coverage on decorated parameters is incorrect when `isolatedModules: true`

Issue :

Coverage on decorated parameters is incorrect when isolatedModules: true

Expected behavior :

Coverage on decorated parameters are correct when isolatedModules option is not set. If isolatedModules: true, coverage is incorrect with branch not covered.

Minimal repo :

https://github.com/hoonoh/jest-decorator-coverage/tree/master

[vad3x]

I’ve ended up with this workaround (similar to what @LoicMahieu has):

// fix-istanbul-decorators.js

const { default: tsJest } = require('ts-jest');

module.exports = fixIstanbulDecoratorCoverageTransformer();

function fixIstanbulDecoratorCoverageTransformer() {
  const transformer = tsJest.createTransformer();
  const process = transformer.process.bind(transformer);
  transformer.process = (...args) => {
    let result = process(...args);
    // Ignore decorators on methods and properties
    result = result.replace(
      /__decorate/g,
      '/* istanbul ignore next */__decorate',
    );

    // When constructor parameters have decorated properties (eg @inject), TS adds
    // a typeof branch check, which we don't want to instrument
    result = result.replace(
      /(?<=__metadata\("design:paramtypes".*?)(typeof \(_\w\s*=)/g,
      '/* istanbul ignore next */$1',
    );

    return result;
  };

  return transformer;
}

// jest.config.js

...

  globals: {
    'ts-jest': {
      isolatedModules: true,
    },
  },
  transform: {
    '\\.(ts|tsx)$': '<rootDir>/fix-istanbul-decorators.js',
  },
...

Ref: https://github.com/istanbuljs/istanbuljs/issues/70#issuecomment-778275926

[mandarini]

We are facing the same issue, and this happens only when we’re using

    'ts-jest': {
      isolatedModules: true,
    },

The solutions provided in the comments (eg. this and this) do not seem to have any effect.

@micalevisk @vad3x these solutions do not seem to have an effect on my end.

Edit:

This is how I solved the issue, in an Angular app. I had to specifically add /* istanbul ignore next */ before every single constructor parameter, and add /* istanbul ignore next */ before the constructor, too.