Today I Learned

If you use webpack aliases for your imports you need to let Jest know about them before you can test your code. You can do this in a jest configuration file - jest.config.js for instance:

module.exports = {
  moduleNameMapper: {
    //an import like: '@/interfaces/Foo.ts' 
    //becomes '[your-root]/src/interfaces/Foo.ts'
    '@/(.*)$': '<rootDir>/src/$1',
  },
  ...
};

If you want to learn how to configure webpack aliases you can read this post