JSFiddle - React, Tailwind, and code Playground

by birdie2019

HTML

<div class="container">
  <h2>註冊帳戶</h2>
  <form>
    <div class="user box">
      <input type="text" name="username" id="name" required>
      <label for="user-name">名稱</label>
      <span>不能為空白、必須在40字以內！</span>
    </div>
    <div class="email box">
      <input type="text" name="email" id="mail" required>
      <label for="user-mail">信箱</label>
      <span>不能為空白、必須包含 @ 符號！</span>
    </div>
    <div class="password box">
      <input type="password" name="password" id="password" required>
      <label for="user-password">密碼</label>
      <span>不能為空白、必須在8字以上、必須在100字以內！</span>
    </div>
  </form>
  <button type="submit" id="submit">註冊</button>
</div>

SCSS

* {
    box-sizing: border-box;
    margin: 0;
    padding: 0;
    font-size: 10px;
    letter-spacing: 1px;
    color: #fff;
}

body {
    background-color: #44474e;
}

.container {
    display: flex;
    flex-direction: column;
    align-items: center;
    width: 400px;
    position: absolute;
    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%);
    margin: 0 auto;
    padding: 50px;
    border-radius: 5px;
    background-color: rgb(20, 29, 49);
    box-shadow: 0 0 15px rgba(0, 0, 0, 0.6);
    h2 {
        margin-bottom: 50px;
        font-size: 2.5rem;
        letter-spacing: 3px;
    }
    button {
        width: 50%;
        padding: 10px;
        background-color: transparent;
        border: 1px solid #fff;
        border-radius: 50px;
        cursor: pointer;
        font-size: 1.6rem;
        color: #fff;
        transition: all .5s;
        &:hover {
            border: 1px solid aqua;
            color: aqua;
            transform: scale(1.05);
        }
    }

    .box {
        position: relative;
        margin-bottom: 28px;
        label {
            position: absolute;
            top: 8px;
            left: 0px;
            font-size: 1.6rem;
            transition: all 0.5s;
        }
        input {
            width: 300px;
            padding: 10px 0px;
            margin-bottom: 3px;
            background-color: transparent;
            border: 0;
            border-bottom: 1px solid rgba(255, 255, 255, .5);
            outline: none;
            font-size: 1.6rem;
            &:focus ~ label {
                top: -20px;
                font-size: 1.4rem;
                color: aqua;
                background-color: transparent;
            }
            &:valid ~ label {
                top: -20px;
                font-size: 1.4rem;
                color: aqua;
                background-color: transparent;
            }
        }
        span {
            color: rgb(255, 117, 117);
            transition: all 0.3s;
     ...

JavaScript

var subName = document.querySelector('#name');
var subMail = document.querySelector('#mail');
var subPassword = document.querySelector('#password');
var submitBtn = document.querySelector('#submit');
submitBtn.onclick = fromRequired;
var spans = document.querySelectorAll('span');
for (var span of spans) {
    span.style.opacity = '0';
}
var inputs = document.querySelectorAll('input');
for (var input of inputs) {
    input.onclick = styleChange;
}

function fromRequired() {
    var subNameValue = subName.value.trim();
    var subMailValue = subMail.value.trim();
    var subPasswordValue = subPassword.value.trim();
    if (subNameValue == '' || subNameValue.length >= 40) {
        subName.closest('.box').querySelector('span').style.opacity = '1';
        subName.style.borderBottom = '1px solid rgba(255, 117, 117, .5)';
    } 
    if (subMailValue == '' || subMailValue.includes('@') == false) {
        subMail.closest('.box').querySelector('span').style.opacity = '1';
        subMail.style.borderBottom = '1px solid rgba(255, 117, 117, .5)';
    }
    if (subPasswordValue == '' || subPasswordValue.length < 8 || subPasswordValue.length > 100) {
        subPassword.closest('.box').querySelector('span').style.opacity = '1';
        subPassword.style.borderBottom = '1px solid rgba(255, 117, 117, .5)';
    } else {
        console.log('資料符合規定')
    }
}

function styleChange(e) {
    e.target.closest('.box').querySelector('span').style.opacity = '0';
    e.target.style.borderBottom = '1px solid rgba(255, 255, 255, .5)';
}