jest: The coverage percent of babel-jest and @swc/jest are different.

[Sh031224]

Problem

I are migrating from babel-jest to @swc/jest.
And I confirmed that the test worked successfully, but I found that the coverage was different from when using babel-jest.

I wrote a new function for simple reproduction.

// index.ts
export const anyFunction = (): number => {
  if (Math.random() > 0.5) {
    return 1;
  }

  return 0;
};

// index.test.ts
import { anyFunction } from '.';

jest.mock('.');

test('testing mock', () => {
  (anyFunction as jest.Mock).mockImplementation(() => 3);

  expect(anyFunction()).toBe(3);
});

And then run the command: jest --coverage

Result

Each has a different percentage of statements and functions.

@swc/jest

babel-jest

Config

swc

// jest.config.js
const fs = require('node:fs');

const swcConfig = JSON.parse(fs.readFileSync('./.swcrc'));

module.exports = {
  coverageDirectory: 'coverage',
  moduleDirectories: ['node_modules'],
  moduleFileExtensions: ['js',  ts'],
  transform: {
    '^.+\\.(j|t)sx?$': ['@swc/jest', { ...swcConfig }],
  },
};

// .swcrc
{
  "$schema": "https://json.schemastore.org/swcrc",
  "jsc": {
    "parser": {
      "syntax": "typescript",
      "decorators": false
    },
    "loose": true,
    "externalHelpers": true
  },
  "module": {
    "type": "commonjs"
  },
  "minify": true
}

babel

// babel.config.js
module.exports = {
  presets: [
    [
      '@babel/env',
      {
        modules: false,
      },
    ],
    '@babel/preset-typescript',
  ],
  plugins: [
    '@babel/plugin-transform-modules-commonjs',
  ],
};

// jest.config.js
module.exports = {
  coverageDirectory: 'coverage',
  moduleDirectories: ['node_modules'],
  moduleFileExtensions: ['js',  ts'],
  transform: {
    '^.+\\.(j|t)sx?$': 'babel-jest',
  },
};

[Sh031224]

moved: swc-project/jest#166