Create 2-layered dropdown menus for custom taxonomy and custom post type

Question

I want to display one dropdown menu with a list of values for a custom taxonomy. My taxonomy is "issue-year" and the dropdown would look like this:

<select name="issue-year">
    <option value="#">Please select a year</option>
    <option value="2016">2016</option>
    <option value="2015">2015</option>
    <option value="2014">2014</option>
</select>

Once a year is selected, a second dropdown would appear below the first to display posts in a custom post type that match the selected taxonomy value. My custom post type is "newsletter."

<select name="newsletter">
    <option value="#">Please select an issue</option>
    <option value="postname-1">Post Name 1</option>
    <option value="postname-2">Post Name 2</option>
    <option value="postname-3">Post Name 3</option>
</select>

I'm currently using "get_terms_dropdown" to generate the first field, but I'm not sure how to get from step one to step two. Also, if there's a better way than what I did in the code below I'd love to hear it.

function get_terms_dropdown($taxonomies, $args){
    $myterms = get_terms($taxonomies, $args);
    $output ="<select name='issue-year'>";
    $output .="<option value='#'>Please select a Year</option>";
    foreach($myterms as $term){
        $root_url = get_bloginfo('url');
        $term_taxonomy=$term->taxonomy;
        $term_slug=$term->slug;
        $term_name =$term->name;
        $link = $term_slug;
        $output .="<option value='".$link."'>".$term_name."</option>";
    }
    $output .="</select>";
return $output;
}

Answer 1

With a combination of jQuery / JavaScript and WP AJAX; you could look for an onchange event on the select box, send an ajax request to your admin-ajax.php with the year and then do a custom WP Query to return the html for the secondary drop down and append it to your dom.

https://codex.wordpress.org/Plugin_API/Action_Reference/wp_ajax_%28action%29

http://api.jquery.com/jquery.ajax

Once it is appending you can target your new select box with an onchange event as well.