Transform to inherit default transformer if no match

[danm]

Pre question

Am I right in thinking that if no transform property is provided in the Jest config, Jest will use its default compiler (babel-jest)?

If so, should the docs be updated that if a transformer isn't defined, instead of being undefined, it should use babel-jest?

🚀 Feature Proposal

Entries in the transform property use the provided compiler, but if no file is matched, then it uses the default compiler.

Motivation

It isn't clear what the default transformer is, so if adding your own to transformer, for example, an SVG, it is not obvious imo that you have to re clarify other files should use babel-jest.

Additionally, recently Jest has changed babel-jest from an external dependancy to an internal one. This means, if I am using a transformer for just SVGs, I also have to tell the transformer to specifically use babel-jest. But what if Jest changes its mind and prefers to use a different compiler in the future, I'd have to change all transformers to make use of the new compiler.

Example

No transformer selected, uses default (babel-jest), compiles without issue

// package.json
{
  "jest": {
     "clearMocks": true
  }
}

Transformer selected to deal with SVGs, but everything else fails.

// package.json
{
  "jest": {
     "clearMocks": true,
     "transformer": {
        "^.+\\.svg$": "jest-svg-transformer"
     }
  }
}

Transformer selected to deal with SVGs and babel-jest defined - compiles without issue.

// package.json
{
  "jest": {
     "clearMocks": true,
     "transformer": {
        "^.+\\.(ts|tsx|js|jsx)$": "babel-jest",
        "^.+\\.svg$": "jest-svg-transformer"
     }
  }
}

Proposed Example

Transformer selected to deal with SVGs and any files that do not match the transformer pattern, use default compiler. This also means that if Jest moves away from using babel-jest, it doesn't need to be re clarified.

// package.json
{
  "jest": {
     "clearMocks": true,
     "transformer": {
        "^.+\\.svg$": "jest-svg-transformer"
     }
  }
}

I could be missing something somewhere - it might just be an update to the docs. Thoughts?

[danm]

somewhat related #2076

[jeysal]

Hey @danm, thanks for the suggestion!
Your understanding of transform is right as far as I can tell, and I agree that the docs saying undefined are confusing - happy to accept a PR changing this :)
With regards to the feature request:

• The transform docs have a section with the information that you'd need to specify babel-jest manually, if you can think of a way to make that clearer that would be awesome too
• I'm not sure the default changing is such a big deal - that would probably already be a big breaking change without some more people having to update the config when upgrading
• If setting transform does not suffice, we'd need a way to nullify the default transform instead

[thernstig]

@danm I also found this confusing, great if you can submit a PR :)

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