solve

function startApp() {
    sessionStorage.clear()
    showHideMenuLinks()

    $('#registerButton').on('click', showRegisterView)
    $('#registerUser').on('click', registerUser)
    $('#loginButton').on('click', showLoginView)
    $('#logUserBtn').on('click', loginUser)
    $('#myListings').on('click', myListingView)
    $('#logout').on('click', logoutUser)
    $('#signUpBtn').on('click', showRegisterView)
    $('#signInBtn').on('click', showLoginView)
    $('#home').on('click', showHomeMenu)
    $('#allListings').on('click', allListings)
    $('#createListings').on('click', createListingsView)
    $('#createCars').submit(createCar)
    $('#editCars').submit(saveEditedCar)
    $("form").submit(function (event) {
        event.preventDefault()
    });
    // $('#registerUser').on('click', function (event) {
    //     event.preventDefault()
    // })
    // $('#registerButton').on('click', function (event) {
    //     event.preventDefault()
    // })


    function hideAllViews() {
        $('#main').hide();
        $('#login').hide();
        $('#registerForm').hide();
        $('#createCars').hide();
        $('#editCars').hide();
        $('#myCarListings').hide();
        $('#myCars').hide();
        $('#listDetails').hide();
        $('#car-listings').hide();
        $('#carMyListings').hide()
    }


    $(document).on({
        ajaxStart: function () {
            $('#loadingBox').show()
        },
        ajaxStop: function () {
            $('#loadingBox').hide()
        }
    })
    $('#infoBox', '#errorBox').on('click', function () {
        $(this).fadeOut()
    })

    function showInfo(message) {
        $('#infoBox').show()
        $('#infoBox > span').text(message)
        setTimeout(function () {
            $('#infoBox').fadeOut()
        }, 3000)
    }

    function showError(error) {
        $('#errorBox').show()
        $('#errorBox > span').text(error)
        $('#errorBox').on('click', function () {
            $(this).fadeOut()
        })
    }

    function handleAjaxError(response) {
        let errorMsg = JSON.stringify(response);
        if (response.readyState === 0)
            errorMsg = "Cannot connect due to network error.";
        if (response.responseJSON && response.responseJSON.description)
            errorMsg = response.responseJSON.description;
        showError(errorMsg);
    }

    const kinveyBaseUrl = "https://baas.kinvey.com/";
    const kinveyAppKey = "kid_rkMtKform";
    const kinveyAppSecret = "53badeb768df46aaa0e0782d3c99a442";
    const kinveyAppAuthHeaders = {
        'Authorization': "Basic " + btoa(kinveyAppKey + ":" + kinveyAppSecret),
    };

    function showHideMenuLinks() {
        hideAllViews();
        if (sessionStorage.getItem('authToken') === null) {
            $('#main').show();
            $('#allListings').hide();
            $('#carListings').hide();
            $('#createListings').hide();
            $('#car-listings').hide();
            $('#profile').hide();
            $('#myListings').hide();
            $('#carMyListings').hide()
        } else {
            $('#main').hide();
            $('#allListings').show();
            $('#myListings').show();
            $('#createListings').show();
            $('#profile').show();
            $('#carListings').hide();
            $('#car-listings').show();
            $('#carMyListings').hide()
            $('#welcm').text(`Welcome ${sessionStorage.getItem('userName')}`);
        }

    }

    function saveAuthInSession(userInfo) {
        sessionStorage.setItem('authToken', userInfo._kmd.authtoken)
        sessionStorage.setItem('userId', userInfo._id)
        sessionStorage.setItem('userName', userInfo.username)
    }

    function getKinveyUserAuthHeaders() {
        return {
            'Authorization': "Kinvey " + sessionStorage.getItem('authToken'),
        };
    }

    function showHomeMenu() {
        showHideMenuLinks()
    }

    function createListingsView() {
        hideAllViews()
        $('#createCars').show()
        $('#createCars').trigger('reset')
    }

    function showRegisterView() {
        hideAllViews()
        $('#registerForm').show()
        $('#registerForm').trigger('reset')
    }

    function showLoginView() {
        hideAllViews()
        $('#login').show()
        $('#formLogin').trigger('reset')
    }

    function loginUser() {
        let username = $('#formLogin input[name=username]').val()
        let password = $('#formLogin input[name=password]').val()
        let usernameRegex = /[A-Za-z]{3,}/
        let passRegex = /[A-Za-z0-9]{6,}/
        if (!usernameRegex.test(username)) {
            showError('User name length has to be at least 3 characters long and contains just english letters.')
        }
        else if (!passRegex.test(password)) {
            showError('Password length has to be at least 6 characters long and contains just english letters and digits.')
        }
        else {
            $.ajax({
                method: "POST",
                url: kinveyBaseUrl + "user/" + kinveyAppKey + "/login",
                data: {username, password},
                headers: kinveyAppAuthHeaders
            }).then(function (res) {
                saveAuthInSession(res)
                showHideMenuLinks()
                allListings()
                showInfo('Login successful.')
            }).catch(handleAjaxError)
        }
    }

    function registerUser() {
        let username = $('#registerForm input[name=username]').val()
        let password = $('#registerForm input[name=password]').val()
        let repeatPass = $('#registerForm input[name=repeatPass]').val()
        let usernameRegex = /[A-Za-z]{3,}/
        let passRegex = /[A-Za-z0-9]{6,}/
        if (!usernameRegex.test(username)) {
            showError('User name length has to be at least 3 characters long and contains just english letters.')
        }
        else if (password !== repeatPass) {
            showError('Password and repeat pass have to be equal.')
        }
        else if (password === '' && repeatPass === '') {
            showError('Password field is required')
        }
        else if (!passRegex.test(password)) {
            showError('Password length has to be at least 6 characters long and contains just english letters and digits.')
        }
        else {
            $.ajax({
                method: "POST",
                url: kinveyBaseUrl + "user/" + kinveyAppKey + "/",
                headers: kinveyAppAuthHeaders,
                data: {username, password}
            }).then(function (res) {
                saveAuthInSession(res)
                showHideMenuLinks()
                allListings()
                showInfo('User registration successful.')
            }).catch(handleAjaxError)
        }

    }

    function logoutUser() {
        $.ajax({
            method: "POST",
            url: kinveyBaseUrl + "user/" + kinveyAppKey + "/_logout",
            headers: getKinveyUserAuthHeaders()
        });
        sessionStorage.clear()
        $('#welcm').text('')
        showHideMenuLinks()
        showInfo('Logout successful.')
    }

    function allListings() {
        showHideMenuLinks()
        $.ajax({
            method: "GET",
            url: kinveyBaseUrl + "appdata/" + kinveyAppKey + "/cars?query={}&sort={\"_kmd.ect\": -1}",
            headers: getKinveyUserAuthHeaders()
        }).then(function (resp) {
            $('#car-listings').show()
            $('#listings').empty()
            if (resp.length === 0) {
                $('#listings').append('<p class="no-cars">No cars in database.</p>')
            }
            else {
                for (const res of resp) {
                    let mainDiv = $('<div class="listing">')
                    let p = $('<p>').text(res.description)
                    let img = $(`<img src="${res.imageUrl}">`)
                    let h2 = $('<h2>').text(`Brand: ${res.brand}`)
                    let divInfo = $('<div class="info">')
                    let divData = $('<div id="data-info">')
                    let h31 = $('<h3>').text(`Seller: ${res.seller}`)
                    let h32 = $('<h3>').text(`Fuel: ${res.fuel}`)
                    let h33 = $('<h3>').text(`Year: ${res.year}`)
                    let h34 = $('<h3>').text(`Price: ${res.price} $`)
                    $(divData).append(h31).append(h32).append(h33).append(h34)
                    let divBtn = $('<div id="data-buttons">')
                    let ul = $('<ul>')
                    let liDetails = $('<li class="action">')
                    let aDetails = $('<a href="#" class="button-carDetails">Details</a>').click(detailsCar.bind(this, res))
                    $(liDetails).append(aDetails)
                    $(ul).append(liDetails)
                    $(divBtn).append(ul)
                    if (sessionStorage.getItem('userId') === res._acl.creator) {
                        let liEdit = $('<li class="action">')
                        let aEdit = $('<a href="#" class="button-carDetails">edit</a>').click(editCar.bind(this, res))
                        let liDelete = $('<li class="action">')
                        let aDelete = $('<a href="#" class="button-carDetails">Delete</a>').click(deleteCar.bind(this, res))
                        $(liEdit).append(aEdit)
                        $(liDelete).append(aDelete)
                        $(ul).append(liEdit).append(liDelete)
                    }
                    $(divInfo).append(divData).append(divBtn)
                    $(mainDiv).append(p).append(img).append(h2).append(divInfo)
                    $('#listings').append(mainDiv)

                }
            }

        }).catch(handleAjaxError)
    }

    function editCar(car) {
        hideAllViews()
        $('#editCars').attr('carId', car._id)
        $('#editCars').show()
        $('#editCars input[name=title]').val(car.title)
        $('#editCars input[name=description]').val(car.description)
        $('#editCars input[name=brand]').val(car.brand)
        $('#editCars input[name=model]').val(car.model)
        $('#editCars input[name=year]').val(car.year)
        $('#editCars input[name=imageUrl]').val(car.imageUrl)
        $('#editCars input[name=fuelType]').val(car.fuel)
        $('#editCars input[name=price]').val(car.price)

    }

    function saveEditedCar() {
        let seller = sessionStorage.getItem('userName')
        let title = $('#editCars input[name=title]').val()
        let description = $('#editCars input[name=description]').val()
        let brand = $('#editCars input[name=brand]').val()
        let model = $('#editCars input[name=model]').val()
        let year = $('#editCars input[name=year]').val()
        let imageUrl = $('#editCars input[name=imageUrl]').val()
        let fuel = $('#editCars input[name=fuelType]').val()
        let price = Number($('#editCars input[name=price]').val())
        let id = $('#editCars').attr('carId')
        if (title.length > 33) {
            showError('Title lenght is too long.')
        }
        else if (description.length < 30 || description.length > 450) {
            showError('Description name should be between 30 and 450 chars.')
        }
        else if (brand.length > 11 || fuel.length > 11 || model.length > 11) {
            showError('Brand, Fuel and Model should be less than 11 chars.')
        }
        else if (year.length !== 4) {
            showError('Year has contains 4 chars.')
        }
        else if (price > 1000000) {
            showError('Price is invalid')
        }
        else if (!imageUrl.startsWith('http')) {
            showError('Image url has to start with "http"')
        }
        else {
            $.ajax({
                method: "PUT",
                url: kinveyBaseUrl + "appdata/" + kinveyAppKey + "/cars/" + id,
                headers: getKinveyUserAuthHeaders(),
                data: {seller, title, description, brand, model, year, imageUrl, fuel, price}
            }).then(function (res) {
                showHideMenuLinks()
                showInfo(`Listing ${title} updated.`)
                allListings()
            }).catch(handleAjaxError)
        }
    }

    function detailsCar(car) {
        hideAllViews()
        $('#listDetails').show()
        $('.my-listing-details').empty()
        let p = $('<p id="auto-title">').text(car.title)
        let img = $(`<img src="${car.imageUrl}">`)
        let divHeaders = $('<div class="listing-props">')

        let h2 = $('<h2>').text(`Brand: ${car.brand}`)
        let h31 = $('<h3>').text(`Model: ${car.model}`)
        let h32 = $('<h3>').text(`Year: ${car.year}`)
        let h33 = $('<h3>').text(`Fuel: ${car.fuel}`)
        let h34 = $('<h3>').text(`Price: ${car.price} $`)
        $(divHeaders).append(h2).append(h31).append(h32).append(h33).append(h34)

        if (sessionStorage.getItem('userId') === car._acl.creator) {
            let divHrefs = $('<div class="listings-buttons">')
            let a1 = $('<a href="#" class="button-list">Edit</a>').click(editCar.bind(this, car))
            let a2 = $('<a href="#" class="button-list">Delete</a>').click(deleteCar.bind(this, car))
            $(divHeaders).append(a1).append(a2)
            $('.my-listing-details').append(divHrefs)
        }

        let p1 = $('<p id="description-title">Description:</p>')
        let p2 = $(`<p id="description-para">${car.description}</p>`)

        $('.my-listing-details').append(p).append(img).append(divHeaders).append(p1).append(p2)
    }

    function deleteCar(car) {
        $.ajax({
            method: "DELETE",
            url: kinveyBaseUrl + "appdata/" + kinveyAppKey + "/cars/" + car._id,
            headers: getKinveyUserAuthHeaders()
        }).then(function (res) {
            allListings()
            showInfo('Listing deleted')
        })
    }

    function createCar() {
        let seller = sessionStorage.getItem('userName')
        let title = $('#createCars input[name=title]').val()
        let description = $('#createCars input[name=description]').val()
        let brand = $('#createCars input[name=brand]').val()
        let model = $('#createCars input[name=model]').val()
        let year = $('#createCars input[name=year]').val()
        let imageUrl = $('#createCars input[name=imageUrl]').val()
        let fuel = $('#createCars input[name=fuelType]').val()
        let price = Number($('#createCars input[name=price]').val())
        if (title.length > 33) {
            showError('Title lenght is too long.')
        }
        else if (description.length < 30 || description.length > 450) {
            showError('Description name should be between 30 and 450 chars.')
        }
        else if (brand.length > 11 || fuel.length > 11 || model.length > 11) {
            showError('Brand, Fuel and Model should be less than 11 chars.')
        }
        else if (year.length !== 4) {
            showError('Year has contains 4 chars.')
        }
        else if (price > 1000000) {
            showError('Price is invalid')
        }
        else if (!imageUrl.startsWith('http')) {
            showError('Image url has to start with "http"')
        }
        else if (title === '' || description === '' || brand === '' || model === '' ||
            year === '' || imageUrl === '' || fuel === '' || price === '') {
            showError('All fields have to be full!')
        }
        else {
            $.ajax({
                method: "POST",
                data: {seller, title, description, brand, model, year, imageUrl, fuel, price},
                headers: getKinveyUserAuthHeaders(),
                url: kinveyBaseUrl + "appdata/" + kinveyAppKey + "/cars"
            }).then(function (res) {
                showHideMenuLinks()
                showInfo('Listing created.')
                allListings()
            }).catch(handleAjaxError)
        }
    }

    function myListingView() {
        hideAllViews()
        $.ajax({
            method: "GET",
            url: kinveyBaseUrl + 'appdata/' + kinveyAppKey + `/cars?query={"seller":"${sessionStorage.getItem('userName')}"}&sort={"_kmd.ect": -1}`,
            headers: getKinveyUserAuthHeaders()
        }).then(function (cars) {
            $('#carMyListings').show()
            $('#myCars').empty()
            if (cars.length === 0) {
                $('#myCars').append($('<p class="no-cars">').text(" No cars in database."))
            } else {

                for (let car of cars) {
                    let div = $('<div class="my-listing">');
                    div.append($('<p id="listing-title">').text(`${car.title}`));
                    div.append($('<img>').attr('src', `${car.imageUrl}`));

                    let secondDiv = $('<div class="listing-props">');
                    secondDiv.append($('<h2>').text(`Brand: ${car.brand}`));
                    secondDiv.append($('<h3>').text(`Model: ${car.model}`));
                    secondDiv.append($('<h3>').text(`Year: ${car.year}`));
                    secondDiv.append($('<h3>').text(`Price: ${car.price}`));

                    let thirdDiv = $('<div class="my-listing-buttons">');
                    thirdDiv.append($('<a href="#" class="my-button-list">').text('Details').on('click', function () {
                        detailsCar(car);
                    }));
                    thirdDiv.append($('<a href="#" class="my-button-list">').text('Edit').on('click', function () {
                        editCar(car);
                    }));
                    thirdDiv.append($('<a href="#" class="my-button-list">').text('Delete').on('click', function () {
                        deleteCar(car);
                    }));
                    div.append(secondDiv);
                    div.append(thirdDiv);
                    $('#myCars').append(div);
                }
            }
            $('#myCars').show();


        }).catch(handleAjaxError)
    }
}