

Unzip and cd into the directory. Then run

npm i
npm run test # see that normal unit testing works
npm run test:mutation # see that Stryker works

[swist]

@nicojs here's a MRE: https://github.com/swist/stryker-flow-bug

Just follow the steps you outlined in your comment :)