JSFiddle - React, Tailwind, and code Playground

HTML

<form id="form" action="">
    <ul>
      <li> 
        <label for="name">Введите имя</label>
        <input id="name" type="text"></li>
      <li>
        <label for="phone">Введите телефон</label>
        <input id="phone" type="text">
      </li>
      <li>
        <label for="email">Введите почту</label>
        <input type="email">
      </li>
    </ul>
    <button id="btn">Результат</button>
</form>

JavaScript

let form = document.getElementById('form')
let input = form.getElementsByTagName('input')
const validation = function () {
    let nextBtn = document.getElementById('next')
    form.addEventListener('click', function (e) {
            e.preventDefault()
            if (e.target === document.getElementById('btn')) {
                for ( let i = 0; i < input.length; i++ ) {
                    if (input[i].value === '') {
                        return alert("Заполните все поля")
                    } else return alert("Все поля заполнены")
                }
            }
        })
}
validation()