Testing child process with Jest mock

test-cmd.ts

jest.mock('child_process');

describe('Test execute commands in a spawn child process', () => { 
  test('execCommand should return command stdout/stderr', async () => {
    // create a "fake" child process 
    const process = mockChildProcess();

    // return the "fake" child process object when calling child_process.spawn
    jest.spyOn(child_process, 'spawn').mockReturnValueOnce(process);

    // note that execCommand returns a promise, so this fake child process was
    // not spawned yet
    const cmdExec = execCommand('no-such-command', '/path/to/nowhere');

    // since we control the fake child process, we can emit the data events on
    // stdout/stderr streams
    process.stdout?.emit('data', 'Process stdout');
    process.stderr?.emit('data', 'Process stderr');

    // since we control the fake child process, we can end it
    process.emit('close', 0);

    // "execute" the command: spawn a child process and wait for it to end
    const { stderr, stdout, code } = await cmdExec;

    // validate the command output
    expect(code).toEqual(0);
    expect(stderr).toEqual('Process stderr');
    expect(stdout).toEqual('Process stdout');
  });
});