JQuery - Tried to connect server but ajax returning Code 0

I'm trying to check if same email exist in server db, but ajax is returning code 0 even my server is opened. I checked it by using URL in browser.

if (input == email.find(".form_input").val()) {
    emailChecking = true;
    $.ajax({
        url: "https://localhost:8888/check?target=" + input,
        async: true,
        type: "GET",
        dataType: "text",
        success: (data) => {
            console.log(data);
            if (emailChecking == true) {
                emailChecking = false;
                inputField(email, error);
            }
        },
        error: (request, status, errorLine) => {
            if (emailChecking == true) {
                emailChecking = false;
                error = "cannot check email - code " + request.status;
                inputField(email, error);
            }
        }
    });
}