Insert data into res.body back to client

with my client js i have perform a fetch and send along some data. And i've attempted to do a res.send or res.json to send a result back.

Do I need to create a new json and send it?

This is the data being sent

let fetchData = {
  method: "post",
  headers: {
    'Accept': 'application/json',
    'Content-Type': 'application/json'
  },

  body: JSON.stringify({
    username: this.state.user,
    password: this.state.password,
    message: null
  })
}

Express server side will try to send a message to the client. but when I do this i did a console.log(res) it shows that the body is { null, null, null}

if(pw == result.password){
    console.log("User and Password Correct!");
    res.json({ message: 'correct' });
  }

via user3307553