我找不到這個反應代碼有什么問題，但是當嘗試 console.log 該項目時，除了注釋字段之外的所有字段都未定義。我決定使用基于類的組件，希望是函數組件導致了錯誤，但類組件聲明 typeError firstName 為 null。這是功能組件和錯誤消息Objectaddress: undefinedbirthday: undefinedcompany: undefinedemail: undefinedfirstName: undefinedlastName: undefinednote: "d"phone: undefinedsocialProfile: undefinedurl: undefined__proto__: Objectimport React, {useState} from 'react'function Phonebookform() {  const [phonebook, setPhonebook] = useState({    firstName:'',    lastName:'',    company:'',    phone:'',    email:'',    note:'',    url:'',    address:'',    birthday:'',    socialProfile:''  })  const handleChange = (e) =>{    setPhonebook({      [e.target.name]:e.target.value    })      }  const handleSubmit = (e) =>{    e.preventDefault()    const item = {      firstName: phonebook.firstName,      lastName: phonebook.lastName,      company: phonebook.company,      phone: phonebook.phone,      email: phonebook.email,      url: phonebook.url,      note: phonebook.note,      address: phonebook.address,      birthday: phonebook.birthday,      socialProfile: phonebook.socialProfile          }    console.log(item)      }    return (        <form id='form' onSubmit={handleSubmit} >           <input name='firstName' value={phonebook.firstName} onChange={handleChange} type="text" placeholder="First Name" />                 <input name='lastName' value={phonebook.lastName} onChange={handleChange} type="text" placeholder="Last Name" />                <input name='company' value={phonebook.company} onChange={handleChange} type="text" placeholder="Company" />           <input name='phone' value={phonebook.phone} onChange={handleChange} type="text" placeholder="Phone" />