script.js

var form = document.querySelector("form");
var nextBtn = form.querySelector(".nextBtn");
var backBtn = form.querySelector(".backBtn");
var allInput = form.querySelectorAll(".input-field");

nextBtn.addEventListener("click", () => {
    allInput.forEach(input => {
        if(input.value !== ""){
            form.classList.add('secActive');
        }else{
            form.classList.remove('secActive');
        }
    })
})

backBtn.addEventListener("click", () => form.classList.remove('secActive'));