How come "allowJs" is required in tsconfig.json when using jsWithTs?

[javoire]

Issue :

The flag seems to be programmatically set when using ts-jest/presets/js-with-ts yet the config docs says it is required. As is it when I run my tests (see log below).

Expected behavior :

I would expect ts-jest to process *.js files when setting ts-jest/presets/js-with-ts without having allowJs in tsconfig.json.

Debug log:

Without allowJs in tsconfig.json

log file content

# content of ts-jest.log :
TS_JEST_LOG=stdout yarn test -i                                                                                   1 ↵  INSERT
yarn run v1.13.0
$ web-scripts test -i
ts-jest[jest-preset] (DEBUG) creating jest presets handling JavaScript files
 FAIL  src/math.test.js
  ● Test suite failed to run

    Jest encountered an unexpected token

    This usually means that you are trying to import a file which Jest cannot parse, e.g. it's not plain JavaScript.

    By default, if Jest sees a Babel config, it will use that to transform your files, ignoring "node_modules".

    Here's what you can do:
     • To have some of your "node_modules" files transformed, you can specify a custom "transformIgnorePatterns" in your config.
     • If you need a custom transformation specify a "transform" option in your config.
     • If you simply want to mock your non-JS modules (e.g. binary assets) you can stub them out with the "moduleNameMapper" config option.

    You'll find more details and examples of these config options in the docs:
    https://jestjs.io/docs/en/configuration.html

    Details:

    /Users/javoire/dev/test-module-serve/src/math.test.js:1
    ({"Object.<anonymous>":function(module,exports,require,__dirname,__filename,global,jest){import { add } from '.';
                                                                                                    ^

    SyntaxError: Unexpected token {

      at ScriptTransformer._transformAndBuildScript (../node_modules/@jest/transform/build/ScriptTransformer.js:471:17)
      at ScriptTransformer.transform (../node_modules/@jest/transform/build/ScriptTransformer.js:513:25)

ts-jest[Importer] (DEBUG) creating Importer singleton
ts-jest[jest-preset] (DEBUG) creating jest presets not handling JavaScript files
ts-jest[versions] (DEBUG) checking version of jest: OK
ts-jest[jest-transformer] (DEBUG) created new transformer
ts-jest[jest-transformer] (DEBUG) computing cache key for /Users/javoire/dev/test-module-serve/src/math.test.js
ts-jest[jest-transformer] (INFO) no matching config-set found, creating a new one
ts-jest[backports] (DEBUG) backporting config
ts-jest[config] (DEBUG) normalized jest config
ts-jest[config] (DEBUG) normalized ts-jest config
ts-jest[config] (DEBUG) babel is disabled
ts-jest[Importer] (DEBUG) loaded module typescript
ts-jest[Importer] (DEBUG) patching typescript
ts-jest[versions] (DEBUG) checking version of typescript: OK
ts-jest[config] (DEBUG) readTsConfig(): reading /Users/javoire/dev/test-module-serve/tsconfig.json
ts-jest[config] (DEBUG) normalized typescript config
Test Suites: 1 failed, 1 total
Tests:       0 total
Snapshots:   0 total
Time:        1.039s
Ran all test suites.
error Command failed with exit code 1.
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.

With allowJs in tsconfig.json

log file content

TS_JEST_LOG=stdout yarn test -i                                                                                     ✔  INSERT
yarn run v1.13.0
$ web-scripts test -i
ts-jest[jest-preset] (DEBUG) creating jest presets handling JavaScript files
 PASS  src/math.test.js
  ✓ adds two numbers (3ms)
  ✓ adds three numbers

ts-jest[Importer] (DEBUG) creating Importer singleton
ts-jest[jest-preset] (DEBUG) creating jest presets not handling JavaScript files
ts-jest[versions] (DEBUG) checking version of jest: OK
ts-jest[jest-transformer] (DEBUG) created new transformer
ts-jest[jest-transformer] (DEBUG) computing cache key for /Users/javoire/dev/test-module-serve/src/math.test.js
ts-jest[jest-transformer] (INFO) no matching config-set found, creating a new one
ts-jest[backports] (DEBUG) backporting config
ts-jest[config] (DEBUG) normalized jest config
ts-jest[config] (DEBUG) normalized ts-jest config
ts-jest[config] (DEBUG) babel is disabled
ts-jest[Importer] (DEBUG) loaded module typescript
ts-jest[Importer] (DEBUG) patching typescript
ts-jest[versions] (DEBUG) checking version of typescript: OK
ts-jest[config] (DEBUG) readTsConfig(): reading /Users/javoire/dev/test-module-serve/tsconfig.json
ts-jest[config] (DEBUG) normalized typescript config
ts-jest[jest-transformer] (DEBUG) computing cache key for /Users/javoire/dev/test-module-serve/src/index.js
ts-jest[jest-transformer] (DEBUG) computing cache key for /Users/javoire/dev/web-module-skeleton/test-module-serve/src/math.js
Test Suites: 1 passed, 1 total
Tests:       2 passed, 2 total
Snapshots:   0 total
Time:        0.924s
Ran all test suites.
✨  Done in 2.39s.

Minimal repo :

Cheers!

[ahnpnl]

from what I understand about allowJs flag is it allows tsc to compile js files. When you don't set this flag to true and use preset js-with-ts, tsc cannot compile your js files to pass them to jest, therefore it passes a non-compiled version to jest and jest throws error.

[javoire]

Hey @ahnpnl! Yes that's my understanding as well, but allowJs is already set here https://github.com/kulshekhar/ts-jest/blob/master/presets/index.js#L5. So what I don't understand is why it is also required to set in your project's tsconfig, according to https://kulshekhar.github.io/ts-jest/user/config/#the-3-presets :)

[ahnpnl]

Hmm you are right, have you tried to debug to check what is the generated config that ts-jest creates ? Maybe the documentation is outdated 😀

updated: the code set allowJs in ts-jest actually isn't related to final config. It is just for internal codes when creating the preset. Therefore the documentation is correct that you need to set allowJs in tsconfig.

[LoganTann]

Please be aware that setting this property might cause a performance issue : #4294

ts-jest suggests another workaround : changing the transform regex in your jest config to select only ts files, and not ts and js