表單少不了的要做一下錯誤題式訊息，在之前的登入註冊頁面使用firebase時then後面接的catch就是用在這邊。

//login.js

const [emailErrorMessage,setEmailErrorMessage]=useState('');
const [passwordErrorMessage,setPasswordErrorMessage]=useState('');

function onSubmit(){
        firebase.auth().signInWithEmailAndPassword(email, password)
        .then((userCredential) => {
            var user = userCredential.user;
            navigate("/")
        })
        .catch((error) => {
            var errorCode = error.code;
            console.log(errorCode)
            switch(errorCode){
                case 'auth/invalid-login-credentials':
                    setEmailErrorMessage('帳號或密碼錯誤');
                    setPasswordErrorMessage('帳號或密碼錯誤');
                    break;
                case 'auth/user-not-found':
                    setEmailErrorMessage('尚未註冊此信箱');
                    break;
            }
        });
 }
return(
//--略
	<div className="form-group">
	    <label className="form-label" htmlFor="email">帳號<span>*</span></label>
	    <input className="form-input" id="email" type="email" placeholder="請輸入您註冊的Email" value={email} onChange={ (e) => {setEmail(e.target.value)	}}/>
	    <small>{emailErrorMessage}</small>
	</div>
	<div className="form-group">
	    <label className="form-label" htmlFor="password">密碼<span>*</span></label>
	    <input className="form-input" id="password" type="password" placeholder="至少6字元以上" value={password} onChange={ (e) => {setPassword(e.target.value)}}/>
	    <small>{passwordErrorMessage}</small>
	</div>
)

//signup.js

const [emailErrorMessage,setEmailErrorMessage]=useState('');
const [passwordErrorMessage,setPasswordErrorMessage]=useState('');

function onSubmit(){
        firebase.auth().createUserWithEmailAndPassword(email, password)
        .then(userCredential => {
            var user = userCredential.user;
            navigate("/")
           
        })
        .catch((error) => {
            var errorCode = error.code;
            // var errorMessage = error.message;
            console.log(errorCode)
            switch(errorCode){
                case 'auth/email-already-in-use':
                    setEmailErrorMessage('信箱已存在');
                    break;
                case 'auth/invalid-email':
                    setEmailErrorMessage('請輸入正確的Email格式');
                    break;
                case 'auth/weak-password':
                    setPasswordErrorMessage('至少6字元以上');
                break;
            }
        });
    
    };
return(
//--略
	<div className="form-group">
      <label className="form-label" htmlFor="email">帳號<span>*</span></label>
      <input className="form-input" id="email" type="email" placeholder="請輸入您常用的Email" value={email} onChange={(e) => {setEmail(e.target.value)	}}/>
      <small>{emailErrorMessage}</small>
  </div>
  <div className="form-group">
      <label className="form-label" htmlFor="password1">密碼設定<span>*</span></label>
      <input className="form-input" id="password1" type="password" placeholder="至少6字元以上" value={password} onChange={ (e) => {setPassword(e.target.value)}}/>
      <small>{passwordErrorMessage}</small>
  </div>
)

這邊(連結)是所以錯誤訊息errorCode，如果有想添加其他的可以在這找找。

今天是幸運的一天，沒有出現錯誤訊息!!