NestJS @swc/jest test coverage

This repo reproduces a difference in test coverage between ts-jest and @swc/jest, with a workaround for it.

Steps to reproduce

$ nvm use
$ npm install

npm run test:tsjest

Coverage should be 100%.

npm run test:swc

Notice that Branch coverage dropped to 50%.

Run tests with ./create-swc-transformer.js, a wrapper around @swc/jest to workaround this.

npm run test:swc-transformer

Coverage should be 100%.

This seems to be happening because of how swc transpiles the Typescript metadata for constructors.

Take this constructor:

constructor(
  @Inject(AppService)
  private readonly appService: AppService,
) {}

The design:paramtypes metadata for the constructor gets transpiled by swc to:

_ts_metadata("design:paramtypes", [
    typeof _appservice.AppService === "undefined" ? Object : _appservice.AppService
])

Notice the typeof ternary. This gets flagged as an "uncovered branch" by istanbul, the coverage calculator used by jest.

Compare this to how ts-jest outputs this:

__metadata("design:paramtypes", [app_service_1.AppService])

No ternary, so no impact to code coverage.

./create-swc-transformer.js is a workaround that tells istanbul to ignore these lines that incorrectly impact code coverage.

It inserts an /* istanbul ignore next */ comment in the necessary places, resulting in transpiled code like this:

_ts_metadata("design:paramtypes", [
    /* istanbul ignore next */typeof _appservice.AppService === "undefined" ? Object : _appservice.AppService
])