Javascript 如何在输入为空时禁用下一步按钮

我正在try 创建一个多登录表单，每一个都很好，但我想禁用下一步按钮，如果邮箱输入为空. 输入class="邮箱" 和按钮CLASS="btn-Next" 如有任何帮助，我将不胜感激，谢谢.

<sub>const pageBox = document.querySelector('.page-box');
const btnNext = document.querySelector('.btn-next');
const btnBack = document.querySelector('.btn-back');
const checkboxPass = document.querySelector('.checkbox-pass');
const passwordInput = document.querySelector('.password');
const loginTitle = document.querySelector('.loginTitle-text');
const userEmail = document.querySelector('.user-email');
const emailInput = document.querySelector('.email');

btnNext.onclick = (e) => {
    e.preventDefault();
    pageBox.classList.add('active-pass');
    setTimeout(() => passwordInput.focus(), 500);
    loginTitle.innerHTML = 'Welcome';
    userEmail.innerHTML = ''  + emailInput.value;
};

btnBack.onclick = (e) =>{
    e.preventDefault();
    pageBox.classList.remove('active-pass');
    loginTitle.innerHTML = 'Sign in';
    userEmail.innerHTML = 'Please continue,';
    emailInput.focus();
};

checkboxPass.onclick = () => {
    if(checkboxPass.checked) {
        passwordInput.type = 'text';
    }
    else{
       passwordInput.type = 'password';
    }
};
emailInput.addEventListener("btnNext", (e) => {
    preventDefault();
    btnNext.disabled()
    if(emailInput.value.length  > 0) {
        btnNext.disabled = 'true';
     }
     else{
        btnNext.disabled = 'false';
    } 
});
</sub>