a user state machine that should transition to either "registered" or "guest" via a guard...

user.js

// User may be in one of:
// * idle: no user. no registration or access attempts
// * guest: a present user, not registered or registration/access failed
// * registered: existing user
// * processing: a register/access fetch machine is in flight
const machine = Machine({
  id: 'user',
  initial: 'idle',
  context: {
    id: undefined,
    email: undefined,
    name: undefined,
    fetchResource: undefined,
    fetchOpts: undefined,
    error: undefined,
  },
  states: {
    idle: {},
    guest: {},
    registered: {
      entry: 'registered'
    },
    processing: {
      invoke: {
        id: 'fetch',
        src: fetch,
        data: {
          resource: ctx => ctx.fetchResource,
          opts: ctx => ctx.fetchOpts
        },
        onDone: [{
          target: 'registered',
          cond: 'userRegistered',
          actions: 'fetched'
        }, {
          // an unregistered user
          target: 'guest'
        }]
      },
      entry: send(ACTIVATE, { to: 'fetch' })
    },
  },
  on: {
    REGISTER: {
      target: 'processing',
      actions: 'register',
    },
    ACCESS: {
      target: 'processing',
      actions: 'access',
    },
  }
}, {
  guards: {
    userRegistered: (_, e) => !!e.data.id,
  },
  actions: {
    register: assign({
      email: (_, e) => e.email,
      fetchResource: (_, e) => e.fetchResource,
      fetchOpts: (_, e) => e.fetchOpts,
    }),
    access: assign({
      // would have to be some auth/password or something... TBD
      email: (_, e) => e.email,
      fetchResource: (_, e) => e.fetchResource,
      fetchOpts: (_, e) => e.fetchOpts,
    }),
    fetched: assign({
      id: (_, e) => e.data.id,
      name: (_, e) => e.data.name,
      error: (_,e) => e.data.error,
    }),
    registered: ctx => {
      console.log(`user ${ctx.id} registered`);
    }
  }
});

snippet from a spec that never goes beyond the "processing" state...

const listener = state => {
  if (state.matches('registered')) {
    expect(state.context.id).toBe('abc-123');
    done();
  } else if (state.matches('guest')) {
    console.warn('in guest state');
    done();
  } else {
    console.warn('IDK...');
    console.log(state.value);
  }
};
service.onTransition(listener);
service.send(REGISTER, { email: '[email protected]', fetchResource: '/users/new' });