add-custom-matchers.ts

// config/jest/add-custom-matchers.ts
import toBeValidDot from '../../test/unit/customMatchers/toBeValidDot';

expect.extend(toBeValidDot);

customMatchers.d.ts

// test/unit/customMatchers.d.ts
declare namespace jest {
  interface Matchers<R, T> {
    toBeValidDot(): R;
  }

  interface Expect {
    toBeValidDot: () => any;
  }

  interface InverseAsymmetricMatchers {
    toBeValidDot: () => any;
  }
}

jest.config.js

module.exports = {
  preset: 'ts-jest',
  testEnvironment: 'node',
  verbose: true,
  collectCoverage: true,
  setupFilesAfterEnv: [
    '<rootDir>/config/jest/add-custom-matchers.ts',
  ],
};

toBeValidDot.ts

// test/unit/customMatchers/toBeValidDot.ts
async function toBeValidDot(this: any, dot: string): Promise<jest.CustomMatcherResult> {
  return {
    pass: true,
    message: () => '',
  };
}

export default { toBeValidDot };