jest-transform: testRegex value shows empty object instead of RegExp object

🐛 Bug Report

Custom transformer receives jest config string via getCacheKey. In ts-jest recently we rely on testRegex to do some logic. However, the value of testRegex which is passed to transformer is unusable. Instead of receiving the real value of the regex, transformer receives it as [{}]

Related to https://github.com/kulshekhar/ts-jest/issues/1509

To Reproduce

Steps to reproduce the behavior:

• Clone the repo and switch to branch test-regex-transformer .
• Install dependencies with yarn, run yarn test and see the exception
• Trace the exception back to getCacheKey and inspect jestConfigStr, observe the value in the picture below (red circle)
• Trace back to ScriptTransformer.js constructor, observe the input value of config is
• The value after this line https://github.com/facebook/jest/blob/2edd5c4724cfadb944a6a233a7ad8fb5a3e05996/packages/jest-transform/src/ScriptTransformer.ts#L72 becomes [{}] and it is unusable.

Expected behavior

testRegex value which is passed to transformer should be usable and retain its original value.

Link to repl or repo (highly encouraged)

https://github.com/ahnpnl/ts-jest-babel-example/tree/test-regex-transformer

envinfo

System:
    OS: macOS 10.15.4
    CPU: (8) x64 Intel(R) Core(TM) i7-7700HQ CPU @ 2.80GHz
  Binaries:
    Node: 12.16.1 - /usr/local/bin/node
    Yarn: 1.22.4 - /usr/local/bin/yarn
    npm: 6.14.4 - /usr/local/bin/npm
  npmPackages:
    jest: ^25.2.7 => 25.2.7 

[SimenB]

or what do you suggest for us regarding to the usage with string config ?

Yeah, feel free to ignore it if you want - it’s just there as a convenience

[ahnpnl]

I discovered that toMatchInlineSnapshot and toMatchSnapshot can handle serializing RegExp object correctly. Maybe the codes behind those 2 functions for serializing can be applied to this situation ?

Updated I’ve tested serialize from jest-snapshot. serialize produces a nice string, but then JSON.parse doesn’t work with that string.