@redocly/openapi-core/src/__tests__/output.test.ts

See https://github.com/Redocly/redocly-cli

import { output } from '../output';

describe('output', () => {
  it('should write all parsable data to stdout', () => {
    const spyingStdout = jest.spyOn(process.stdout, 'write').mockImplementation();
    const data = '{ "errors" : [] }';

    output.write(data);

    expect(spyingStdout).toBeCalledTimes(1);
    expect(spyingStdout).toBeCalledWith(data);

    spyingStdout.mockRestore();
  });
});