Javascript Js错误："；未捕获（承诺中）语法错误：输入意外结束“；

我遇到了错误：

Uncaught（in promise）语法错误：输入意外结束

on尝试将引用发布到我的专用express服务器，你们能帮我吗

function create() {

    event.preventDefault()

    firstName = document.getElementById('firstName').value
    lastName = document.getElementById('lastName').value
    username = document.getElementById('username').value
    password = document.getElementById('password').value

    const userInfo = {
        firstName: firstName,
        lastName: lastName,
        username: username, 
        password: password
    }

    const config = {
        method: "POST",
        mode: "no-cors",
        body: {userInfo},
        headers: {
            "Content-Type":"application/json"
        }
    }

    fetch('https://marcelochat.herokuapp.com/create', config)
    .then(res => res.json())
    .then(resp => {
        console.log(resp)
    })
}

错误在这里：

。然后（res=>res.json（））

注意：firstName、lastName、username和password常量是输入值

    firstName = document.getElementById('firstName').value
    lastName = document.getElementById('lastName').value
    username = document.getElementById('username').value
    password = document.getElementById('password').value

这个变量是否超出范围

如果没有，试试这个

  let  firstName = document.getElementById('firstName').value
  let  lastName = document.getElementById('lastName').value
  let  username = document.getElementById('username').value
  let  password = document.getElementById('password').value

config.body userInfo是否应该位于对象内部