Testing GraphQL API

I'm testing my GraphQL API, but I would like to clean it up a little bit. It's worth noting I'm using chai-http for the network requests. Here's what I'm currently doing (which works):

let createUser = (()=> {
  return new Promise((resolve, reject) => {
    chai.request(server)
      .post('/api/graphql/')
      .set('content-type', 'application/json')
      .send({ 'query' : 
        'mutation users { \
          user : addUser(inputs: { \
            firstName: \"Test\", \
            lastName: \"User\", \
            email: \"test@test.com\", \
          }) { \
            id, \
            firstName \
          } \
        }'
      })
      .end((err, res) => {
        if (err) { reject(err) }
        let data = res.body.data;
        let user = data.user;
        resolve(user);
      })
  });
});

However, I would like to clean it up a bit and do something like this:

let createUser = (() => {
  let newUser = {
    firstName: 'Test',
    lastName: 'User',
    email: 'test@test.com'
  };

  return new Promise((resolve, reject) => {
    chai.request(server)
      .post('/api/graphql/')
      .set('content-type', 'application/json')
      .send({ 'query' : 
        'mutation users { \
          user : addUser(inputs: ' + JSON.stringify(newUser) + ') { \
            id, \
            firstName \
          } \
        }'
      })
      .end((err, res) => {
        if (err) { reject(err) }
        let data = res.body.data;
        let user = data.user;
        resolve(user);
      })
  });
});

However, this style does of placing the object inputs does not work and returns a bad request error. Here is what part of the returned error object reads:

Any ideas how I can accomplish this task? Thanks in advance!

via Thomas