NodeJS Mock 3rd party service

I have an example endpoint, when a request is submitted to it, I do another request to a third party service which turns out to be malfunctional at times. I want to simulate this service being malfunctional so my tests can be relevant. Here is some example code

it('should handle malfunctional 3rd party service', done => {
  Frisby.post(Endpoints.randomEndpoint, {
    email: 'johndoe@gmail.com',
    firstName: 'John',
    lastName: 'Doe'
  })
  .expect('status', 400)
  .expect('jsonTypes', Common.Types.ErrorModel)
  .done(done);
});

On my server side, I have something like..

app.post('randomEndpoint', (req, res) => {
  request('http://3rdpaty.com/api')
    .then(data => {
      res.send(200);
    })
    .catch(err => {
      res.send(500);
    })
})

My goal is tomock the response from the thrid pary service. Any ideas?